"Why aren't my plants blooming?" is a question often asked of nursery workers or others with special knowledge of plants. The question may be about ornamental trees and shrubs such as crabapples or lilacs, or about house or garden plants such as begonia, poinsettia or chrysanthemum. The answer to the problem is, in general, related to one of six causes: the age of the plant, temperature, alternate flowering, light, nutrition or pruning practices.

Age:

Many woody plants pass through a period of growth, called the juvenile stage, in which the plant does not flower. This stage occurs early in the life of the plant, and is sometimes characterized by a different leaf shape than is found on older plants of the same species. During the juvenile period, the plant produces an abundance of leaves and new shoots but is prevented from flowering by a complicated chemical balance within the plant. "Juvenility," as this phase of growth is called, may last 2 or 3 years on some flowering shrubs or 5 to 10 years on certain tree species. Simple patience is required when juvenility is the cause of failure of the plant to flower. Century plant (Agave americana), for example, has a juvenile period of about 10 years. Crabapple and flowering cherry also have substantial juvenile periods.

Plants that have been budded or grafted may have flowering delayed or hastened, depending on the type of rootstock onto which the plant was grafted. In general, rootstocks that restrict growth (such as those used on dwarf trees) produce plants that flower at a younger age than plants on rootstocks that do not limit growth.

Temperature:

Temperature, particularly cold temperature, plays an important part in the flowering of many plants. When winter temperatures drop extremely low, flower buds may be killed, resulting in sparse flowering. This condition is commonly associated with forsythia grown in northern parts of the country. Some years the plant will bear flowers only on the lower part of the plant, which was protected by snow cover during the low temperature period in winter. The flower buds of flowering dogwood will quite often have the two outer bracts (petals) injured during the winter--the resulting flower has only two bracts rather than the usual four. If you suspect that excessively cold temperatures are the cause of your plant's failure to flower, examine the plant closely in the spring for brown and dead flower buds that may still be present.

A certain amount of cold temperature (usually at least as low as 45 degrees F) is required for many ornamentals and houseplants to flower properly, however. "Vernalization" is the term applied to this cold temperature requirement, and it is necessary for such plants as tulips, crocus, daffodil, Christmas cactus and many garden biennials.

Alternate flowering:

Some plants, such as some flowering crabapples and flowering dogwood, are subject to a phenomenon called "alternate flowering." This type of plant will frequently flower very heavily in one year and then fail to flower for one or two additional years. This is a natural phenomenon. Selection of plants that do not exhibit this tendency would be the best way to correct this problem.

Light:

Many plants are "photoperiodic," meaning that their flowering is controlled by the number of hours of light and dark each day to which they are exposed. "Short-day" plants require a prolonged dark period (15 to 18 hours) in order to flower because it is during the dark period that the chemical transformations occur in the plant that eventually cause flowering. Poinsettia, chrysanthemum and gardenia are three common "short-day" plants. Short-day houseplants may be supplied with their required dark period by putting the plants into a closet at 5 p.m. and removing them the following morning at 8 a.m. for about 12 weeks.

"Long-day" plants require a light period of 15 hours or longer in order to flower. Garden shrubs that are "long day," consequently flowering during the long days of summer, include Shrubalthea (Hibiscus syriacus), Glossy Abelia (Abelia grandiflora) and Wiegela (Wiegela florida).

Many plants which are "photoperiodic" per se do respond to light with regard to the number of flowers produced by the plant. Flowering shrubs or garden plants planted in a very shady area often produce lush growth and appear very healthy, but produce few if any flowers. Again, this is related to a chemical balance within the plant. Move the plant to a sunnier location to promote flowering.

Nutrition:

It has often been reported that many plants which are growing vigorously, with a great deal of foliage and new shoots, fail to bloom. Such vigorous growth is often caused by excessive fertilization, particularly with nitrogen, an element that promotes vegetative growth rather than flowering. To induce a plant under such conditions to bloom, decrease the rate of fertilization and water thoroughly to wash the excess nitrogen from the root area. Water infrequently from then on. It may require a year or two before the effect will be apparent on the trees or shrubs.

Pruning:

Lack of flowering in plants can also be related to improper pruning methods. Many woody shrubs or ornamental trees produce flower buds in fall that bloom the following spring. Pruning in late fall or very early spring may remove these flower buds. For full floral effect, prune flowering plants in the landscape just after bloom. These six conditions can account for or explain the lack of flowering in most ornamental plants. However, remember that a plant is a complex biological organism and the flowering process is controlled by many factors in the environment--many of which are not fully understood.

Adapted from Leonard P. Perry, University of Vermont Extension