A new weed

Cardamine oligosperma

New to us in the northeast, anyway. This is Cardamine oligosperma, or little western bittercress. It’s a member of the mustard family, as is quite clear from its four-petaled (cruciform) flowers. It’s native to western North America but not to the east. The plant is all of 2 inches high, and I first noticed it two years ago, when, after a very mild winter, it was suddenly everywhere and blooming with all its might in January. Last spring, after it finished blooming and turned an ugly dusty brown, people kept asking me how to control it in their lawns, but that was the wrong time to do anything about it, as you’ll learn if you read on below. This year it’s in bloom right now. It likes wet places (like well-watered lawns).

The key to controlling this or any other weed is to understand its life cycle. This particular plant is a winter annual, meaning its seeds germinate in summer or fall, and the seedlings remain dormant throughout the winter and bloom in early spring. Like many other mustards, this one sets seed very quickly (the straight lines at the top of the plant are the seedpods), so to prevent a new generation from coming back to plague you next spring, you need to pull it now, today, before it produces seed. Notice in the photo above that the plant has both mature seedpods and flowers at the same time. Pull it up the second you see those white flowers begin to open. Because it’s an annual, it’s very easy to remove.

The other control technique you can use is to apply a pre-emergent herbicide in the fall before the seeds germinate. As with any control plan, timing is key. A pre-emergent won’t work if the seeds have already germinated; pulling the plants out won’t work it they’ve already scattered their seeds (more about that next month when the garlic mustard is in bloom). And a pre-emergent won’t work on other weeds that aren’t winter annuals or that don’t set seed at the same time. So in this as in all cases, you need to weigh carefully the cost versus the benefit of using potentially hazardous chemicals. And remember, this plant likes moist areas. Water your lawn less (your lawn doesn’t need all that water anyway), and you’ll have fewer weeds.

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