An ornamental tree has one job in the garden: to bring joy into the gardener's heart because of its beauty. "Black Tulip" magnolia (Magnolia × soulangeana "Black Tulip" accomplishes this task easily, showcasing wine-red, tulip-shaped flowers that burn brightly above long lime-green leaves. Unfortunately, the tree's susceptibility to problems such as pests and diseases can make it difficult for "Black Tulip" to do its job. Providing proper cultural conditions is key to having a healthy tree.
Magnolias require well-draining soil and loathe having "wet feet." Poorly draining soil can quickly lead to root rot, which will kill the tree. While core aeration can help remedy the problem, it is best to start by planting the tree in well-draining soil enhanced with organic matter. Water slowly and deeply to prevent soil from becoming flooded. Avoid any injury to the trunk, which may invite wood decay, and always prune diseased or dead limbs with clean, smooth cuts. Sterilize pruning tools prior to use to forestall the spread of disease.
2. Foliage Disease
The striking dark flowers of "Black Tulip" will be less spectacular if set against a backdrop of disease-ridden leaves. Cases of powdery mildew and bacterial leaf spot are common. Avoid overhead irrigation when watering young trees, as wet leaves invite disease, and water early in the day to allow leaves to dry before evening. Remove all fallen leaves and destroy them to prevent the pathogen from living on. Do not compost diseased leaves. Fungicide is not usually necessary.
"Black Tulip" may be attacked by pests such as greenhouse thrips or magnolia scale. Scale and thrips can be controlled by using a narrow-range horticultural oil, applied evenly to both sides of the leaves as per label recommendations. You may need to make multiple applications. This may not be practical for larger trees without the help of a professional. Avoid using broad-spectrum insecticides in the garden, which kill beneficial insects that would otherwise help combat harmful pests.
"Black Tulip" is suitable for U.S. Department of Agriculture plant hardiness zones 5 to 9, where it requires full sunlight for optimal flowering. Magnolia trees have sensitive roots; once planted, the tree should not be removed. Avoid gardening under the tree, as you risk injury to the roots and other plants will compete with the tree for water and nutrients. Provide a neutral to acidic soil and prune as little as possible to avoid injury to the tree.