If you have noticed yellow leaves on your Southern magnolia (Magnolia grandiflora) and your balled-and-burlapped specimen has sparse flowering and yellow leaves dropping off of the tree, it might need some special attention to bring it back to health.
Southern magnolia trees are broadleaf evergreens that can grow up to 80 feet tall. Their white fragrant flowers bloom in late spring. They are indigenous to the American Southeast and are a popular specimen plant. The tree is best grown in USDA plant hardiness zones 7 through 9 in full sun or partial shade. They thrive in slightly acidic, fertile soil, benefiting from soil enriched with compost. If you live in the North and wish to grow a Southern magnolia, look for 'Bracken's Brown Beauty,' a cold-hardy cultivar suitable for growth down to zone 5b.
Here are some potential issues that cause leaf yellowing in Southern magnolia trees and how you can troubleshoot them.
Possible Reasons for Yellowing Leaves
Southern magnolia trees are relatively easy trees to grow; they are large trees that need a lot of space. Transplant shock is common with this species, so if you've just moved it to a new site, don't worry if some leaves turn yellow and it loses a few leaves in its first growing season. However, other problems might be more cause for concern.
Soil Composition Issues
Yellow leaves on Southern magnolias indicate that your soil has insufficient nitrogen or iron. The only way to determine whether this is the problem is to run a soil test on a sample of the plant's soil. If the test results show a nitrogen deficiency, you can fertilize accordingly to raise the nitrogen level. The nitrogen content of fertilizer is indicated by the first number in the NPK sequence, which is listed on the fertilizer container.
If the Southern magnolia tree has an iron deficiency, that condition manifests as leaf chlorosis with irregular growth and leaf loss. The leaves of the plant turn yellow, but the veins of the leaves stay green. Leaf chlorosis starts at the tips of new growth and works its way to older leaves as it worsens.
Southern magnolia trees do best when the soil is neutral to slightly acidic. To check your soil, get a pH testing gauge for your soil, try some do-it-yourself methods to test your dirt, or bring a soil sample to your local county extension office for evaluation. If your soil is alkaline with a pH higher than 7.0, consider amending the soil to make it more acidic. These methods include adding sulfur to the soil, using acidic fertilizer, or mixing sphagnum peat moss into the soil. If that doesn't work, you might need to move the tree to a location with more acidic soil.
If you've ruled out nitrogen deficiency, sometimes Southern magnolia trees develop yellow leaves (often accompanied by the presence of spots) as a prelude to natural shedding, which happens periodically and can occur throughout the year. The natural shedding period seems to occur more often in the spring. If new leaves grow in place of a shed leaf, it's not a sign of a problem; it's the expected life cycle of that leaf.
Water or Soil Drainage Issues
If you see yellow leaves in fall, your Southern magnolia may have water or drainage problems. Too much or too little water are both possibilities, whether you irrigate yourself or rely on rainfall. Water issues are inextricably intertwined with drainage. Even if sufficient water percolates through the ground too rapidly, the Southern magnolia tree can't take full advantage of it. Conversely, if tight layers of clay soil impede drainage, and you get a lot of rain or overwater your specimen, the roots can "drown."
Too much or too little sun can cause magnolia leaves to turn yellow. The tree needs good sunlight, but even too much direct, pounding sun on the hottest summer days with insufficient water can give the tree sunburn, causing leaves to turn yellow.
If the tree has an infestation of aphids, mealybugs, or spider mites, the pests will suck the sap out of the leaves, causing leaf discoloration, and eventually, the leaves will fall out. Aphids and mealybug are visible with the naked eye, while spider mites are tougher to spot, but their telltale webbing indicates their presence. To treat these leaves, spray horticultural oil or insecticidal soap on the affected foliage to eliminate the pest problem.
Leaves that have brown or black spots rimmed with yellow edges are a potential sign of a fungal disease. As it worsens or becomes more systemic, it leads to yellow leaf discoloration and leaf drop. To prevent the condition, ensure the plant has adequate space and ventilation between plants. Also, avoid overwatering, which can lead to root rot. Use a broad-spectrum fungicide to control the fungus and remove all dead or dying foliage.