An Examination of Magnolia grandiflora under Varying Management Conditions
Magnolia grandiflora L., commonly known as the Southern magnolia or evergreen magnolia, is native to much of the southeastern United States. Its native range encompasses the middle and southern sections of Georgia, South Carolina, Alabama and Louisiana and the northern portions of Florida and Texas. The species' large leaves are shiny green on top and fuzzy orange-brown below, while its large buds are also a fuzzy or pubescent red-brown color. Southern magnolia has large (often eight or more inches in diameter) white flowers that typically bloom in late June or July. Typically, Magnolia grandiflora begins blooming at about fifteen years of age, though many cultivars bloom at a younger age. The species is somewhat unpredictable in form, but usually has a single dominant trunk.
The large number of cultivars used in this and other regions makes assessing the plants somewhat problematic. Differences in size, growth form, and general health may be at least partially attributed to the use of different cultivars. However, maintenance of the tree and environmental conditions are the primary factors influencing the health and success of Magnolia grandiflora. Proper management techniques are the same for all cultivars of the species.
I studied twenty Magnolia grandiflora plants in a variety of locations throughout the Pacific Northwest. The sites included the University of Washington campus, the Washington Park Arboretum, parking lots, shopping centers, and private yards.
In order to assess the management of the trees, I created a checklist of items to look at in each location. The studied items included: soil texture, compaction level, drainage, amount and direction of solar radiation, whether or not mulch was used, growing space, nearby vegetation, pruning wounds, broken branches, evidence of pathogens, and overall health of roots, trunks, branches, and foliage.
The soil environment is one of the most critical factors influencing a plant's health and success. Nutrient and water availability, as well as root health, are impacted by the soil a plant grows in. To determine an optimal soil environment, I looked at several factors relating to soil. I first checked soil texture with a feel-test in the field (grab a handful of soil, dampen it, and see if it forms a ribbon and how coarse or fine the particles seem). I studied soil at a depth of approximately four inches. I next looked to see what type of mulch, if any, was used around the M. grandiflora. Mulch helps to suppress weed growth, retain water, and regulate soil temperatures among many other benefits. I also looked at the root system to check for any visible problems, such as kneeing or circling roots. Finally, I tried to determine drainage rates in the area by studying the plants on both dry and wet days. I looked for any standing or pooled water, and dug down a couple inches into the soil to see how wet it was. Not surprisingly, the heavily compacted soils, as well as the more clayey soils, had worse drainage than the other soils.
The general health of a tree is also important, and can indicate how well cared-for a plant is (or isn't). In studying the trees, I examined their trunks, branches, and canopies. I looked for and noted any cankers, galls, foliar blight, insects, or other signs and symptoms of plant pathogens. I watched for types and patterns of foliar damage or death, including leaf spots, insect damage, holes, discoloration, or necrosis. All of these problems can be signs of serious disease or nutritional problems, though some, such as leaf spots, don't do much to harm the trees. The growth habit (natural or trained) was noted, as was the size of the tree. Any barriers to growth, such as buildings, power lines, or competing vegetation were noted; if the tree lacks sufficient room for growth branches may break off, roots may damage sidewalks and other impermeable surfaces, or the plants' health may suffer. I also looked for pruning wounds to see if pruning had been done, and also to see if it was performed correctly. Any improperly done pruning may stress the trees and allow an entryway for pathogens and diseases. Finally, I checked for broken off branches, either from wind or humans or other animals. These wounds allow pathogen entry and may not properly heal.
Appendix one contains tables summarizing the general health and maintenance of the trees I studied.
1 The first Magnolia grandiflora I assessed is located on the north side of Winners restaurant, in a retail corridor near Southcenter Mall. It is one of a row of five Magnolia grandiflora plants in a small alcove. The trees were planted less than a foot from the building, approximately one foot from some large shrubs, and about three feet from each other, all conditions that restrict the trees' horizontal growth. An overhead awning roughly twelve feet high restricts vertical growth; several of the trees' branches are already growing into the overhang and bending outward to find sufficient growing room. The trees receive very little sunlight, with most of the light blocked or filtered by the restaurant building. To further compound their problems, the trees are staked using wood that rubs against the trunk, as well as plastic ties placed too high on the plant, restricting trunk movement. The stakes are beginning to girdle the trees, creating wounds and reducing taper. Planting this species here is an excellent example of poor plant selection. The planting site is far too small, and although the general landscape is well maintained, these trees are quite stressed and already performing poorly at a fairly young age. It won't be at all surprising if they die within a few years.
