This year, I have a patch of weeds that won’t go away. I killed them with weed killer (and the grass) and repeated when they recurred. I then re-seeded the grass and pulled any new weeds that sprouted and, a week later, they completely took over again. Enclosed is a picture of this demon plant. Any idea what they are? More importantly, how do I kill them?
– Scott Honda, Sacramento
Your demon plant most likely is cheeseweed or little mallow ( Malva parviflora), a very pesky and determined garden invader. This annual is common throughout California. If left to grow unchallenged, it will reach 5 feet tall. Its close cousin, common mallow ( Malva neglecta), grows closer to the ground and tends to grow out instead of up.
It gets its cheesy nickname due to the shape of its seeds, which resemble little wheels of cheese. The seed can lie dormant in the soil for several seasons before sprouting. The plant is a relative of several attractive garden mallows (a favorite of beneficial insects). But cheeseweed can be a real pest.
The reason it’s so hard to kill is its long and woody taproot. That long root helps it survive in times of drought, but also makes it difficult to eradicate. It’s hard to pull out the whole thing and the plant – as you’ve documented – re-sprouts easily from the remaining crown.
According to the University of California’s integrated pest management program, no herbicides are approved for home use that are effective against this weed, so save your money.
Instead, take a hoe and whack these plants down to below the ground’s surface. Then, smother them with organic mulch (such as wood chips or bark) at least 3 inches deep. That will keep them from re-sprouting.
Because your cheeseweed has invaded your lawn, you may be limited to just pulling the plants out instead of mulching. Attack them when they’re young and tender. Get as much of the root out as possible. A cultivator tool that cuts roots below the surface also can be useful.
You can learn more about cheeseweed on the UC’s IPM website, www.ipm.ucdavis.edu; look for “Pest Note 74127: Mallow.”
More on mystery tree
Reader Matt Toland was curious about a white-flowered tree he saw in Washington, D.C. Possible suggestions for its identity offered in a recent Garden Detective column included the honey locust.
Jim Rosenfield, an associate with the campus arboretum at California State University, Sacramento, is pretty sure the mystery tree is a black locust, not honey.
“Honey locust is a good shade tree for Sacramento but does not have white, fragrant flowers,” Rosenfield said. “They are inconspicuous, greenish and tiny in small clusters at the base of the leaves. If your reader likes honey locust, local nurseries sell a fine thornless selection, ‘Sunburst,’ whose new leaves are yellow through the summer and has nice fall color.
“I believe the tree in the picture may well be black locust ( Robinia pseudoacacia), which has the composite leaves and a very nice profusion in late April of white, fragrant flowers, which up close look like miniature wisteria,” he added. “There are several in the community.”
Black locust are native to the Southeast (and grow great in Washington, D.C.), but have been widely planted throughout the United States.
UC Cooperative Extension master gardener Fran Clarke had another suggestion: Cladrastis kentukea, also known as yellowwood.
“There used to be one at the CSUS Arboretum, but it fell victim to breakage from uncorrected narrow weak branch crotches,” Clarke said. “I never saw that particular specimen bloom spectacularly, but I was privileged see a spectacular one at a private home near Placerville.”