Extension Educator, Horticulture
I'm sure I'm not the only one anxiously anticipating spring flowers. Actually I would settle for just a bit of green life poking through the ground. We may not always notice the spring flowers of some trees and shrubs. Maples and oaks, for example, do flower, but they usually go unnoticed by most individuals except for allergy sufferers. Other trees and shrubs, such as crabapples and lilacs, are planted specifically for their attractive flowers.
Apples, cherries and other fruit trees are grown for their edible fruit. Many gardeners become concerned when their flowering tree or shrub fails to produce blossoms. The failure of woody plants to bloom may be due to several factors.
All plants must be physiologically mature before they are capable of blooming. During the juvenile stage of growth, plants are strictly vegetative and do not bloom. For annuals, such as marigolds and petunias, the juvenile stage may last for only a few weeks. Trees, however, may not be physiologically mature for 10 or more years. Apple and other fruit trees planted in the backyard garden may not flower and bear fruit for four to six years. The actual length of time from planting to flowering varies tremendously. Differences exist among varieties or cultivars.
Generally, a 'Jonathan' apple tree will bear fruit sooner than a 'Red Delicious.' Dwarf and semi-dwarf fruit trees bear earlier than standard-sized trees. Lilacs may not bloom for five or more years after planting.
The flower buds of most plants are generally less hardy than the leaf buds. Low winter temperatures may kill the flower buds without damaging the leaf buds. For example, temperatures below -20°F. will kill the flower buds on peach trees. As a result, those peach trees that survive in Illinois often fail to produce a crop. Many forsythia varieties often fail to bloom because of low temperature injury. Two forsythia varieties that bloom reliably in Illinois are 'Meadowlark' and 'Northern Sun.' The flower buds on these two varieties have survived temperatures of -30 to -35°F.
Some trees, such as fruit trees and crabapples, bloom heavily one year and then sparsely the following year. Hand thinning of excess fruit on fruit trees will help to overcome this tendency to flower and bear fruit in alternate years. 'Bob White,' 'Dolgo,' 'Selkirk' and 'Red Splendor' are four crabapple varieties that tend to flower heavily in alternate years.
Heavy pruning and excessive nitrogen fertilization promote vegetative growth and inhibit the production of flower buds. Generally, fertilization of trees and shrubs is unnecessary if the plants are growing well and possess good leaf color. Spring-flowering shrubs, such as forsythia, lilac and some hydrangeas bloom from buds formed during the previous season's growth. Pruning these shrubs heavily in late winter or early spring will remove much of the flowering wood.
Many trees and shrubs require at least 6 to 8 hours of direct sunlight to bloom properly. Generally, the amount of flowering decreases as the shade increases. Lilacs, for example, bloom heavily in full sun, but bloom sparsely in shaded sites. Even many shade tolerant plants bloom poorly in heavy shade.
These are some of the reasons trees and shrubs may fail to bloom. Good plant selection, proper planting and care should help to ensure flowering. Gardeners, however, should be patient. Some plants are truly late bloomers and just need a few years to "grow up".