Plant your cold-sensitive plants after average last front date

You can encourage more flowers next year on rhododendrons and azaleas if you deadhead them right after they finish flowering. Very carefully pinch off 1/2 of the sticky, new green growth that is emerging from the area where the flowers were generated this year.

This is also the correct time to prune rhododendron and azalea branches to reduce the size of the plants as needed. Be conservative when pruning them, and don't prune them back too far.

Keep the average last frost date for your area in mind as you consider when to plant cold-sensitive plants. The average last frost date at the Chicago Botanic Garden in Glencoe is May 15, but this average date can be earlier or later elsewhere in the Chicago region.

It is best not to install tropical plants and warm-season annuals and vegetables early if there still is a risk that the weather will reach freezing or there will be extended periods of temperatures in the 40s. Some plants, such as impatiens and coleus, would likely be badly damaged or killed by a frost. Tender plants can be set back by periods of cold and wet weather even if there is no frost, which may require them to be replaced.

Warm-season vegetables such as tomatoes also will not do well in typical early May cold spells. Do not buy them until later in the month, even though you may find them for sale. Go ahead and plant trees, shrubs, perennials and ground covers.

If you have a lot of digging to do in your garden, it's important to have the locations of underground utility lines marked before you start. This is a free service that can prevent dangerous accidents. In the suburbs, call JULIE at (800) 892-0123 or 811 or visit illinois1call.com. In Chicago, call DIGGER at (312) 744-7000. It generally takes two working days for these services to locate your utility lines, so allow plenty of time before digging. These locating services will not mark any lines you have installed yourself, such as a gas line to a grill or wires for landscape lighting.

Dig carefully by hand if you are working within 18 inches on either side of any marked underground utility lines. Cable television and phone lines are often just 1 to 2 inches below the surface.

Be careful, too, when digging in gardens with underground sprinklers. Black polyvinyl piping, used in many irrigation systems, is very easy to cut with a spade.

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