Why Flowering Plants Fail to Bloom

Top "Ten +1" Reasons Why Flowering Plants Fail To Bloom

1.Maturity 7.Temperature
2.Light 8.Planting Depth
3.Excess Nitrogen 9. Root Environment
4.Pruning 10.Alternate Flowering
11. Overall Health
PHOTOS

1. Maturity

Flowering plants typically undergo a juvenile period before they begin to produce
flowers. During this juvenile period, the plant produces stems and leaves but no flowers.
The plants will not produce flowers (reproductive structures) until they are mature. 
The juvenile period can range from weeks in annual plants to years in woody plants,
depending on the species and cultivar in question.  Because annual plants complete their
life cycle in one growing season, they undergo a relatively short juvenile period before
flowering. Biennial plants complete their life cycle in two years. The first year yields only
vegetative growth and the second year yields flowers and seeds.   Perennial plants have
one vegetative year followed by repeated flowering years. The juvenile period can last for
years in woody ornamentals and fruit trees. Markets now favor the production of certain
dwarf apple trees that have a reduced juvenile period. These dwarf plants may mature up
to five years earlier than a standard apple and will, therefore, produce flowers and fruits
sooner. Even flowering houseplants may undergo a juvenile period before flowering. A
bird-of-paradise plant, for example, will not bloom until it is at least four or five years old.
If the flowering plant in question has never bloomed before but appears to be
quite healthy, immaturity may be a probable cause. On the other hand, a plant that has
bloomed beautifully in the past but lately is producing only small, weak flowers may
simply be exhibiting signs of old age. This type of decline occurs quite frequently in
herbaceous perennials. Dividing old perennials and advancing only the youngest plantlets
will usually encourage a healthy and vibrant floral display. Even hardy bulbs such as
daffodils may decline with age.  In this situation, it may be easier to simply plant new bulbs.

2. Light

All plants have preferences when it comes to light exposure and all flowering plants have
minimum light requirements necessary for flower production. Lilacs, for example, require
a minimum of six hours of full sun daily in order to produce a good show of flowers. If
shade is a problem in the garden, select herbaceous and woody plants that will adapt to
and flower in reduce light conditions. It is important to realize, however, that even shade
plants require at least few hours of morning sunlight (or dappled sunlight throughout
the day) in order to set flowers. This is one of the principle reasons why it can be
impossible to grow turf and other plants beneath large trees. Most plants cannot even
survive in severe dense shade, let alone set flowers. Insufficient sunlight is often a huge
problem with flowering houseplants and may warrant a supplement of artificial light.
Plants that exhibit spindly growth and yellowish leaves are likely deprived of adequate
light.

Many plants flower in response to photoperiod, which is essentially daylength. For these
plants, the flowering process is triggered when a minimum light/dark requirement is met.
Long day (short night) plants flower in response to increasing daylength. Examples of
long day plants include delphinium and coneflower. Short day (long night) plants flower
in response to decreasing daylength. Examples of short day plants include chrysanthemum
and poinsettia. Day neutral plants do not flower in response to photoperiod; they simply
flower when they are mature. Examples include geraniums, pansies and certain strawberries.
If plants fail to bloom, it could simply be that it is the wrong time if year. Indoor plants
can be induced to flower by manipulating photoperiod using artificial light or dark treatments.
This is how growers ensure that poinsettias are flowering in time for Christmas and lilies in
time for Easter. If using artificial light to remedy inadequate sunshine, it is important to realize
that this interferes with photoperiod and may inhibit the flowering process in certain
houseplants.

3. Excess Nitrogen

A plant that is grown in the correct exposure and exhibits healthy, green, vigorous growth
but fails to set flowers could be subject to excess soil nitrogen. Excess nitrogen in the soil
promotes a flush of green, vegetative growth at the expense of flowers. Flowering is
essentially delayed or inhibited by the continuous production of new leaves and stems. It is
one of the leading causes of poor fruit production. No flowers means no fruit. The effect of
excess nitrogen on flowering is commonly seen in trees and shrubs that inhabit lawns or other
grassy areas. High nitrogen fertilizers are applied to turf in order to encourage lush, green,
vegetative growth. Although desirable for grasses and other foliage plants, nitrogen
applications tend to have an adverse effect on flowering plants. A possible remedy is to
water deeply and thoroughly to flush away the excess nitrogen and fertilize later with a
high  phosphorus or high potash formula. In fact, most tomato foods are ideal for fertilizing
flowering plants. A related but opposite scenario, nutrient deficiency can also contribute
to poor flowering. Nutrient deficiency is quite common in containerized plants whereby
the roots have only a limited amount of soil area in which to explore and draw nutrients from.
The nutrients inside containers are depleted rapidly and need to be replenished on a regula
r basis in order to maximize bloom time and flower quality.