2. The second M. grandflora I studied is also located near Winners restaurant in Tukwila, on the west side of the building. This plant, however, has significantly more growing space and is planted in a more appropriate location. Its trunk is located about seven feet from the parking lot and the only nearby vegetation is small shrubs and flowers. Nothing restricts the horizontal growth of this tree, but vertical growth could eventually be restricted by the edge of the building. The tree is properly pruned, but has some broken branches that haven't yet been pruned back, though they presumably will the next time a landscaping crew is working at the site. With its more suitable location, this tree is performing far better than the trees on the north side of the building, and appears to be in far better health.
3. The third tree I examined is growing near an Azteca restaurant in the parking lot of a strip mall south of Southcenter Mall. The tree is located on the southwest corner of the building. It was planted approximately three feet from the walkway and five feet from the building. Nearby vegetation includes small shrubs and flowers, which don't really cause problems for the magnolia. The tree has been pruned to keep branches away from the building, but there are no other major restrictions on its growth. Currently, a nearby expanse of grass is torn up by construction equipment. The construction activity is far enough away from the plant that no major roots were damaged by the digging, though some smaller, outlying roots are being damaged or removed. Most of the plants in the area, including this tree, look fairly healthy and have been pruned, mulched, and generally well maintained.
4. The next tree I assessed is one of a group of trees planted alongside the road west of Boeing Field. The trees are in a gently sloped grassy area approximately fifteen feet wide. A swath of turf about three or four feet in diameter was cleared around the base of each tree, leaving bare soil near the trunk. At least seven or eight feet of permeable surfaces exist around the trees, though farther out the roots must grow under a parking lot or the road. Overhead power lines could eventually restrict vertical growth, but are far enough (at least five feet horizontally and thirty feet vertically) away that they shouldn't present a problem. These were the healthiest M. grandifloras I saw being used as street trees, with some leaf spotting and minor chlorosis being their only obvious health problems.
5. Tree number five is located on the University of Washington's campus, planted on the south side of the atmospheric sciences/geophysics building. The tree was planted about ten feet from the building, and some of the branches touch the building. A sidewalk and road within ten feet of the trunk limit root growth. This tree is in a construction area, with no significant protection from construction activities. All the nearby vegetation was either removed or pruned severely, leaving only small stumps where large bushes once stood. Chunks of concrete, pieces of plywood, and other rubble were thrown atop the tree's roots and the area's soil has been heavily compacted by construction activity. A chain link fence was erected on the south and western sides of the tree, but this seems more for the benefit of the construction company than for the tree. Before the construction activity began, this was a fairly healthy tree. It now has large branches and numerous leaves dying along with mechanical damage to the trunk, and is generally in poorer health.
6. This magnolia is growing in front of Hall Health on the UW campus. Its trunk is located four or five feet from both an asphalt walkway and a brick-paved entryway, which both compact soil beneath them, restricting root growth. A large nearby tree and a light post restrict horizontal branch growth, though there are no barriers to vertical growth, allowing the tree to grow to a height of approximately thirty feet. Hall Health casts a shadow that limits sunlight from the east, though the tree receives fairly large amounts of both direct and indirect light from the west and the south. Although this tree appears fairly well maintained, it has a circling root that could some day girdle the trunk, as well as some decayed branches that should be pruned out. I noticed multiple older leaves turning completely yellow and falling off the tree, but was unable to tell if this was the result of a nutrient deficiency or other problem or was simply normal annual leaf drop.
7. The seventh tree I looked at is located at the Center for Urban Horticulture. This tree was one of the bushiest I saw, with many branches growing throughout the trunk and a canopy that started just a foot or so above the ground. Branches growing outward could eventually hit a nearby sign, and large concrete pavers located several feet beyond the dripline on the west and east sides of the tree could impede root growth. The only vegetation within the tree's dripline is small herbaceous plants. This tree also had older leaves becoming chlorotic and falling off the tree. Many newer leaves were full of brown or black leaf spots, but the tree generally appears to be well cared for and in good health.
8. This magnolia is growing on the UW campus near Pend Oreille Road, south of McMahon Hall. This is also a rather bushy tree, with the lowest branch about a foot and a half above the ground. It is nearly as wide as it is tall, growing about twelve feet in both height and width. There are no barriers to aboveground growth for this tree, and the nearest impervious surface is at least ten feet away from the trunk. The tree receives filtered sunlight throughout most of the day, with some fairly strong sunlight in the morning. Gaultheria shallon growing up to the base of the trunk made it difficult to examine the tree's roots. The Gaultheria may be competing with the magnolia for resources, though neither showed much sign of stress or nutrient deficiency.
