Divide perennials to manage their growth

Most perennials are best divided in the spring when they are showing a couple inches of growth.

A sharp, flat spade works well for dividing perennials. It can be very difficult to break up a clump of perennials if your spade is dull. Use the spade to cut around the perennial's root ball, about 2 to 4 inches around where the shoots are coming up and at an angle that goes under the plant.

Once you have cut the roots, gently pry the clump out of the ground. At this point, use the spade to cut off any excess soil that is on the clump.

Some perennials will have very dense roots and will take some work to get out while others will be more delicate and will take some care not to break them apart as you lift them. Use the spade to cut up the clump into smaller sections.

When dividing a plant, choose the strongest-looking parts of the plant to replant and discard the rest. Use a tarp to put the divisions on to make clean up easier. Plants that bloom in April and May can also be divided after they bloom. It is best to avoid dividing perennials when they are in flower.

• Plant cool season annuals such as pansies and primulas that can tolerate a light frost in early April to add color to your garden. Ideally, the flowers would have been hardened off before you plant them. If not, then wait to plant them if there are temperatures below 30 degrees forecast within the week.

If the plants have been acclimated to being outside, then it will be OK to plant them. As temperatures drop below 28 degrees there will be more risk of damage to the pansies, so it may be best to cover them for those nights. These annuals can be planted under bulbs to complement their flowers and carry color over until summer annuals are planted in mid- to late May. Choose plants that are well-developed with lots of flowers and buds. The cool season annuals do not have much time to develop after planting.

• Install bare root plant materials as soon as you can after they arrive in the mail. Unpack plants and make sure the packing around the roots is moist. Store the plants in a cool place that will not freeze until they can be planted.

• It is a good idea to soak the roots of trees and shrubs in water for a short period of time before planting. Do not let the roots dry out. Prune only broken branches and roots before planting. There is no need to prune to compensate for transplant shock.

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