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Flowering on old wood

Q: What does it mean when you hear that a shrub flowers on old wood?

Jenny Pfifer, Fort Dodge, IA

A forsythia flowers on old wood. (Old wood is any branch or stem that was produced during the previous growing season.) A forsythia flowers on old wood. (Old wood is any branch or stem that was produced during the previous growing season.) Photo/Illustration: Jennifer Blume

A: Wayne Cahilly, an arborist and manager of the Institutional Mapping Department at the  New York Botanical Garden , responds: The term “flowering on old wood” refers to the age of the twigs on which flowers bloom each year. Old wood is any twig, branch, or stem that was produced during the previous growing season. If you were examining a plant in March, the old wood would be the growth from the previous summer. Some species of plants produce flower buds in mid-to-late summer, and these buds overwinter in a dormant condition. In the early spring, the flower buds begin to bloom. Because these flowers are located on twigs that were formed the previous year, they are said to be blooming on old wood. Some examples of plants that flower on old wood are forsythias (Forsythia spp. and cvs.), big-leaf hydrangea (Hydrangea macrophylla), and English holly (Ilex aquifolium).

By contrast, some species of plants produce their flower buds during the spring flush of growth. Buds produced during spring growth do not go through a winter dormant period. Instead, they bloom as soon as they mature. Since these flowers are positioned on brand-new twigs that have just been produced, they are said to be blooming on new wood. Examples of plants that flower on new wood are most varieties of butterfly bushes (Buddleia spp. and cvs.), smokebushes (Cotinus coggygria cvs.), and potentillas (Potentilla spp. and cvs.).

When pruning, it is wise to know when flower buds are produced. Plants that bloom on old wood should be pruned immediately after they flower. This gives the plant the remainder of the growing season to generate flower buds for blooming the following year. Plants that bloom on new wood can be, and often are, pruned severely during the late winter, since the new growth stimulated by pruning will generate flower buds a month or so after growth begins. Some woody plants, like butterfly bushes, may be cut to within a foot of the ground each spring and the new growth trained for a spectacular bloom in mid-to-late summer. This treatment applied to a rhododendron, however, would result in a temporary loss of flowers.

From Fine Gardening 86, pp. 82