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Originally published Thursday, July 10, 2014 at 6:15 AM

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Q&A: Dealing with rose black spot and sod webworm

Garden writer Ciscoe Morris on help for diseased roses and lawns.


Special to The Seattle Times

Gardening Events

Ciscoe’s Picks

Sequim Lavender Weekend: July 18-20, 9 a.m.-7 p.m. Friday and Saturday, 9 a.m.-6 p.m. Sunday. Music, arts and crafts. Tour of seven farms 10 a.m.-5:30 p.m. each day; free (360-683-6197 or www.visitsunnysequim.com).

Woodinville Garden Club’s Tour of Gardens: 10 a.m.-4 p.m. July 19. Five private local gardens. Tickets are $20 and include wine tasting from 3:30-5 p.m. at Molbak’s after the tour (www.woodinvillegardenclub.org).

20th annual West Seattle Garden Tour: 9 a.m.-5 p.m. July 20. Ten gardens, with a noontime lecture by Susie Egan. Ticket $18, free age 12 and under (www.westseattlegardentour.org).

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In the Garden

Q: My roses are looking diseased and lots of leaves are falling off. Will cutting back now help to make them attractive again?

A: This wet spring has made roses highly susceptible to black spot. This is a fungus disease that begins with spotted leaves that soon turn yellow. In severe infestations most of the leaves fall off, leaving the shrub bare and unsightly.

It can’t hurt to cut the rose back by a third. Hopefully that will stimulate the buds at nodes just below the cut to put out new growth. Feed your rose with a mix of alfalfa meal and organic rose food. Make sure your rose has adequate water, but use care not to wet new leaves or they will likely become infected with black spot as well.

As a precaution, spray the new growth weekly during rainy weather with environmentally friendly neem oil, available at nurseries and garden centers. If all goes well, the leaves will grow back and your rose will put on a nice display of blooms by fall.

Consider replacing your susceptible variety with a ‘Knock Out’ rose. These roses are tested and only those varieties that are highly resistant ever reach the market. They lack fragrance, but they make up for it with attractive disease-free foliage and practically nonstop flowering all summer long.

Q: I’ve noticed swarms of little moths hovering over my lawn and lots of small brown, dead spots are appearing. Are the two related?

A: There are a lot of possible causes for brown spots in a lawn, but the fact that you’ve noticed numerous moths above the lawn makes it likely that your lawn is infested with sod webworm.

The troublemakers are the offspring of the moths, ½-t o 1-inch-long caterpillars that hide in silken tunnels during the day and then come out at night to chew grass blades off at the base.

Just to be sure webworms are causing the damage, examine the grass. If you see silken tunnels and green frass (polite word for pelletlike bug poop) your lawn has a sod-webworm infestation.

Fortunately, I can tell you from experience that you don’t have to reach for the spray bottle to control this problem. We had a severe sod-webworm infestation in one lawn during my tenure as grounds director at Seattle University.

After researching the problem, I learned that thatch encourages sod webworm. We rented a power dethatching machine and removed the thatch, which probably did in quite a number of the caterpillars in the process. Now is not the time to dethatch your lawn, but if the problem persists, it would be a good option in mid-September.

Avoid the temptation to spray the lawn with a nonselective pesticide. That could kill or harm the beneficial critters, such as birds, ants, spiders and ground beetles that feed on sod webworms and help keep populations of the caterpillars from reaching damaging levels.

The good news is that if you’re willing to accept a bit of short-term damage, even if you do nothing within a year or two, it’s highly likely that the good guys will take care of the problem for you.

Ciscoe Morris: ciscoe@ciscoe.com “Gardening With Ciscoe” airs at 10 a.m. Saturdays on KING 5.



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