9. This Magnolia grandiflora is part of a group of magnolias planted throughout the parking lot of the Lewis and Clark theater/bowling alley in Tukwila. The care of these trees varies, but none of the trees looks particularly healthy or well maintained. The trees are located in planting islands ranging between three and six feet in diameter; this tree is in a bigger planter with a diameter of approximately six feet. This tree (along with the others in the parking lot) is subjected to incredibly stressful conditions. The magnolia receives unrestricted direct sunlight, along with reflected light and heat from the parking lot and automobiles. It is continually exposed to car exhaust and other atmospheric pollution. There is no other vegetation growing in the planting strip (not even weedy species), and the soil is heavily compacted. Most of the root zone is covered with the asphalt of the parking lot, and this tree has little suitable or uncompacted soil in which to grow. It looks as though the tree has been topped, with much of the tree's growth concentrated outward just below its uppermost point. There is no irrigation system in the area, and it is highly doubtful that the tree receives any supplemental water in the dry season. Based on the overall condition of the lot's vegetation, I doubt that it receives pruning, watering, or other general maintenance more than once a year, if that often. I also wouldn't be surprised if the plant was not transplanted correctly or had root problems that were not fixed prior to planting the tree.
10. This tree is growing in a private yard at the corner of 15th Ave. S. and S. Angeline St. on Beacon Hill. It was one of the healthiest, most well maintained magnolias I saw. The tree is located on the north side of the home, but receives ample sunlight from the east and the west. Unfortunately, the tree was planted only a few feet from overhead power lines, which are about fifteen feet high. The main trunk of the tree is already as tall as the lines, and the power line going into the home travels through the tree's canopy. The branches may eventually be considered hazardous nuisances, causing the homeowner or power company to resort to a bad pruning or topping job to keep the tree out of the lines. At the moment, however, the tree is not creating or experiencing any serious problems. It is being properly pruned and trained to have a somewhat rounded shape, with lower branches removed to prevent them from growing into a fence or the house. The tree appears to receive regular and proper maintenance.
11. The next tree I assessed is located in the Washington Park Arboretum, near the Graham Visitors' Center. This tree is planted roughly twelve feet from the parking lot and seven from the greenhouse and a path leading to it. At the moment, there are no restrictions on the growth of this young tree, but if it grows taller and wider the tree will likely hit the building and the roots may grow under the parking lot and push up on the pavement. The tree is growing in very wet, heavy, clay soil and is performing worse than most of the nearby vegetation. Until a few years ago, the tree looked very nice. This magnolia was being trained gradually and correctly, with the canopy being slowly raised and a few branches removed to shape the tree. Unfortunately, the tree started losing many leaves, with other leaves becoming quite chlorotic. According to arboretum staff, fertilizing and other cultural practices haven't improved the tree's health. The WPA staff believes that the tree has root problems and should probably be removed.
12. This magnolia is also growing in the Arboretum, this time near Loderi Valley. The tree was randomly chosen from a grouping of about a half-dozen Magnolia grandifloras. The tree is growing rather close to a large Liriodendron tulipifera (tulip tree), and the two trees are competing for space. It appears that the L. tulipifera is winning; the magnolia bends away from the tulip tree and has very few branches on the side closest to the tulip tree. Another younger magnolia is also growing close to this magnolia, and the two magnolias will likely begin competing for resources, including space, in the near future. Aside from the space restrictions, the magnolia appears to be in good health with many strong branches and shiny, healthy leaves.
13. The thirteenth tree I studied was located in front of the Riverton Heights (Tukwila) church. This tree was perched atop a short hill with about a 30-degree slope. The trunk and any roots within the dripline were on level ground at the top, but roots farther out were under the sloped portion. The magnolia was planted about seven feet from a building, with some branches only inches away from the structure. It appeared that some branches had been pruned to prevent them from hitting the building, though they might have been pruned to thin the canopy--it was difficult to guess the intentions of the pruning. The tree has multiple leaders, and other leaders have been pruned out, though the stumps left by the pruning are too large and not healing well. It looks like people are trying to care for the tree, but don't understand good horticultural practices, such as proper pruning or nutritional regimes.
14. This magnolia is growing in Clackamas, Oregon, near a shop in the Clackamas Center. Four magnolias were planted in a grassy planting area between the store's parking lot and a sidewalk. The tree I assessed was planted about ten feet from both the parking lot and the sidewalk, with a row of Buxus sempervirens and a small slope between the magnolia and the parking lot. The tree has limited growing space in this area (though more than most trees in retail areas), but no restrictions on aboveground growth. It is growing on the southeast side of the building and receives much direct sunlight and no real protection from wind or other potentially damaging elements. This tree appears fairly young, but has already had several major branches chopped out, leaving a rather sparse canopy. This pruning seemed excessive, and did not appear geared toward training the tree or improving its health. The tree's leaves appear rather stressed, with many chlorotic leaves indicating that the plant needs to be fertilized. This tree apparently gets the same poor maintenance that too many other plants in retail areas receive.