4. Pruning

Improper pruning techniques are frequently responsible for poor flowering, particularly in
woody plant species. Pruning a plant at the wrong time of year can result in the actual removal
of flower buds. Simply put, if the flower buds are removed, the plant will not flower. This type
of pruning error is common in spring flowering shrubs such as lilac and forsythia. Spring
flowering shrubs typically flower on last year's wood. If they are pruned in fall, late winter,
or even early spring, the flower buds will be removed. If this happens, there will be no
flowers in the following spring. Winter-feeding animals can actually remove the flower buds,
creating the same outcome. A good time to prune spring flowering shrubs is immediately
after flowering, before they have had a chance to set next year's buds. Summer flowering
shrubs typically bloom on current year's wood. These should be pruned after flowering or
in the winter but not after new growth has begun. Excessive pruning can also have a negative
effect on flowering in the same way that excess nitrogen can. Heavy pruning stimulates a
flush of vegetative growth that, in turn, delays or inhibits flowering.

Hardy bulbs are also common victims of pruning mistakes. Even though the flowers may
fade early in the spring, the foliage must be maintained throughout the growing season in
order for the bulb to accumulate enough food to support next year's growth. Many gardeners
think that the foliage is an unsightly nuisance and will cut it back prematurely. In some cases,
lawnmowers or other garden equipment remove the leaves accidentally. Failure to allow
the foliage to die back naturally jeopardizes the success of next year's floral display.

5. Severe Winters and Late Spring Frosts

Prairie winters are typically characterized by extremely low temperatures and little
snowfall. Severely cold winters without adequate snow cover can challenge
even the hardiest plants. Perennials with only marginal hardiness often succumb
to such winter conditions and fail to come back the following spring. Spring
flowering shrubs such as forsythia produce flowers on year old wood, meaning
that the flower buds are set prior to winter. Without adequate snow cover, these
flower buds are extremely susceptible to winter injury and may fail to bloom in the
spring. In this situation, the dead buds often remain on the plant, making it easier to
identify winter injury as the cause. It is not uncommon for only the lower portion of
a shrub to flower in the spring. In this case, higher buds exposed to the elements
are killed but lower buds insulated by snow are able to survive. If winter injury has
affected the bloom on one plant, it has likely affected other plants of the same type.
Ask friends and neighbours if they have experienced a similar problem.  If you seem
to be the only gardener without lilacs, winter injury is perhaps not the problem!
Late applications of fertilizer, namely nitrogen, should be avoided as it interferes with
a plant's ability to shut down for the winter and, therefore, increases the potential for
winter injury.

Late spring frosts can also be devastating, especially for early flowering shrubs and
even fruit trees. Leaf buds tend to be much hardier than flower buds. Therefore, after
a late spring frost, the leaves may open up as usual but there will be no flowers. Leaves
that had already begun to expand prior to the frost may become blackened, a common
symptom of frost injury. Sometimes gardeners can be a little overzealous when it comes
to transplanting annuals into the garden, especially after a long winter. Annual plants
are extremely delicate and cannot tolerate frost as well as most perennial species.
Dependingon the severity of the frost, some annuals will recover (and flowering will
be delayed) but some will not. Pansies are perhaps the hardiest of the annual flowers
and can be planted fairly early. As for the others, it is best to wait until the last frost
has passed. Hardy bulbs such as tulips and daffodils are generally frost tolerant but
may be affected if the frost is severe enough.

6. Deadheading

The goal of any flowering plant is to perpetuate itself by setting seed. Seeds come
from flowers and once seed is set, flower production is terminated. The term
"going to seed" is especially applicable to annual plants. Annual flowers are grown
for their extremely showy flowers and their continuous bloom. Although the flowers
of some species fall cleanly and naturally from the plant, some require constant
deadheading. Deadheading is essentially the manual removal of spent flowers in an
effort to prevent seed production and maximize bloom time. If spent flowers are
removed prior to seed set, the plant directs its energy into producing more flowers,
energy that would otherwise be spent on seed production. Geraniums are extremely
common landscape plants that, if not deadheaded, will go to seed and stop producing
flowers. If the spent flowers are removed regularly, they will continue blooming until
frost. Impatiens, on the other hand, does not require deadheading and will continue to
bloom throughout the summer. Some perennials will also benefit from the removal of
spent flowers. Deadheading is an especially useful technique for perennials that
have a short blooming time. Some perennials, including shasta daisy and dianthus,
will continue blooming if the spent flowers are removed promptly. For other perennials,
such as delphinium and balloonflower, deadheading can even induce a repeat bloom
later in the season.  Although deadheading can be used to maximize bloom time and
induce repeat flowering, it can also be considered a good way to keep the garden clean
and tidy. Deadheading will only enhance flowering in some plants. Proper techniques for
removing faded blossoms vary according to plant species. In some cases, only the
flower head needs to be removed but for others, removal of the flowering stalk
may be necessary.

