Weeds growing in cracks easy to eliminate with an herbicide

This is the time of year to check closely for magnolia scale, an insect about the size of a pencil eraser. Typically found on the underside of magnolia branches, the scale is responsible for creating an overall thin condition on the tree.

Magnolia scale exudes a clear, sticky honeydew after feeding on the plant. Black, sooty mold is a fungus that grows on the honeydew. Gardeners often notice the black mold on or under the magnolia before noticing the actual scale.

The mold itself is not a real problem. On small trees, simply pick off the scales and squash them. Other control measures include spraying with summer-weight oil, or applying an insecticide when the scale is in the active crawler stage, generally in early September.

For trees with a history of magnolia scale, a late winter to early spring application of dormant oil will provide good control for the scale. If your tree is affected, provide extra water during summer to minimize stress.

• Weeds growing between cracks in brickwork or sidewalks are unsightly but easy to eliminate. Treat them with a nonselective herbicide when they are small to avoid having to pull them out by hand.

• If your hybrid roses have been losing their lower leaves and the remaining leaves have yellowish foliage with dark spots, then it is likely you have blackspot, which is a common fungal disease. Begin a spray program with approved fungicides immediately.

The fungicides need to be applied once every seven to 10 days as they work to prevent the disease and do not cure what is already infected. Be sure to clean up any leaves that have fallen from the plants. Many landscape shrub roses are resistant to black spot so do not need to be on a spray program.

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