15. The next M. grandilfora I assessed is located in a sidewalk-level planter at University Village. This is one of many trees scattered throughout the shopping center; this particular tree is just west of the Azteca restaurant. The tree is in a small (about 2 1/2 feet by 2 1/2 feet of soil) planting hole with a metal grate above it. The trunk is widening, but currently has sufficient room to grow in the hole in the metal grate. The only major barrier to root or shoot growth is the limited soil space. Although no stakes are currently on the tree, it obviously had been staked, and the stakes were left on too long. Large areas of raised bark show where wires with rubber ends had been; it looks like the bark grew up on both sides of the rubber portion's edge and was close to growing over the rubber. A long strand of white lights also encircles the tree. It appears as though the lights were put up over the holiday season and were never removed. The lights may well end up girdling the trunk of the tree if left on year-round.
16. The next tree I studied was also located at U. Village. This magnolia, however, is in a rather large ceramic container (the container is about 2 1/2 feet tall with a top diameter of about 3 feet), though too small for such a large tree. This container leaves very limited space for the tree's roots to grow, and they likely will start to circle the container in search of more room, if they haven't already done so. To further constrict rooting space, several small shrubs were also planted in the same small container, and the whole thing was set several feet from a building. Whoever planted the tree was apparently quite concerned about its stability. The tree has three or four guy wires as well as a long wooden stake connected to the trunk by green plastic material that is beginning to girdle the trunk. The system is excessive and is doing more harm than good. To the untrained observer it probably looks okay, but in reality this tree isn't well maintained.
17. This magnolia is located in Kirkland, in a business park on Northup Way. It was planted approximately ten feet from a building and about five feet from a walkway, both of which moderately restrict root and branch growth. A few branches are close to growing into the building, and it looks like other branches have been pruned to keep them away from the building and the sidewalk. All branches on the bottom five feet of the trunk have been pruned out to provide clearance and a view from the nearby offices. Unfortunately, the pruners cut about three inches outside of the branch collars, leaving large stumps and wounds that will take a long time to heal. The planting strip is a mix of bare soil and grass; this magnolia is in the grassy section and has turf growing to the base of its trunk. The grass is likely competing with the magnolia for resources and root space, and should be cut back, as it has been around nearby plants. This magnolia's trunk has a large crack, likely caused by sunscald, and splits to two leaders directly above the cracked area. The tree seems fairly well maintained, but better care could be provided.
18. The next tree I assessed is located near the intersection of 45th and NE 15th, on the western (front) side of the University Youth Center. This tree was planted very close to both the building and a large lilac bush (about a foot and a half away from both), leaving the tree less than ample growing room. The magnolia stands at the top of a slope that has been leveled slightly with small retaining walls creating several layers of planting strips. This tree's trunk leans somewhat, and its branches are concentrated on the sides away from the building. This appears to be another situation where people are trying to take good care of their plants, but don't understand or utilize proper horticultural techniques.
19. This M. grandiflora is located in a private yard in Burien, WA. It is planted approximately four feet from a driveway, but has no obvious vertical or horizontal restrictions on growth. The tree is on the east side of the yard and able to receive direct sunlight from the east and south. The vegetation in this yard, although sparse, is fairly well maintained and the tree appears to be well cared for and healthy.
20. The final tree I looked at was obviously misplaced. It is growing just off "The Ave." in the University district in a small raised (about six inches high) planting strip very close to both a building and a brick wall. The tree's trunk was less than a foot from the wall, with a major bend leaving it mere inches from the wall in several places, and about two feet from both the building and the edge of a parking lot. To further restrict growing space for the magnolia, bamboo was planted between the magnolia and both the wall and the building. Part of the tree's crown was also growing into some power lines, and several branches had been badly pruned to keep them away from the electrical wires. A large black plastic container marked "Poison--Do not touch!" sits inches from the base of the tree, and what little soil exists is covered in weeds, trash, and broken glass. Considering the inappropriate nature of the site (for the magnolia or almost any vegetation) the tree is performing remarkably well. It is, however, not a very healthy or happy tree.
In assessing these twenty Magnolia grandifloras, I looked for both helpful and harmful management techniques. Not surprisingly, the trees that received the most care, or the most horticulturally sound care, were the healthiest. The trees that received little care or improper maintenance, particularly those in parking lots or retail areas, performed the worst.
Providing ample and appropriate growing space is definitely a good idea for Magnolia grandiflora, especially considering the species' large mature size. Unfortunately, few people seem to consider the ultimate size of these trees when planting them, often leaving too little room for them to grow or forcing the magnolias to compete with nearby vegetation for space and other resources. Southern magnolias growing in small planting strips or parking lots tended to have slow growth rates, few branches and leaves, and kneeing roots that push up at the soil's surface in an attempt to find growing space. The trees also often appear stressed and weakened, leaving them more susceptible to attack by canker-causing fungi, scale, and other pathogens.
