Subject: Pruning Lilacs
Inquiries about lilacs seem to be at an all time high. After questions about cultivar selection, the next most common category of questions seems to be pruning.
Most of the large flowered 'lilacs that look and smell like lilacs' in the Genus Syringa are pruned the same way. These include Syringa vulgaris, Syringa oblata and Syringa x hyacinthiflora cultivars. Plants that may commonly be called lilacs like Ceanothus or California lilac as well as the large variety of different plants called southern lilacs do not fit into the following pruning recommendations.
Pruning is necessary for continued healthy, vigorous lilacs. Remove 1/4 to 1/3 of largest stems annually in mid-winter allowing no more than 6 to 12 major stems to remain. Stems should be spaced so none are allowed to rub. Remove all but one or two replacement suckers an inch or so below the ground following flowering and all small twiggy growth annually. Damaged or diseased wood should be removed when it is noticed.
Height control can additionally be accomplished by pruning a foot or so lower than the maximum height desired. This is best done immediately following flowering. Deadheading or removing spent flower heads may increase the number of flowers the following season slightly but this practice is more cosmetic than necessary. A healthy lilac will flower in most years whether deadheading is done or not.
Tree lilacs (Syringa reticulata and Syringa pekinensis) should have basal suckers removed and be limited to one to three main trunks. The smaller statured lilacs like Syringa meyeri and Syringa patula, including the popular cultivar 'Miss Kim' rarely need much pruning.
The littleleaf lilacs like Syringa microphylla 'Superba' and the hybrid cultivar 'Tinkerbelle,' as well as the shrubby Syringa laciniata need little pruning as well but we have occasionally seen terminal dieback after winter similar to that seen with Caryopteris. This dieback should be removed back to healthy living tissue when apparent in spring. If this lilac type gets overgrown, prune it back as you would most other large, deciduous shrubs by removing about 1/3 at a time following flowering.
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