Magnolias flower in the spring and summer and bear striking white, pink or red flowers. They can be either deciduous or evergreen. Magnolias can suffer from a number of problems such as yellowing leaves, lack of flowering and leaf loss. They may also suffer from brown or black flower buds. These may either open to show ugly brown, rotting petals, or fail to open at all.
The most common cause of brown or blackened magnolia petals is frost damage. Spring flowering trees and shrubs are particularly susceptible. Magnolia flower buds are surrounded by two outer furry coatings that fall as the flower opens. These protect the flower to an extent, but frost that is particularly hard can still penetrate. Strong winds can similarly cause the petals to turn brown, especially after the flower begins to open.
The best prevention is to choose a late flowering plant, especially if growing in the north of England or Scotland. Magnolia soulangeana does not come into flower until May. Choose a shorter tree or a shrub for windy sites. Do not plant your tree at the bottom of a hill or valley, as frost tends to flow downhill and form frost pockets in these areas. Smaller trees and shrubs can also be covered with horticultural fleece if frost threatens.
Another possible cause of brown, rotting petals in magnolias is a disease relatively new to the U.K. called Phytophthora kernoviae. Gardeners have observed this infecting a range of magnolia shrubs and trees in the U.K. for the last two years, since 2009. Symptoms vary from plant to plant. Some show only brown or black spots on leaves, while others have blackened and shrivelled flowers in spring. According to the Forestry Commission, all affected plants have been growing in gardens with infected rhododendrons, which have probably passed the disease on.
Sometimes trees fail to flower entirely. This may just be because the plant is young. Trees can take several years to come into flower. Another reason may be that the site is not sunny or warm enough, or that the tree has been pruned too severely. Yellowing leaves are caused by failing to plant the tree in the slightly acidic soil that magnolias prefer. Occasionally, evergreen magnolias will lose a lot of leaves all at once in the early summer, but this is normal behaviour.