Pruning to shape or thin magnolias aids the trees' overall health, but only when the pruning is performed correctly. If large stubs are left behind or branches are broken off rather than cut cleanly, the wounds tend to be slow to heal, and may never completely seal over. Several trees had cankers or pockets of decay near pruning wounds, suggesting the pathogens entered through the wounds or attacked the weakened tree. Excess pruning that leaves the tree with very few branches, generally clustered in the upper quarter of the tree, is also a problem. This leaves the tree with fewer leaves with which to photosynthesize. The unevenly distributed limbs also leave the trees less stable and more prone to failure, especially in a storm. I noticed many broken or diseased branches in trees with excessively pruned canopies. In training some of the young trees, people seem to raise the trees' canopies too quickly, leaving spindly trees with reduced photosynthetic ability that look quite bad while they try to recover and add new branches and leaves. Magnolia grandiflora generally flowers from summer to early fall (July to September). To prevent the pruning out of flower buds, this species should be pruned in the fall or winter, before spring bud set. If the tree is pruned in the winter, cold damage may occur, though cold damage is not a frequent problem in the Pacific Northwest or in the species' native range.
Magnolia grandiflora seems to like or tolerate some shade, but can also do well in large amounts of sunlight. However, trees exposed to lengthy periods of direct sunlight or reflected light and heat are rather stressed and don't perform very well. These trees often have large cracks or damage from winter sun scald. The damage may also have occurred when the plants were very young and exposed to excessive light, as sometimes happens in nurseries. The magnolias exposed to the most sun and heat generally had more spotted or chlorotic leaves, perhaps indicating that the pathogens leading to the problems attacked stressed plants. Excessive light and UV exposure may also have caused the leaf chlorosis and necrosis by frying the plants' chloroplasts. Younger or smaller plants seemed to be more adversely affected by lengthy periods of direct and indirect solar exposure than were bigger or older plants.
Soil texture doesn't appear to have much of an impact on M. grandiflora health or success. The Southern magnolias I assessed performed equally well in both sandy and clayey soils. The compaction and drainage levels of the soils were better indicators of success than was the soil texture. Plants in heavily compacted soil all performed rather poorly and tended to be small and have kneeing roots. These plants also often had chlorotic and necrotic leaves and leaf spot was a bigger problem in compacted soils. The greater stress placed on the roots and the plant in general was likely showing itself in the foliage problems. I also noticed that trees in poorly drained soils were more likely to have yellowing, chlorotic leaves, suggesting that nutrients were leached out of the soil or that the waterlogged roots weren't very efficient at taking up nutrients. The small rooting zones of some of the plants, combined with the heavy compaction, led to some spindly, sickly trees, such as those in the Lewis and Clark parking lot or around University Village.
Without digging up large quantities of soil and disturbing the plants it was difficult to assess root damage in the magnolias. However, I did inspect the root crown and soil surface for visible root problems, including kneeing or circling roots. Not surprisingly, the trees with the smallest viable rooting space and the most heavily compacted soil had the most problems. These were the trees most likely to have kneeing roots or roots that were pushing up on the soil to try to find adequate rooting space. Several trees had circling roots that should have been removed at transplanting, but weren't, and will likely end up girdling the trees.
The most commonly used mulches around M. grandiflora appear to be wood chips and bark. The mulches, with the exception of turf, seem to benefit the trees, probably by retaining moisture, increasing soil fertility, and modifying soil temperatures. The non-composted, woody mulches are probably decreasing soil nitrogen levels, which could help explain leaf chlorosis in some of the trees with wood chip mulch. Most of the unhealthy, poorly maintained trees had no mulch or had old, nearly decomposed mulch that needed to be replaced.
The majority of the trees I assessed appeared to have been transplanted correctly. However, there were some that probably were not, including several with circling roots that were not removed. It's also possible that people created problems such as kinked roots or soil interface problems when transplanting. In the more compacted soils, people may have planted without loosening or aerating the soil first, making it difficult for the trees' roots to grow through the soil and leading to slow growth of the trees. At least one tree had been planted too deeply, burying the root crown under excessive amounts of soil. Two of the trees were currently staked, and a third obviously had been, and in all cases the stakes were left on too long and girdled the trunks.
None of the Magnolia grandifloras appeared to be suffering from a lack of water. However, several plants, especially those in poorly drained soils, were exposed to excessive moisture levels. The roots were probably inundated with water, lacking oxygen, and having difficulty respiring or taking up nutrients. This could account for the slow growth rates and small sizes of several plants in extremely wet soils.
