I have been led to believe it would be fall, when the tree.
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While I'd not hesitate to do rejuvenation pruning on a typical common lilac now, one that is grown in a tree form (single trunk rather than multiple stems/canes from the ground) should be treated a bit differently.
Too severe a pruning at this time of year might well jeopardize its future, specially in. Nov 17, Others, such as lilacs, bloom only on old wood from the previous season. Thus, if you prune at any time other than immediately after flowering, you will cut off next year's blooms. For lilacs, the recommended method is generally to remove one third of the old wood each year immediately after flowering and shape the shrub over a number of seasons.
Of course, you can prune in the fall if absolutely required. Keep in mind, though, that you are cutting Estimated Reading Time: 2 mins. Apr 28, To prune lilacs, get to work in the spring: cut fresh flowers for bouquets, nip off the longest stems, and thin out any weak stems at the base. Avoid pruning late in the summer or fall since doing so can limit future growth. Part 1 Doing an Annual Pruning75%(4). Jul 14, A couple strong suckers can be retained to replace main branches, but all weaker suckers should be cut at the soil line so that they don’t steal away nutrients.
Finally, there are a few of rules of thumb to follow when pruning lilacs: Trim mature lilac bushes so that they remain within 6 to 8 feet. Aim to have 10 to 12 main stems, each of them 1 to 2 inches thick. Never prune more than one-third of the bush.
Create a balance of older stems and new flowering shoots. Do you. Timing is the Decisive Factor in Pruning Lilacs If you want to have your blooms next year, it is crucial to prune in those first few weeks after blooming ends. Ideally, this is when you would do maintenance pruning each year. But I get so lazy busy, I end up with a ten foot bush with ten blooms on top.