Trimming Korean Lilac Trees
Thank you for the question. Lilacs can be fertilized after the blooms are done. Use a 5-10-5 or 10-10-10 fertilizer at the base of the plant, no more than the package directs. Using too much will result in lots of green growth and not many blooms next year.
Whatever tool is chosen for pruning, make sure it's sharp, makes clean cuts and is easy to control. A string trimmer shouldn't be used. It tears rather than cuts. Learn all about caring for lilacs here: http://learningstore.uwex.edu/Assets/pdfs/A3825.pdf
Thank you for contacting Extension.