Every Southern magnolia I've seen in the Pacific Northwest has had at least some leaf spotting. This doesn't seem to have any harmful effect on the trees, though stressed or declining trees tend to have more leaf spots. Treatment doesn't appear to be necessary or very commonly used. Something (probably an omnivorous looper) is eating the leaves of several of the magnolias I studied, but doesn't appear to be doing significant damage. If people were concerned about the aesthetic damage (holes), they could bring in predaceous birds and beetles or a parasitic wasp to kill the looper or spray the plant with Bt (Bobbitt et al 1996). However, I saw no indication that the infected trees were being treated, so apparently people are living with the damage. The only other pathogen I saw on the trees was a species of scale, which could cause serious damage to the trees by slowing growth or killing branches. It was difficult to tell how or if people were managing the scale, though presumably they would use some form of treatment, perhaps horticultural soap or a dormant oil spray.
Assessing nutrient deficiencies or additions was somewhat difficult. Some of the trees were so poorly maintained that a lack of fertilizers or nutrients was the least of their problems. Other trees appeared much better cared for and generally had no sign of nutritional problems. Several plants did have chlorotic leaves, which might have been caused by nutritional deficiencies, but may also have been caused by a number of other problems. In general, the Magnolia grandifloras that were best managed were the least likely to show chlorosis, necrosis, or other signs of nutrient deficiency or toxicity. These plants were probably properly fertilized.
In reviewing both popular and primary literature, I found management suggestions that both differed from and agreed with what I found to be good management techniques. Occasionally, these suggestions also differed from other authors' recommendations.
Southern magnolia can grow sixty to eighty feet in height and forty feet in width, with a diameter at breast height (d.b.h.) of two to three feet. Dirr (1998) emphasizes the need to provide these trees with ample growing room in order to maximize their health and beneficial aspects, including shade and natural beauty.
According to Parker (1997), most magnolias don't need much pruning. She recommends pruning Magnolia grandiflora in late winter or early spring, though not late enough in the year to interfere with dormancy. Parker also makes the common-sense recommendation to remove dead branches, suckers, or branches crowding each other. Her advice also includes "shortening long branches" (p.11), which could be interpreted as heading or topping the tree. Improper "shortening" could lead to a hydra effect, with the tree using all its energy to create new branches to compensate for those lost through excessive pruning. Finally, she states that Southern magnolia will "take care of its own litter problems" (p.12) if all the low branches are left on the tree. Presumably the branches will hide the leaf litter or prevent it from blowing away while the leaves are decomposing and fertilizing the soil.
Magnolia grandiflora performs well in full sun to partial shade. Seedlings can grow in fairly heavy shade, but adults need ample sunlight. In forests, Southern magnolias may begin as understory species, but gradually push their way into canopy openings as they mature and need more solar exposure (Outcalt 1990). Peters and Platt (1996) speculate that Magnolia grandiflora in shady understory conditions may compensate for decreased photosynthesis in the summer by increasing photosynthesis in winter months when overstory deciduous trees lose their leaves, providing the magnolias with more solar radiation. M. grandiflora does not like large amounts of reflected light and heat, such as commonly occur in parking lots, and should be protected from harsh winter sunlight (Dirr 1998). Southern magnolia is hardy to USDA zone 6 (-10 to 0° F). In its natural range, temperatures below 15°F (-9° C) or above 100 °F (27° C) are rare (Outcalt 1990). The high temperatures and harsh sunlight common in parking strips or along streets can stress the trees, leaving them wilted and possibly killing leaves and branches.
Southern magnolia grows best on loamy, acidic, rich, moist (but well drained) soils (Dirr 1998; Outcalt 1990). It also grows on mesic upland sites where fire is a rare occurrence (Coladonato online). M. grandiflora is said to be a good urban tree because it can withstand acid deposition (Ibid.). Southern Magnolia is a deep-rooted species, except on sites with a high water table. Seedlings quickly develop one major taproot, then many thick laterals with few root hairs develop as the tree matures. Southern magnolia should not be propagated in or transplanted into poorly drained soil as this will reduce rooting (McCracken et al 1996). None of the literature I found discussed soil compaction directly. However, McCracken (1985) notes that trees growing in heavy clay soils (which are often quite compacted) tend to perform poorly. Diseases may enter magnolias through stressed rooting systems, such as those found in compacted soils. Nowak et al (1990) also state that soil compaction is one of the five most common or serious problems for urban trees, including M. grandiflora.
Several authors (Hensley et al 1988; Brenzel 1996) recommend mulching Southern magnolias. Hensley et al (1988) found that trees mulched with hardwood bark were slightly smaller and shorter than unmulched trees at the end of one growing season, likely due to competition for nitrogen from the decomposition of the mulch. However, applying nitrogen fertilizer to the mulched plants eliminated the differences. After the first growing season, the mulched trees were taller, had larger calipers, significantly higher growth rates, and more branches. Hensley et al (1988) believe that the mulch application especially benefited newly transplanted trees by reducing resource competition by and allelopathic effects of nearby turf.
