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University
of Rhode Island GreenShare Factsheets
Why
Plants Fail to Bloom
"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.
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, 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.
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.
"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.
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.
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
Pesticides
are poisonous! Read and follow all safety precautions on labels.
Handle carefully and store in original containers out of reach
of children, pets or livestock. Dispose of empty containers
immediately, in a safe manner and place. Pesticides should never
be stored with foods or in areas where people eat.
When trade names are used for identification, no product endorsement
is implied, nor is discrimination intended against similar materials.
Be sure that the pesticide you intend to use is registered for
the state of use.
The user of this information assumes all risk for personal injury
or property damage.
For more
information, call the URI CE Gardening and Food Safety Hotline
at 1-800-448-1011 or (401)874-2929 from outside Rhode Island;
Monday-Thursday between 9 am and 2 pm.
University
of Rhode Island Cooperative Extension provides equal program
opportunities.
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