Wait until early to midsummer, then examine the Miss Kim lilac bush for branches that have few or no leaves. Cut these branches off near the base of the bush with hand pruners or loppers. This will redirect the plant's energy to its healthier portions.
Jun 17, Prune right after the blooming period because 'Miss Kim' blooms on old wood. If you prune too late in the season, you will remove next year's blooms; pruning that is too severe might even reduce flowering for up to three stumpgrind.buzzcal Name: Syringa pubescens subsp. patula 'Miss Kim'.
Jan 29, Otherwise, best to prune while the lilac is fully dormant. Pruning when the weather is warmer encourages young new growth that might not survive the still-cold months. It is best to prune out longer individual branches back to the"joint" to prevent the inside of the bush from being bare and void of leaves.
I learned the hard way on that. Firstly you prune after flowering. That will preserve the blooms for next year on the parts you leave intact. Secondly you prune lilacs from their base. You take about a third of the oldest branches out by cutting them with a pruning saw at the base of the plant.
The next year you take another third and the next year the final third.