1 - Prune lilacs after blooming if light pruning, now for heavy pruning.

Yard and Garden - February 5, 2011

Q.

Can I prune my lilacs now? They have become quite large and produce only small clusters of flowers. I read that I should wait until after they bloom to prune them.

A.

Conventional wisdom is to prune lilacs after they bloom. The reason for this is that pruning in the fall or winter removes the preformed flowers. However, if the lilacs are doing poorly and need to be rejuvenated by removing large trunk-like stems, the dormant season is the best time. You will lose some of the flowers, but the plants will respond better since you will not remove the newly formed leaves that are needed to support new, healthy growth. If you wait to prune, you will remove these important leaves.

Lilac plants, such as you have described, should be rejuvenated by pruning one-third of the old stems as close to the ground as possible. This will encourage new growth from the base of the plant. Such growth will be vigorous and be producing flowers within about 3 years. As you prune, remove the oldest, weakest trunks. By leaving two-thirds of the old growth, you will still have some flowers. Next year you can remove the next one-third of the oldest growth and the final one-third in the third year. By then the new growth produced in response to the pruning in the first year will be forming flowers.

After the third year, you can continue removing one-third of the old growth, or wait for several years before beginning the rejuvenation process again. The lilac shrubs should now look like shrubs and produce more and larger flower clusters. Water the plants well at least twice a month during the growing season to encourage production of healthy new growth and flowers. Fertilizer high in phosphorus will help stimulate flower development in late summer. August is a time to be sure the plants are not water stressed if you want good flowering the next spring.

Mulch and other good cultural techniques conserve water and help the plants remain healthy and floriferous.

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.

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