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On knockout roses, plus a daisy as plant of the week

My Knockout roses are not doing well. First I had to treat them for rose slugs eating the leaves. Now the leaves of two trunks are fading to a pale green. Are they dying? How is that possible with a Knockout?

Knockout roses are highly resistant to black spot, but that doesn't mean they're immune to everything. We get calls primarily about rose slugs, rose rosette disease, and stem cankers. Japanese beetles were heavy this year, too. Some Knockout colors are more prone to problems than others. Check the stems of your rose for cankers (darkened areas) and prune them off below the canker. Contact us if you see other symptoms.

Crabgrass is terrible this year. It grows sideways and roots wherever it touches, so one plant becomes huge. What can I do?

Crabgrass is ubiquitous in Maryland lawns. This annual grassy weed is generally managed by applying pre-emergent herbicide granules in early spring, so that the seeds cannot develop. Some pre-emergent herbicides have limited ability to help after the weeds have grown. There are post-emergent herbicides, too, but they can only kill young crabgrass plants. Since your plants are full-grown, you can hand-pull or dig it out or keep it cut to limit seed formation. It will die at the end of summer. To help prevent crabgrass next summer, reseed and thicken up your lawn in the fall, maintain mowing height of at least 3 inches high (crabgrass need sunlight to germinate), and fertilize in the fall.

For better season-long crabgrass control, consider applying your pre-emergent twice, but make sure the product you choose does not contain fertilizer. Apply first in early spring then again about six weeks later. Also see our crabgrass control publication, "TT43: Herbicides for Crabgrass and Goosegrass Control in Turf"on the HGIC website under "Publications" in the lawn category.

University of Maryland Extension's Home and Garden Information Center offers free gardening and pest information. Call 800-342-2507 or send a question to the website at extension.umd.edu/hgic.

Plant of the Week

Shasta Daisy 'Becky'

Leucanthemum x superbum 'Becky'

It's easy to see why Shasta daisy cultivar 'Becky' earned the 2003 "Perennial Plant of the Year" award. Showy and reliable, this clump-forming perennial is a summer must-have. The large, clear, white flowers have strong stems requiring no staking. Unbothered by Maryland's heat and humidity, they bloom from July to September if deadheaded. Drought-tolerant once established, they attract butterflies while resisting deer and rabbits. They're a favorite in borders, beds, and containers. Try shastas in cuttings gardens, too, since the flowers have a long vase life. About 3 feet tall and wide, plants thrive in full sun in a moist, well-drained soil. Divide every two to three years and share this beauty with friends. — Marian Hengemihle

Copyright © 2014, The Baltimore Sun
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