Landscape - Why Plants do not Flower

A common question I receive from landscapers is why a certain plant does not flower or flowers very little. There are many possibilities. The following is a short article on the subject.

Plants can take several years before they will flower. Often there is a juvenile period where the plant produces leaves and shoots but does not set flower buds. This juvenile stage may last 2 or 3 years on some flowering shrubs or 5 to 10 years on certain tree species. Certain propagation techniques can also speed or delay flowering in resulting plants. Grafted or budded plants often flower sooner than plants on seedling rootstocks. Some plants are genetically disposed to flower at a later stage than others. Another common problem limiting flowering is excessive shade for sun loving plants. Excessive nitrogen fertilization will often reduce flowering, especially in herbaceous perennials. Improper pruning is probably the most common cause of poor flowering. In spring bloomers, flower buds are generally produced on last year’s growth. Heavy winter or early spring pruning may remove that flowering wood. In summer and fall bloomers, flowers are most commonly produced on new growth. Excessive late spring and summer pruning will remove flowering wood. Some plants are prone to alternate year flowering, producing heavy loads one year and few the next. Short day perennial plants (daylength sensitive) may not flower if there is excessive stray light such as outside security lights. Winter flower bud kill due to low temperature is common, especially in species that are marginally hardy in this area. Drought stress, injury, pest damage, and other stress factors the previous year can limit food reserves and thus flowering the following year. Of course, certain insects and diseases can also attack flowers.

Gordon Johnson, Extension Horticulture Agent, UD, Kent County