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Garden Q&A: How to treat asters and plant new perennials

Q: My asters have developed bleached-looking leaves lately. I’ve been checking the soil for moisture before watering, so I think I’m giving it enough water, and only when it needs it. Is this a disease? Some of the leaf edges look a bit burned.

A: This is feeding damage from a population of chrysanthemum lace bugs. They’re named after mums, but they feed on many members of the aster family, especially asters themselves.

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Lace bugs are named for the delicate, lacy pattern of veins on their partially clear wings that resemble frosted window panes. (Wouldn’t frosty weather be lovely right about now?) Anyway, I actually find them quite attractive when magnified; at a quarter-inch or so long, lace bug adults are fairly small. Don’t confuse them with the similarly named, unrelated, larger and clear-winged lacewings, which are beneficial insects. Over a dozen other lace bug species occur in Maryland, their hosts ranging from azalea or rhododendron to oak, walnut, linden or eggplant.

Like other “true bugs,” lace bugs have piercing-sucking mouthparts, which they jab into plant cells (leaves in this case) and suck out the contents. Picture a wall-to-wall array of lidded takeout cups or juice boxes that you pierce and empty one-by-one with a straw, and you’ve got a decent mental image of a plant-feeding bug’s dining habits.

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This gradual removal of the leaf’s chlorophyll causes that bleached look over time. When large areas of feeding coalesce, they make the whole leaf, and eventually most of the plant, look pale and stressed. I have seen some leaf-edge browning and drying associated with these lace bugs, even though the roots have enough moisture.

Adult and juvenile lace bugs on the underside of a goldenrod leaf. The black spots are frass (poop) glued to the leaf, a common trait of lace bug colonies.

Water can be a solution here, though not for the roots (unless the soil is dry a few inches down). Use a garden hose and aim a strong spray of water at the aster leaf undersides. This will blast off most of the lace bugs, especially if you repeat it about once a week. Adults can fly and might return, but the juveniles cannot fly. Or, just give the plant a good, vigorous shake and see what falls off, which you can then dispatch with a shoe or trowel. It’s not perfect and won’t eradicate them all, but it doesn’t have to — the plant will live and regrow just fine next year unless an unrelated issue has weakened the roots.

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We want to avoid using insecticides on plants as popular with pollinators as aster family members are, though if need be, horticultural oil or insecticidal soap might give you a leg up on the situation. You’ll have to thoroughly coat both leaf surfaces with the spray, though, and this can be difficult to do well on short or dense plants. You also shouldn’t spray while it’s above 85 degrees or you risk damaging the plant even more, so that puts a damper on this tactic. Plus, contact insecticides like these need reapplying every one to two weeks for a few sessions to catch missed individuals.

Since treatment won’t reverse the existing leaf damage, I typically wouldn’t bother with trying one, aside from the hose method. Besides, this is a native insect, so it’s part of the ecosystem services we’re trying to provide by planting these species.

Everything feeds something else: predatory insects and other beneficial organisms will take out some of the lace bugs on their own. Chrysanthemum lace bug overwinters in leaf litter and other debris, so while leaf litter has many benefits (and may shelter lace bug predators as well), this is one risk with keeping it around plants previously infested.

Q: Can I plant new perennials now, or move some I want to rearrange? I hear fall is ideal, aside from waiting until spring, but I can deal with the heat if I can make progress on my garden now instead of waiting.

A: Fall can be ideal in several respects — human comfort level, lower plant moisture needs, cooler temps reducing the demands on roots, late-season plant discounts, etc. — but you can also do these planting tasks throughout summer. Perennials are still in active growth in warm weather and should establish well if their watering needs are monitored.

You might risk more transplant stress from moving an existing plant compared with planting a new container-grown plant, but midsummer landscape adjustments can be successful. If wary, research the particular species in question to check if it’s sensitive to root disturbance (either in general, or at certain stages of growth).

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University of Maryland Extension’s Home and Garden Information Center offers free gardening and pest information at extension.umd.edu/hgic. Click “Ask Extension” to send questions and photos.


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