Time to start spraying crab apple trees

It is nearly time to begin spraying crab apples that are susceptible to apple scab (typically this disease affects older varieties of crab apples). If your tree's leaves become covered with black spots and fall off in late summer, it is in need of a protective spray program, if you want to try to control this disease, or it should be replaced with a new disease-resistant cultivar.

Begin spraying after the buds open and treat once every seven to ten days until the leaves are fully open with three treatments generally being adequate. The sprays may not be completely effective in years when there are extended damp or rainy periods of weather that favor this disease. Apple scab spreads rapidly when it is 55 to 75 degrees. Call the Chicago Botanic Garden's Plant Information Service at (847) 835-0972 for recommended fungicides and timing for applications.

• If you have had problems with crabgrass in your lawn, then apply a pre-emergent herbicide for controlling crabgrass. There is no need to apply this product if you have a thick, healthy lawn without crabgrass. These types of herbicides work by preventing weed seeds from germinating and do not work for perennial weeds.

Once applied, you need to avoid breaking the barrier this herbicide forms, so forgo maintenance practices such as power raking and core aerating. In most years, you should apply this herbicide to lawns in early to mid-April before weeds germinate. The window is much later this year due to the cold spring.

Complete this work before lilacs begin to flower.

• Improve the health of your lawn by core aerating. Leave the plugs on the lawn as they will break down in a short period of time.

Do this before applying any pre-emergent herbicides, which form a barrier at the soil surface to prevent weeds from germinating. Pre-emergent herbicides will also prevent grass seed from germinating, so coordinate any seeding with use of a pre-emergent herbicide.

• When turning on your irrigation system for the first time this year, monitor each zone to ensure uniform coverage. Sprinkler heads can become plugged or not rotate properly, leaving dry areas.

Soft, excessively wet spots in the lawn can indicate a leak in the system.

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