Diagnostic questions usually inspire a question in response. My response to "Why don't my shrubs flower?" is usually something like: "Have they ever flowered?" "How long have they been planted?" "Did they start not flowering this year?" "Have they gradually declined?" Answers to the original question can depend on the answers to the follow up questions because shrubs often are responding to environmental conditions. Therefore, I usually suggest lots of possible reasons and ask the gardener who wants an answer to come up with the correct answer.

If a plant has never flowered, a number of scenarios present themselves. Is the problem plant maturity, climate, soils, light or something else? The following suggestions are in no particular order. If you have one cultural factor right and another is limiting, correct the one that is limiting. However, correcting one problem can lead to another plus it is likely that more than one thing may be limiting. All the gardener can do is work on a balance where most factors are okay. In a diverse garden with many different plants, some plant is not going to be growing under ideal conditions.

PLANT MATURITY: In an age when we expect to see plants flowering in garden centers people tend to forget that shrubs often go through stages when they are not mature enough to flower. If the shrub purchased is a seedling, maturity may be all that is wrong. Most native azaleas are sold as seedlings and many of the plants sold as native shrubs are grown from seeds as well. They need time to mature. However, even asexually propagated plants that are not provided with the proper cultural conditions may not flower.

CLIMATE: Plants can be hardy without being flower bud hardy. Flower buds are often killed at a higher temperature than vegetative buds, leaves and stems.

In a climate like North Carolina with winter temperatures that alternate warm and cold, another factor can be involved, i.e., hardiness usually depends upon stage of growth. When a plant is dormant it can withstand a lot more cold than when it breaks dormancy and starts to grow. Many plants that break bud early have flower buds killed in the Carolinas but are perfectly hardy and flower well in colder climates where winter weather is consistently cold. For example, bigleaf hydrangeas that are often listed as flower bud hardy in USDA Zone 6 and colder often succumb to late winter freezes that are only Zone 7 cold because the plants are already growing . . . and are susceptible to frost injury. Vegetative growth often will appear later in the season but many hydrangea cultivars do not rebloom because the flower buds were formed the previous year. The result is that they become nice foliage plants without flowers.

SOILS: If plants are stressed because conditions are consistently too wet or too dry, flowering may be impacted. However, the most common soil related reason we see plants that are not flowering as abundantly (or at all) in Carolina landscapes can often be traced back to improper pre-plant soil preparation. It can be as simple as adequate liming or application of phosphorus to the soil before planting. Shrubs like crape myrtles will survive in highly acidic infertile soils but thrive with increased flowers at an earlier age if the soils have been adequately limed.

Shrubs like azaleas, rhododendrons and mountain laurels thrive in acidic soils IF nutrient needs are provided. The most common reason these plants do not flower well, if at all, in native mountain soils is a lack of phosphorus available to roots. Mixing superphosphate with the planting soil can help or, once shrubs are established, using a fertilizer like diammonium phosphate (18-46-0) can improve flowering over a couple of years of use. Our mountain soils are often deficient in phosphorus and most forms of this fertilizer do not move down through the soils to the root system quickly. Therefore, mixing fertilizer phosphorus, such as superphosphate, with the planting soil is the best way to be sure this essential nutrient is where the plant can use it.

In certain sandy areas of the state, late season flowering shrubs and other plants will have their flowering diminished by lack of potassium because the potassium applied as a fertilizer in the spring was leached beyond the root zone of the plants by irrigation or rainfall. Rarely is this a problem in mountain soils.

Over fertilizing with high nitrogen fertilizers can keep newly planted landscapes growing too rapidly for plants to reach the proper stage of maturity for flowering. In addition, over fertilizing can result in excessive growth that needs to be pruned away. Far more shrubs are overfertilized than underfertilized. The prunings removed may contain flower buds.

To know which fertilizers to use and how much, soil test before planting if at all possible. If you can not soil test before planting, collect the soil sample and get it analyzed after planting.

PRUNING: If shrubs have never flowered at all, it is unlikely that pruning is the only culprit. However, there are a number of shrubs that form flower buds in the summer or fall of one year then must go through a chilling period in the winter before they flower the next season. If the person asking the question is in the habit of doing fall and winter pruning on azaleas, rhododendrons, bigleaf hydrangeas, tree peonies and similar plants, they may be pruning off flower buds. If these shrubs need pruning at all, it is usually safer to prune immediately after flowering. Fall and winter pruning will mean sacrificing flower buds on most spring and early summer flowering shrubs.

