June 20, 2015

1 - Lilac plants are quite tolerant of pruning.

Yard and Garden June 20, 2015

Q.

I trimmed my lilacs late last winter, but now they have grown so much that I have a hard time seeing when I back out of my driveway. Can I trim them again? Will it harm the lilac plants? They did not produce many flowers this spring and I do not want to hurt the plants. I love the flowers.

Raton, NM

A.

When trees and shrubs are creating hazardous situations, you should trim them or otherwise remove the hazard. Lilac plants are quite tolerant of pruning, especially in your more northerly region of New Mexico. Plants will regrow to compensate for what was removed by pruning. When you prune your plants, you must consider the compensatory growth that will occur, and prune sufficiently to allow for such growth without recreating the problem. Lilacs often benefit from rejuvenation pruning that cuts at least some older stems to the ground level. This stimulates production of new stems from the base of the plant. These newly produced stems may take two years to flower, but will produce more flowers and higher quality flowers than old stems.

Pruning in mid-summer to remove growth to restore visibility at the end of your driveway should not harm healthy lilac plants if you water well and provide proper care after pruning. Do not prune parts of the plant that are not causing problems. If cutting a few entire stems to the ground will help, you can do that. You can also prune the new growth enough so that when it produces compensatory growth, the problem does not reoccur. Branches will form below the pruning cuts and in late summer these stems will produce flower buds to perfume your landscape next year. Next spring you can completely remove any large stems with growth that could recreate the visibility problems.

If excess growth of the shrubs continues to be a problem at the end of your driveway a wise solution is to remove the large shrubs and replace them with something that will not cause problems. Perhaps you will plant a small tree that has a trunk with few lower branches. This would allow visibility under the branches. Or, you can choose a plant that will not grow tall enough to block your visibility at the end of your driveway. You can try transplanting the lilacs to another location in the landscape, but if the lilacs are old it will be easier to just purchase new lilacs.

You mentioned that you pruned your lilacs in the winter and that they did not produce many flowers. The lack of flowering may be due to the fact that you removed the flowers when you pruned in the winter. Lilacs produce flower buds in late summer. These flower buds remain dormant through the winter and then produce flowers early in the spring. Winter pruning removes the preformed flower buds. Pruning in spring after flowering is often recommended. Extensive pruning, such as rejuvenation pruning can be done in the winter, but flower buds will be lost.

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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