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