Tree & Bush Trimming
I want to know if it is harmful to the tress and bushes to be trimmed in the Summer months. The young man who cuts my lawn want to trim up some of the lilac bushes and forsythia so he can get under them to cut the grass. He also wants to take some branches off the holly tree for the same reason. I normally have my bushes trimmed in September. One other bush/tree he wants to trim up is the Crape Myrtle. I told him that I would want to wait until its done blooming. Thank you.
Baltimore County Maryland
Spring-blooming shrubs such as Lilac are best pruned immediately after flowering, though it is probably not too late to do so now. The purpose for this is to avoid removing next year's flowers, which will be developing later this year. Removal of flower buds will not harm the plant's health, however, so if branches are in the way and at risk of being broken, they can be removed any time. Prune away only what is absolutely necessary, as young stems (which will be the more flexible ones likely to droop in the way) are the growth that flowers best and resists some pests compared to older, thicker, more upright stems.
Forsythia is more forgiving of harsh pruning and can be cut back now, though it is beginning to be a bit late for this without risking next year's flowers. (Again, it will not harm the plant's health to prune later, it just might remove most of the flowers.) The entire plant can be trimmed fairly low and it should rebound well; or, you can remove only what is in the way of the mower.
Hollies respond well to pruning, and while early spring is the ideal time for most, they can be trimmed now if you wish. Autumn pruning is generally not advisable for most trees or shrubs because it can encourage re-growth too close to frost, which is a waste of the plant's energy and could predispose the wounded tissue to infection in the future.
Crepemyrtle is best trimmed in spring as the branches are leafing-out, but doing so now would not be harmful. They should only be pruned as needed to correct crossing branches or to remove branches in the way of a walking path or equipment (like the mower in this case). They should not be topped or otherwise drastically pruned to reduce size, despite the fact that under-trained commercial landscapers do this type of pruning on them regularly.
For any of the larger stems on the holly or crepemyrtle, the correct pruning cuts should be made so they do not endanger the health of the tree. Diagrams showing how and where to cut can be found on this page under the heading "Proper Pruning Cuts": https://extension.umd.edu/hgic/topics/pruning-trees
To avoid the mower coming too close to the base of each of these plants, they should have a mulched zone over their roots to keep the grass at a distance. (This will also minimize competition with the grass.) Weed-whacking strikes to stems and trunks can be more serious than mower bumps, though neither is treatable and can cause drastic dieback. If the mower comes close enough to any of these plants that lower branches are in the way, it's possible the lawn is closer to the shrub/tree base than it should be for injury avoidance.