Why Did My Plant Fail to Flower?
By Joan Allen, UConn Home & Garden Education Center
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Courtesy of UConn |
When a flowering tree, shrub or perennial fails to flower, its owner is likely to feel a bit disappointed. At the UConn Home & Garden Education Center, this is one of the more common problems we are asked about. There are a number of reasons that a plant may fail to flower and most of them are cultural or environmental. Diseases and insect pests will sometimes damage flower buds or flowers, but this article will focus primarily on the problem of few or no flower buds being present. Causes include improper pruning, fertilizing practices, winter or frost injury, plant age, crowding, amount of sunlight, and leaving developing seed pods on the plant.
Spring flowering trees and shrubs produce flowers from buds that were formed in the previous growing season. Examples include rhododendron, azalea, dogwood, lilac, forsythia, and some hydrangeas. Pruning them in late summer through early spring before they flower will remove many flower buds. The best time to prune spring flowering plants is immediately after flowering. New flower buds for the next season will be formed during late summer and fall.
Nutrition and soil pH can also influence flowering. The soil pH affects the availability of nutrients to the plant. If the plant is getting too little or too much of important nutrients, it will be stressed, resulting in the formation of fewer flower buds. Excess nitrogen promotes vegetative growth at the expense of the formation of flower buds. Acid loving plants such as rhododendron and azalea should have a soil pH of 4.5-6.0. Lilacs and most herbaceous perennials prefer a pH of 6.0-7.0. pH can be determined by having the soil tested. Information on having your soil tested at the UConn Soil Test Laboratory is available at http://www.cag.uconn.edu/plsc/soiltest/index.php.
Flower buds and flowers are vulnerable to winter or frost injury. Plants such as the mophead hydrangeas (Hydrangea macrophylla) that are often planted outside of their recommended hardiness zone are most susceptible. H. macrophylla, which includes the popular hydrangeas having large blue or pink flowers, is hardy to zone 6. Away from the shore, Connecticut is in zone 5 and regularly has winter temperatures that will be damaging to these plants. The flower buds for the next season are borne on the ends of the previous season's stems and are very exposed to the weather. Smaller specimens can be protected in the winter by surrounding them with chicken wire or something similar and filling it in with leaves, pine needles, or straw to cover the plant. Remove the covering the next spring after the danger of frost has passed. On any flowering plant, once the flower buds have begun to open, a frost may injure them.
Another factor in the production of flowers is the age of the plant. Many trees and shrubs need a few years to become established before they will produce flowers. A container grown plant with crowded roots from a nursery may have many blooms. Once it is transplanted the plant will use much of its energy for new root development before it will start to flower profusely. Most lilacs begin to bloom at three to six years of age. Even if it has blossoms when purchased it may take a few years to bloom again after recovering from being transplanted.
Crowding is a common cause of reduced flowering in herbaceous perennials and is related to the age of the plants. Established irises may flower less as the years go by. The remedy is to divide the plants and replant them in late summer. Some irises benefit from dividing every three years. Peonies often bloom for many years with little attention. If they begin to produce fewer flowers, they may also benefit from dividing. Following dividing and replanting, perennials may need two or even three seasons to recover before they flower profusely again.
An adequate amount of light is critical for the development of flowers. A plant that requires full sun needs at least six hours per day of light. If you have a plant that is not producing flowers, check to see whether it is getting enough light. Even plants that will tolerate some shade require some sun exposure to flower. The amount of sunlight available in a particular garden location can change over the years as nearby trees and shrubs grow taller. Too much sun can also cause stress that can influence flowering. A plant exposed to full sun and hot temperatures may not have enough water and the resulting stress will reduce flowering. For example, particularly in the south, dogwoods benefit from some shade during hot summer afternoons. They are shallow-rooted and benefit from regular watering when rainfall is insufficient.
Leaving developing seed pods on a plant will draw the plants nutritional resources and can also result in fewer flower buds. This applies to both spring and summer flowering plants. Although it can be time-consuming, the removal of spent flowers will encourage the plant to produce more flower buds for the next season or for the rest of the summer.
In summary, many factors can lead to a lack of flowering. It could be a disease or insect problem, but it is most likely one of the problems described here. For more information on failure to flower or other home and garden questions, contact the UConn Home & Garden Education Center toll free at (877)486-6271 or by email at ladybug@uconn.edu.
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