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Wait to plant cold-sensitive annuals

Be careful not to plant cold-sensitive plants too early. Tropical plants, warm-season annuals such as impatiens and coleus and cold-sensitive vegetables such tomatoes and peppers can be damaged or killed by temperatures that fall to freezing or even by extended periods in the 40s.

A good guideline is the average date of the last frost in your area, which can vary within the Chicago region. At the Chicago Botanic Garden in Glencoe near Lake Michigan, the average last frost date is May 15, but frosts are more likely to occur later farther from the lake. However, the last frost date is only an average, and spring weather can vary from year to year. Monitor long-range weather forecasts before making a final decision on when and what to plant.

Wait to buy warm-season vegetables such as tomatoes until later in May, 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 buried utility lines located before starting the work. In the suburbs, call JULIE at (800) 892-0123 or 811 to have the locations of underground gas and water lines and other utilities marked (this is a free service). Their website is www.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 your project.

These locating services will mark only lines installed by utilities and not those you may have installed yourself, such as a gas line to a grill, wires for landscape lighting or piping for sprinkler systems.

Dig carefully by hand if you are working within 18 inches of either side of any underground utilities. Also, be careful when digging in gardens with underground sprinklers. The black plastic piping used in many systems is easy to cut with a spade.

Rhododendrons, azaleas

It's a good idea to deadhead (manually pinch off spent flowers) of rhododendrons and azaleas after they finish flowering. This is also the correct time to 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 half of the sticky new green growth that will emerge from the ends of branches near the spent flowers.

Trees

Many trees are planted too deeply. To determine the proper planting depth for a new tree, locate the trunk flare, which is the place where the trunk widens to join the roots. You may need to open the burlap and very carefully remove the excess soil above the roots to expose the flare.

Measure the height from the bottom of the root ball to the trunk flare. Then dig a hole that is wide, with sloping sides, and just deep enough so the trunk flare will sit slightly above the level of the surrounding soil. Generally, planting 2 to 3 inches higher than ground level is a good idea in heavy clay soils.

After the tree is positioned in the hole, remove as much of the wire basket and burlap as you can reach.

It is best to refill the hole around the root ball without amending the soil. However, it is OK to amend the soil at the surface around the tree by top-dressing with compost.

Water the tree well and spread mulch in a wide, even layer over the root zone, keeping it away from the bark of the trunk.

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

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