Why you should test soil, locate utilities before spring garden work
Working the typical heavy clay Chicago soil when it is too wet can have a negative impact on its structure and result in rock-hard clumps of soil and a crust. The best way to determine whether your soil is OK to work 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 crumbles easily, which would mean that the bed may be worked. If the soil forms a ribbon when pressing it with your thumb or turns into a squishy ball when you try to break it up, it is too wet to work with.
If you have digging to do in your garden, it’s important to have your utilities located before starting work. In the suburbs, call J.U.L.I.E. at (800) 892-0123 or 811 to have your yard marked for underground utilities (this is a free service). Their website is illinois1call.com. In Chicago, call DIGGER at (312) 744-7000. It generally takes at least 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 of 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.
Keep the average last frost dates for your area in mind as you consider early planting of cold-sensitive plants. I was recently asked what date the Chicago Botanic Garden now considers to be the average last frost date (which has been May 15 for as long as I can remember). We had one surprise year with a hard, killing frost on May 22. After doing some research and getting frustrated with finding a wide range of dates, I have decided to stick with May 15 as the average last frost for the garden. Each spring can vary from average and different areas in Chicago can vary too, so be sure 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 extended periods of time in the 40s Fahrenheit. These are plants such as impatiens and coleus that will be damaged by cold weather in the 40s and killed by a light frost. Warm-season vegetables such as tomatoes do not do well in typical early to mid-May cold spells, so do not plant them until later in the month, even though you may find them for sale. Go ahead and plant trees, shrubs, perennials and ground covers. I started planting in April when they were available to purchase. Shrubs that were brought in from nurseries in a warmer climate with growth far more developed than plants growing here can also be damaged by frosts.
• Tim Johnson is director of horticulture at Chicago Botanic Garden, chicagobotanic.org.