My mystery weed has been identified. While browsing Taylor’s Encyclopedia of Garden Plants, I came across the genus Robinia, which is made up of Locust trees. The plant that has been sprouting in the middle of my lawn is Robinia hispida, bristly locust, sometimes called by the more inviting name of rose acacia.
Apparently some people intentionally plant these thorny buggers. They actually aren’t quite as thorny as they look- the red bristles on the stems are more like fuzz than thorns, but there are thorns hiding near the base of the leaves. I expect that the former owners of my house didn’t plant it in the middle of the lawn- thorny locust is known to aggressively spread by suckering. I’ve been told that some nurseries solve this problem by grafting thorny locust onto Robinia pseudoacacia, black locust, which is a less aggressive plant.
Thorny locust gets pretty, sweet-pea like flowers in spring, which may be the feature that entices people to buy this overly aggressive plant. Robinia pseudoacacia ‘Frisia’, golden acacia, a close cousin of bristly locust, sounds like a prettier and more controllable plant, though I’d still watch out for suckers. Yellow-leaved plants can be less robust than their green-leaved counterparts, though, so it may be a trade-off between disease resistance and pretty foliage.
