"Why do my plants fail to bloom?" is a question that is often asked of nurserymen 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 two or three years on some flowering shrubs or five to ten years on certain tree species. Simple patience is required when juvenility is the cause of failure of the plant to flower. Some plants for which juvenility may be the cause of lack of flowering include: century plant (Agave americana with about a ten-year juvenile period), crabapple, and flowering cherry).
Plants which 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 which restrict growth (such as those used on dwarf trees) produce plants which flower at a younger age than plants on rootstocks which 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 and the plant flowers sparsely. 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 so that 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 which may still be present.
On the other hand, a certain amount of cold temperature (usually at least as low as 45 degrees) is required for many ornamentals and houseplants to flower properly. 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: 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 which 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 that they are exposed to. "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 which will 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 5:00 p.m. and taking them out the following morning at 8:00 a.m. for about twelve weeks. "Long day" plants require a light period of fifteen hours or longer in order to flower. Garden shrubs which are "long day" and consequently flower during the long days of summer, include Shrubalthea (Hibiscus syriacus), Glossy Abelia (Abelia grandiflora), and Wiegela (Wiegela florida).
Some plants, although they are not "photoperiodic," 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 plants in a vigorous growing condition with a great deal of foliage and new shoots fail to bloom. The vigorous growth is often caused by excessive fertilization, particularly with nitrogen, an element which 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 which will 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 account for or can explain the lack of flowering in most ornamental plants. However, it should be remembered that a plant is a complex biological organism and that the flowering process is controlled by many factors in the environment, many of which are not fully understood. But, hopefully, this somewhat abbreviated explanation will help to answer the question "Why do my plants fail to bloom?"
(Adapted from: Harold Davidson, Michigan State University)
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Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation with the United States Department of Agriculture. Lawrence Forcier, Director, UVM Extension System, Burlington, Vermont. University of Vermont Extension System and U.S. Department of Agriculture cooperating, offer education and employment to everyone, without regard to race, color, national origin, sex, religion, age, disability, political beliefs, and marital or familial status.
Last reviewed 3/31/97