Don't work garden soil if heavy clay is too wet

Working the typically heavy clay in Chicago soil when it is too wet can have a negative impact on its structure and result in hard clumps of soil and a crust.

The best way to determine if your soil is workable is to take a handful of soil from the bed and gently squeeze it to form a ball. A garden trowel works well to dig out the soil. Next, try to break it apart using your thumb or fingers to see if it easily crumbles.

If it crumbles, the bed is ready to work. If the soil forms a ribbon when pressing it with your thumb or turns into a squishy ball when trying to break it up with your fingers, it is too wet to work.

• If you have a lot of digging to do in your garden, it's important to locate your utilities before starting the work. In the suburbs, call JULIE at (800) 892-0123 or 811 to have your yard marked for underground utilities (this is a free service). Its website is 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 utilities. Also, be careful when digging in gardens with underground sprinklers. Systems with black polyvinyl piping are easy to cut with a spade.

• Be sure to keep the average last frost dates for your area in mind as you consider any early planting of cold-sensitive plants. The Chicago Botanic Garden's average last frost is May 15, but each spring can vary from the average so it is best to monitor the long-range weather forecasts before making a final decision about when and what to plant.

It is best not to install tropical plants and warm-season annuals and vegetables this early unless you can be sure the weather will not reach freezing or have extended periods in the 40s. Plants such as impatiens and coleus may be badly damaged by cold weather and can be killed by a light frost.

Warm-season vegetables such as tomatoes will not do well in typical early to mid-May cold spells, so do not buy them until later in the month, even though you may find them for sale. Plant trees, shrubs, perennials and ground covers instead.

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