According to Dirr (1998), it's best to transplant young container grown or balled in burlap plants that have been root pruned prior to moving. However, Gilman and Kane (1990) found that root pruning prior to transplanting had no real benefit. In the first year after transplanting, trees that were root-pruned had slightly faster shoot growth rate than unpruned trees; after the first year, the trees had the same rate of shoot growth. Gilman and Kane didn't examine how the roots performed, but assumed that when the root-pruned species caught up to the unpruned species in shoot growth the trees were "well established" and had completely replaced the lost roots.
The best time of year to transplant depends on the environment; in Seattle, magnolias can be planted in fall, while spring planting is best in areas with harsher winters. In their native south, Southern magnolias are said to perform best when planted or transplanted in August (Gilman and Kane 1990). In a study in Kansas, Hensley et al (1988) found that spring planted magnolias had more branches and larger stem diameters than fall planted individuals. Unfortunately, the researchers did not study the roots of these plants or the root to shoot growth ratios to see how balanced growth was throughout the plants. The trees may have had large crowns but small rooting systems, making them less stable and healthy then they appeared. Roots on this species are often thick, fibrous, and easily injured during planting, so extra care should be taken to minimize damage to the trees' rooting systems. M. grandiflora has better survival rates in mild climates and when protected from winter winds and sun (Dirr 1998; Nowak et al 1990). If Southern magnolias are to survive as street or parking lot trees, they must have adequate space for their roots as well as regular watering and fertilizing by neighbors, business owners, or other caretakers (Nowak et al 1990; Sommer and Summit 1996). Finally, if magnolias are to be staked, the stakes should not be placed high on the tree or too close to the trunk (Nowak et al 1990).
Magnolia grandiflora typically grows in areas with forty to sixty inches (102 to 152 cm) of rain annually (Outcalt 1990). This species cannot withstand prolonged periods of water inundation or "wet feet," though it can tolerate occasional periods of soil saturation. Southern magnolia is said to be quite drought tolerant, but prolonged periods of drought can lead to extensive dieback of branches and leaves. Adding a mulch that will decrease evaporation from the soil and slowly release water to the plant, as well as providing supplemental water during the dry season should alleviate this problem (Coladonato online).
Numerous biotic pathogens may infest the Southern magnolia. Some merely look bad and don't require treatment, while others cause serious damage to the trees and should be treated. Several pathogens are responsible for causing leaf spots, a primarily aesthetic problem, in Magnolia grandiflora, including species of Glomerella, Colletotrichum, Cladosporium, Phyllosticta, Septoria, and Glomerella fungi. An alga (Cephaleuros sp.) and a bacteria (Pseudomonas sp) also may cause leaf spots (Goff et al 1996; Outcalt 1990). Goff and others (1996) studied the effects of various fungicides and a surfactant on leaf spot in trees grown for floral arrangements. They found that fungicides were effective in controlling leaf spots and marginal scorch. However, they also discovered that residue from the fungicides and the surfactant created ring spots as well as irregular brown lines that roughly followed the leaf margins. They concluded that the phytotoxicity of the chemicals negated any benefits in controlling leaf spots, so the chemicals should not be used. Numerous species feed on Southern magnolia, causing aesthetic damage and potentially girdling roots or stems. These species include magnolia leafminer (Phyllocnistis magnoliella), leaf weevil (Odontopus calceatus), tulip tree aphid (Illinoia liriodendri), striped mealybug (Ferrisia virgata), a spider mite (Tetranychus magnoliae) and an omnivorous looper (Sabulodes sp.) (Dreistadt et al 1994; Outcalt 1990). Fomes sp. and Polyporus sp. can cause heartrot in Magnolia grandiflora. Perhaps the most damaging pest for Southern magnolia is scale. Several species of scale attack magnolias, including magnolia scale (Neolecanium cornuparyum), tulip tree scale (Toumeyella liriodendri), and oleander pit scales (Asterolecanium pustulans). While all three scales can injure Southern magnolias, magnolia scale is the only species that typically kills branches or trees (Dreistadt et al 1994).
Hensley et al (1988) found that adding fertilizer at planting time improves the growth and health of Magnolia grandiflora. They performed a study in which 75g of 12-5-10 (N-P-K) fertilizer was added upon transplanting M. grandiflora into soil with no indication of nutrient deficiency. Three years after planting, the researchers found that the magnolias receiving fertilizer grew significantly taller and had greater stem diameters than non-fertilized magnolias.
For the most part, my findings on proper and improper management techniques agreed with the literature. There were, however, a few areas in which we disagreed. These differences mostly seem due to environmental differences between Seattle, M. grandiflora's native south, and the midwest, where several research projects on Southern magnolia were performed.
