When is the best time to plant a lilac brush?

Anne, depending on where you are, mostly the early spring or late fall would be the best time.

Spring or Fall would be the best time, when temps are cooler. After planting and watering thoroughly add B1 or RootGrowth to your watering can and water it in around the base of your plant. This will help stimulate the roots. Do not fertilize this year as that would promote top growth and this first season needs to be about getting the roots established.

Early spring or late fall is preferable, depending on what area you are located.

Hi Anne - Ideally plant in the fall, but you can also plant in the spring. They thrive in Zones 3-9. I remember growing up with a lilac tree in the yard and how fragrant it was. When we moved to our homestead, we planted several and it's always my favorite time in spring when they bloom! Hugs, Holly

You need to take as much of the root ball as possible and if they are really big it may be a large chore. Once transfered into a hole with margines top and bottom, potting soil filled in, then water often. Water by leaving hose on a steady drip at least every other day. You may need to wrap cloth and tie around root ball. You'll need some healthy HELP FOR THIS?

https://www.gardeningknowhow.com/ornamental/shrubs/lilac/transplanting-lilacs.htm