Protect your crab apple tree now

If you have a crab apple tree that gets apple scab, it is time to begin spraying it with fungicide if you wish to try to control the disease. On a tree with apple scab, the leaves become covered with black spots and fall off in late summer.

You will need to begin spraying after the buds open, and treat the tree once every seven to 10 days until the leaves are fully open. Generally three treatments suffice. Call the Chicago Botanic Garden's Plant Information Service at (847) 835-0972 for recommended fungicides and timing of applications.

The alternative to yearly spraying is to replace the tree with a newer disease-resistant variety. Typically, the disease affects older varieties.

Plant cool-season annual flowers, such as pansies and primulas, that can tolerate a light frost in early April to add color to your garden. These early spring annuals can be planted under bulbs to complement their flowers and provide color until summer annual flowers are planted in mid- to late May.

Due to unusually cold weather this year, the botanic garden has delayed planting some spring annuals. You may want to consider installing your annuals a bit later if the cold spring weather is predicted to persist.

Temperatures that drop into the 20s can damage even the more cold-tolerant annuals such as pansies. If an exceptionally cold night with temperatures below 28 degrees is predicted, it would be a good idea to cover your early spring annuals. At 25 degrees, the spring flower display will be severely impacted, though the plants themselves may survive.

When you purchase plants, choose those that are well-developed with lots of flowers and buds, because cool-season annuals do not have much time to develop after planting.

Install bare-root plants 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. Do not let the roots dry out. It is a good idea to soak the roots of bare-root trees and shrubs in water for a short time before planting.

Prune branches and roots before planting only if they are broken. There is no need to prune to compensate for transplant shock.

Apply crabgrass control if needed to lawns in early to mid-April before weeds germinate. The window is later this year due to the cold spring. Complete this work before lilacs begin to flower. If you did not have problems with crabgrass last year, you probably do not need to apply crabgrass control this year.

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