7. Temperature

Both air and soil temperature can have an effect on flowering. For example, annual plants
that are transplanted into cool spring soils tend to take longer to become established and,
as a result, may exhibit a delay in flowering. Cool air temperature early in the growing
season also tends to delay flowering in many types of plants. However, certain plants
may respond favorably to cooler temperatures. Pansies, for example, prefer cool and
wet spring conditions and will perform best when exposed to lower temperatures. As
the season progresses and the temperature rises, they tend to show symptoms of decline.
By mid-summer, they often exhibit a spindly growth habit with fewer and less vibrant flowers.
When this happens, it may be worth replacing them with more heat-tolerant annuals.
Portulaca, for example, is an annual that thrives in hot and dry summer conditions.
Excessive heat may also inhibit flowering in certain types of peonies.

Temperature can also have a great impact on the performance of hardy bulbs such as
tulips and daffodils. These bulbs require a chilling period in order to bloom properly in
the spring, a phenomenon known as vernalization. If the minimum exposure to cold is
not provided, such as in areas with mild winters, the bulbs often exhibit a poor floral
display in the spring. This is also important to consider when choosing a planting site in
the garden, for even heat from a house/basement during winter can interfere with spring
flowering.

 8. Planting Depth

Planting depth can have a significant effect on both the flowering process and the overall
health of a plant. Most plants have their own depth specifications and failure to meet these
specifications may result in bloom failure. For example, when planting peonies, the buds
(eyes) must never be more than 4-5 cm below the soil surface. Exceeding this recommended
depth is one of the most common reasons why peonies fail to flower. When planting irises,
ensure that the rhizomes are no more than 5-8 cm below the soil surface, otherwise bloom
failure may occur. On the other hand, spring flowering bulbs that are planted too shallow
may also fail to reach their full bloom potential. If the bulbs are not planted deep enough,
exposure to high temperatures can damage the flower buds. For example, daffodils that
are planted too shallow will often respond with a sparse display of flowers. As a general
rule, the planting depth for bulbs should be approximately three times the width of the bulb.
It is always better to follow individual plant specifications in order to achieve the best floral
display possible.

9. Root Environment

For many plants, the quality and quantity of flowers heavily depends on the amount of soil
area that is available to the root system. Many perennials and bulbs require a lot of space
and will not tolerate overcrowding. Peonies, for example, should be given a separate growing
area, away from all other root systems, in order to optimize their performance in the garden.
Perennials and bulbs that begin to show symptoms of decreased vigor and poor flowering
may be experiencing the effects of overcrowding. In this situation, digging and dividing the
plants will help restore them to their full potential. On the other hand, many plants prefer a
smaller, more confined soil area. This is perhaps more common in flowering houseplants,
where the root systems are contained. African violet, bird-of-paradise, hibiscus and many
common orchids prefer to be slightly potbound and will flower more readily if the roots are
restricted. Transplanting these houseplants into unnecessarily large containers will often
inhibit flowering. It is recommended, for example, that bird-of-paradise not be repotted after
it reaches about 4-5 years of age. At this age, the plant reaches maturity and will begin to set
flowers. Because the plant performs best when it is slightly potbound, repotting at this critical
time can have a detrimental effect on flowering. It is important to realize, however, that these
houseplants respond well to being only slightly potbound and must be repotted before they
become excessively potbound.

10. Alternate Flowering

Alternate flowering is a natural, biological phenomenon exhibited by some flowering shrubs
and fruit trees. Plants that exhibit an alternate flowering pattern will have alternating "on"
and "off" years. They will flower heavily during "on" years but then flower poorly (if at all)
during the following "off" year. Some plant species may actually have two consecutive
"off" years before flowering again. Alternate flowering is especially common in apple and
crabapple trees. Basically, a heavy fruit crop in one year will consume a significant
proportion of the plant's food reserves at the expense of the next year's flowers and fruit.
It has been suggested that hand thinning excess fruits in an "on" year will help to conserve
the plant's resources and reduce the occurrence of subsequent "off" years. Otherwise,
the best thing to do is avoid varieties that are particularly prone to alternate flowering
patterns. Flowering dogwood is a common shrub that exhibits alternate flowering.
Lilac and mockorange may also demonstrate a similar pattern if seed heads are not
promptly removed after flowering.

 11. Overall Health

Poor flowering can also be pest related. It is important to become familiar with the pests
and symptoms commonly associated with the respective plant in order to identify a
problem and implement an appropriate control measure. Plants that have been attacked
by disease or insect pests usually exhibit visually identifiable symptoms that aid in diagnosis.
Keeping plants as healthy and vigorous as possible will help to increase their immunity to
pest invasion.  This means administering a regimented watering plan and a balanced
fertilizer program. Also ensure that stress and injury are minimized throughout the growing
season.  Apply pesticides only if a disease or insect has been confirmed and always follow
the manufacturer's label carefully.








  

U of MDepartment of Plant ScienceContinuing Education