Chamberbitter is the weed people describe to us as "the little mimosa" or "that thing with the seeds under the leaves." It shows up in June in beds, turf, gravel and cracks, and it is one of the few weeds that a well-run spring program routinely lets through. Here is why, and how to close the gap.
What it looks like
An upright plant, a few inches to a foot tall, with a single stem and small compound leaves arranged like a feather along thin side branches. The giveaway is on the underside: a row of tiny round green seed capsules lined up beneath each leaflet. Turn a branch over and there they are. It is sometimes called gripeweed or leafflower. Mimosa seedlings, which it resembles, do not have those capsules and have a woody stem.
Why it gets past a spring program
Chamberbitter is a warm-season annual that germinates late, once soil temperatures are well up into the 70s, which is late May or June around Monticello. A spring pre-emergent applied in late February for crabgrass has often worn thin by then. It also keeps germinating through the summer, so one application does not cover the whole season. And each plant sets seed within weeks and drops it continuously, so by August one plant has become fifty. That combination of late start, long season and fast seeding is what makes it feel like it appeared overnight.
The pre-emergent fix
The single most effective control is a pre-emergent timed for chamberbitter's germination rather than crabgrass's. That means a second application in mid to late May, which is the same timing we recommend in our post on the second pre-emergent application. Products with prodiamine, dithiopyr or pendimethalin are labeled for it in turf. In landscape beds, granular pre-emergents labeled for ornamentals work well; apply them after mulching in spring, or apply to the soil and then mulch, and water in.
Isoxaben-based products are labeled specifically for broadleaf pre-emergent control in beds and are worth looking at where chamberbitter is the main problem. Read the label for plant safety and rates.
Post-emergent in turf
Once it is up, small plants are easier than big ones. Three-way broadleaf herbicides with 2,4-D, dicamba and mecoprop are labeled for chamberbitter in Bermuda and Zoysia at full rates. On centipede and St. Augustine those same products need the reduced rate on the label, and even then they can injure the lawn in summer heat; many labels restrict application above 85°F or so. Products containing atrazine are labeled for centipede and St. Augustine and are more effective on chamberbitter, but there are label restrictions on timing and on use near water. Follow them. Our guide to reading a herbicide label for centipede covers what to check.
Expect to spray twice, two to three weeks apart, because new plants keep germinating. Combine it with the pre-emergent or you are just mowing down a wave that never stops.
Post-emergent in beds
In beds, hand pulling is realistic while plants are small. The taproot is shallow and the whole plant comes up. Pull before the seed capsules ripen; a plant pulled in August scatters seed as you pull it. Bag the plants rather than dropping them in the bed. For larger areas, a non-selective herbicide applied carefully between plants works, with a shield to keep it off ornamentals. Then get a pre-emergent down so the bare spot does not refill.
Mulch and density
Chamberbitter needs light to germinate. A two to three inch layer of mulch in beds, kept topped up, stops most of it. In turf, mowing at the top of the height range for your grass shades the soil and cuts germination. Thin lawns and thin mulch are where it wins. Our mulch depth post covers the bed side.
The seed bank problem
Seed from this year's plants germinates next year and the year after. A bad chamberbitter year means two or three years of pressure even with perfect control. Do not judge a program by one season. Judge it by whether each year is lighter than the last.
FAQ
Is chamberbitter harmful to pets or people?
It is not a concern in the lawn. It is a nuisance weed, not a toxic one.
Why is it worse in my mulched beds than the lawn?
Thin or old mulch lets light through, and beds usually do not get the pre-emergent the lawn gets. Top up the mulch and treat the beds too.
Will it die on its own in winter?
Yes. It is an annual and the first frost kills it. The seed it dropped does not die.
Do you treat beds as well as turf?
Bed maintenance and weed control are part of our landscaping service.
If the little mimosas have taken your beds, get in touch and we'll set up a program that catches them before they seed. We handle bed and lawn work around Monticello and out to Greenville.