Wasps are a beneficial nuisance

Populations of yellow jackets (a yellow-bodied social wasp) build up in summer. They are unwelcome visitors to picnics and are particularly attracted to sweet things like soda, so clean up after eating outside to minimize any pest problems.

Yellow jackets tend to make their nests in the ground, piles of rubble, sides of building and stone walls, so watch these areas as you are working in the garden. When the weather is warm during the day, there may be a lot of activity, with the yellow jackets flying in and out of the nest.

Baldfaced hornets construct large paper nests in trees and large shrubs at head height and higher, so be careful as you are working in the garden. Paper wasps build nests under eaves, signs and fence railings and will sting if disturbed.

These are all beneficial insects, so control them only if they pose a safety hazard.

• It is time to take cuttings of annuals such as geraniums and begonias that are to be kept over the winter. Cool nights in September help harden off the plants, which will make them more difficult to root.

Take a tip cutting 2 to 3 inches long and stick it in a well-drained potting medium such as one part peat moss and one part perlite. Keep the cuttings in humidity and bright light. Cover the cuttings with a plastic bag and ventilate daily by opening and resealing the bag.

Once they are well rooted, typically in two to four weeks, transplant to a 2- to 4-inch pot filled with a growing medium. The size of the new pot will depend on the vigor of the rooted cuttings.

• Do not forget to check the oil and clean out the air filter on your lawn mower on a regular basis. It is easy to overlook this task as you mow throughout the summer. The engine can be ruined by letting it run out of oil.

Dust raised when mowing over dry leaves later in fall can clog up the air filter, which will interfere with the engine's performance.

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