How to know whether it’s time to start planting

Horticulturists at the Chicago Botanic Garden are often asked when the best time is to start planting outdoors. If you hope to plant cold-sensitive plants outdoors, keep the potential for frost in mind. Midwestern gardeners learn to be skeptical of early warm days. Winter may not be quite done with us yet! The Garden’s average last frost is May 15, but not every spring is alike, so monitor the long-range weather forecasts before making a final decision on when and what to plant. Do not install tropical plants or warm-season annuals and vegetables early unless you can be sure the weather will not dip below freezing. Even extended periods of time in the 40s Fahrenheit can do damage to plants like impatiens and coleus. Warm-season vegetables such as tomatoes do not do well in the early May cold spells that can happen in our area. We recommend that you wait to buy them until later in the month, even if they are on sale. Go ahead and plant trees, shrubs, perennials, and ground covers.

• Be sure to locate any utilities in your yard before starting the work if you have any plans to dig in the garden. 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 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 (like 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.

• If you have rhododendrons or azaleas, once they have finished flowering, be sure to pinch off the spent flowers manually. This is known as deadheading. Prune their branches to reduce the size of the plant as needed. It is best to prune lightly. You can increase the flower count for next year by very carefully pinching off one-half of the sticky new green growth that emerges from the ends of branches near the spent flowers.

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

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