My findings generally agreed with available literature in most areas. These included providing the trees with ample growing room, mulch types and benefits, optimal water levels, pest control, and fertilizing and transplanting techniques. Researchers and I also had similar findings about soil, although the literature rarely discusses urban soil problems such as compaction and poor drainage.
My analysis of optimal light levels for Southern magnolia mostly agrees with the literature. However, several researchers seemed to think M. grandiflora could tolerate lengthier and more direct sunlight than I did. Perhaps the researchers conducted studies in more "natural" areas without the reflected light and heat I found in parking lots and other urban sites.
The available literature also somewhat contradicted my findings on proper pruning. While we agreed that dead and damaged branches should be removed, I think training young trees is more important than Parker (1997) apparently does. This could be because Parker has different cultivars than I studied or because she prefers a "wilder" more spreading look than I do or has more space to allow the trees to grow freely. We also differed on proper timing for pruning. This could be attributed to differences in climate between Seattle and the south (where Parker did her study). I would not recommend pruning in the winter because of the risk of cold damage, but that could be less of an issue in the south. Parker also has apparently not received professional training in proper pruning techniques, which could explain her advice to perform heading cuts.
Overall, the differences were minor and my findings on proper maintenance of Magnolia grandiflora agreed with the published literature. While not all landscapers and homeowners practice good plant selection and management techniques, those who do have beautiful, healthy trees that can bring joy for many years.
Anisko, T. 1992. Integrated pest management of magnolia scale. Magnolia 28: 6-11.
Bobbit, V., A. Antonelli, C. Foss, R. Davidson, R. Byther, and R. Maleike. 1996. "Pacific Northwest landscape IPM manual". Washington State University and Washington State Department of Ecology: Puyallup, WA p. 125.
Brenzel, K. (Ed.). 1996. "Sunset western garden book". Sunset Publishing Co.: Menlo Park, CA pp. 365-370.
Coladonato, M. 1991. Species: Magnolia grandiflora In: W.C. Fischer (Ed.) The Fire Effects Information System. USDA Forest Service: Missoula, MT. Accessed May 3, 1999. Available: http://fs.fed.us/database/feis/plants/tree/maggra/
Dirr, M.A. 1998. "Manual of woody landscape plants: Their identification, ornamental characteristics, culture, propagation, and uses (5th ed)". Stipes Publishing: Champaign, IL pp. 598-603.
Dreistad, S.H., J.K. Clark, and M.L. Flint. 1994. "Pests of landscape trees and shrubs: An integrated pest management guide". University of California Division of Agriculture and Natural Resources Publication 3359: Oakland, 327 pp.
Gilman, E.F. and M.E. Kane 1990. Growth and transplantability of Magnolia grandiflora following root pruning at several growth stages. HortScience, 25: 74-77.
Goff, W.D., R.L. Shumack, K.M. Tilt, and A.K. Hagan. 1996. Fungicide sprays affect leaf condition and tree appearance of Southern magnolia. Journal of Arboriculture, 22(5): 201-205.
Hensley, D.L., R.E. McNeil, and R. Sundheim. 1988. Management influences on growth of transplanted Magnolia grandiflora. Journal of Arboriculture, 14(8): 204-207.
Martin, C.A., D.L. Ingram, and T.A. Nell, 1991. Growth and photosynthesis of Magnolia grandiflora 'St. Mary' in response to constant and increased container volume. Journal of American Society of Horticulture Science, 116(3): 439-445.
McCracken, F.I. 1985. Observations on the decline and death of Southern magnolia. Journal of Arboriculture, 11(9): 253-256.
McCracken, T.P., J. Catazzaro, and T.E. Bilderback 1996. Rooting of 'Brown Velvet' Southern magnolia stem cuttings as influenced by medium and auxin treatment. Journal of Environmental Horticulture, 14(3): 158-9.
Nowak, D.S., J.R. McBride, and R.A. Beatty. 1990. Newly planted street tree growth and mortality. Journal of Arboriculture, 16(5): 124-129.
Outcalt, K.W. 1990. Magnolia grandiflora L. (Southern magnolia). In: R.M. Burns and B.H. Honkala (Eds.) Silvics of North America Vol 2. GPO: Washington, D.C. Also available: http://willow.ncfes.edu/silvics_manual/Volume_2/magnolia/grandiflora.htm
Parker, F. 1997. To prune or not to prune... Magnolia, 32: 8-12.
Peters, R. and W.J. Platt. 1996. Growth strategies of main trees and forest architecture of a Fagus-Magnolia forest in Florida, USA. Vegetatio, 123: 39-49.
Sommer, R. and J. Summit. 1996. An evaluation of tree descriptions in a popular garden guide. Journal of Arboriculture, 22(3): 155-159.
Table 1: Analysis of Landscape Conditions
Table 2: General Health of the Trees