Wild violets are plentiful in my lawn even though a company sprayed several times. I also have weeds that look like clover but have tiny yellow flowers. What kills these weeds?

Wild violets are not all bad. They feed wildlife and the flowers make pretty violet jelly. They are admittedly tough plants. Fall is a great time — and sometimes the only effective time — to kill some hard-to-manage weeds. Your clover-type weed is oxalis. Look at life cycle and control options for oxalis and wild violet in the HGIC website’s weed gallery. They are both under the category of broad-leaved perennial weeds. For wild violets, follow the link to "Broad-leaved Weed Control in Established Lawns" for specific chemical recommendations. Apply twice, a few weeks apart, starting in late August-early September. A shady thin acidic lawn encourages wild violets. Fall is the best time to seed and fertilize lawns. Do a soil test now to determine pH and get fertilizer recommendations. Thickening your turf prevents many weed problems from ever occurring. Thick turf shade prevents weed seed germination and weed seedlings are out-competed.

We lost a small flowering tree, apparently to red-headed ash borer. We saw hundreds of reddish insects crawling on it. Now all the leaves on our purple plum tree are dried up and brittle. Is this the work of the same insect? Can you recommend tree specialists to come and evaluate all our trees?

The redheaded ash borer is a secondary pest, meaning it goes into stressed, weakened trees and isn’t actually the cause of death. They don’t prefer plums. Most likely your purple plum has been in decline from other issues (e.g. drought stress, poor soil conditions, and/or physical injury). Purple plums tend to have fungal leaf disease and be problematic here in the Mid-Atlantic. For on-site tree evaluations, we always recommend arborists certified by the International Society of Arboriculture. They are usually associated with tree service companies. To find ones near you, follow the prompts on www.isa-arbor.com.

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