LIGHT: Lack of light, in my experience, is the most common reason a plant will not flower. Too much light usually causes so many stress related problems like leaf scorch, wilting, faded foliage color, minimal growth, etc., for plants sun grown that should be grown in the shade that corrective measures are taken before lack of flowering ever gets to be an issue.

Most flowering shrubs require a certain amount of light to make adequate food through photosynthesis so that the plant has enough energy to flower. Even plants that are best grown in the shade need some light to flower adequately. The more light, usually the more flowers. For plants that have never flowered, they were probably planted in shade that was too dense to start. They either need to be moved, more light provided by thinning or limbing up trees if they are providing excess shade. Without adequate light, accept that your azaleas are going to be foliage plants with only occasional flowers.

PESTS: Rarely are pests the problem when shrubs do not flower. However, anything that can significantly reduce the vigor of plants can impact flowering. Insect infestations, with prime culprits in the western part of North Carolina being aphids and scale insects, can significantly reduce flowering but rarely eliminated it. Diseases that reduce flowering, often later in summer and into early autumn, are usually those that reduce photosynthesis. Powdery mildew is a major problem for some shrubs. Leaf spots like black spot of roses can cause enough defoliation to significantly reduce flowering. Sooty mold should be traced to it's cause, which is often an insect, then the insect managed to prevent black sooty mold from covering leaves and preventing adequate photosynthesis. Diseases like root rots and stem diebacks that actually kill plants or tissue are so dramatic that the lack of flowers is rarely noticed by the gardener.

In a situation where shrubs USED TO FLOWER but don't flower much, if at all, any more, something in the landscape has changed. Most frequently, that something is shade . . . trees grow and provide more shade over time PLUS trees compete for nutrients and water. A competent arborist will know how to thin trees to permit more light and water to enter a landscape either by removing branches, whole trees or limbing up trees so light can enter under the canopy of shade.

New construction can also impact flower and plant health. If you have the luxury of actually seeing the landscape where the problem plant exists, look for new buildings, trenches, evidence of solvents being used on roofs, walls, etc. Changes in light patterns due to construction or tree removal can result in scorched leaves as well as stem and root damage that will impact flowering.

If soils are infertile, regular liming and fertilizing may be needed to stimulate growth that will result in flowering, particularly in situations where there is significant competition from other plants. Soil testing will tell you a lot.

PRUNING: Shrubs can get old, overgrown and fail to produce flowering wood. Removing non-productive, dead and diseased wood from shrubs is essential to keep some shrubs flowering. For example, it is suggested that at least 1/3 of many lilac cultivars be removed every year. This includes old growth and 2/3 of root suckers. If pruning is not done, flowering diminishes.

Everything that has been written about flowers applies to fruit. Without flowers there are no fruit. Most fruiting plants need more light rather than less light with full sunshine often being best for full fruiting.

However, you can have abundant flowers without fruit. If a gardener has fruit some years and not others, then there is adequate pollen nearby . . . or was before the source was cut down or died. Flowers with female parts are generally required in order to get fruit. If your shrub is a male, you will not get fruit. If it is a female, you have a chance to get fruit. Some shrubs are self fruitful and others require pollen from another plant in order to fruit.

Just having a male and female of the same species is not enough. Both need to flower. They also need to flower at the same time and pollen needs to get transferred from the male to the female. This is why it is important with plants like deciduous hollies to get the correct pollinator, not just any male holly.

Fruit set depends upon weather. If the temperatures are too cold or too hot at the time of flowering it is possible that you will not get fruit. If it is rainy and you have a shrub that requires cross pollination by flying insects, you can only get cross pollination when insects fly and they often do not fly when it is raining.

Fruit falling off a tree can be due to changes in the weather, e.g., too wet or too dry. Fruit drop is often due to lack of complete or proper pollination plus if too many fruit set, some plants will drop fruit in a way that seems to balance production, i.e., allow the shrub to mature a crop of fruit. This is natural and normal. Fruit drop can also happen when certain sprays are applied or too much fertilizer is applied at the wrong time.

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North Carolina State University