Invasive buckthorn trees grow rapidly

Buckthorn is an invasive tree and common weed in Chicago-area gardens. It tends to hold green leaves later than other deciduous trees and shrubs in fall so it is easy to spot.

Cut buckthorn at ground level and quickly treat the stump with an herbicide to kill the root system.

Smaller trees can be dug out with a sharp spade.

• Shredded leaves make good mulch for your garden beds. For the serious gardener, a shredder can be rented, but be forewarned that they are very noisy so it is best to use ear and eye protection when shredding the leaves.

Using a shredder is a time-consuming process but results in finer leaf mulch. For the average gardener, a lawn mower with a bag to catch the leaves is adequate for this job. The mower doesn't cut the leaves up as finely as a shredder but works a lot faster.

Ground-up leaves decompose more quickly if you are using them in a compost pile. Shredded leaf mulch can also be purchased from garden centers.

• In general, it is not necessary to mulch established perennial borders. Leave perennials up for winter interest and leave a light layer of leaves in the bed to provide some winter protection. Cut back perennials when they start looking bad.

Any new perennials planted this year and especially those installed in late summer or fall should be mulched for the winter. The freeze-and-thaw cycles in spring can push newly planted perennials out of the ground.

• Tim Johnson is director of horticulture at Chicago Botanic Garden, chicagobotanic.org.