Dallisgrass and Virginia Buttonweed in St. Augustine
Two stubborn weeds general weed killers miss. How to identify dallisgrass and Virginia buttonweed in St. Augustine and the approaches that actually work.
The short version
- Dallisgrass is a clumping grassy weed. There's no safe selective spray for it in St. Augustine, so you spot-kill the clumps and replug.
- Virginia buttonweed is a spreading broadleaf weed. It takes several applications of the right product, starting in spring.
- Both love wet, poorly drained spots.
Dallisgrass
How to identify it
- Coarse, gray-green clumps that grow faster than the lawn and spread outward in a circle.
- Tall seed stalks with several flat spikes along the stem, dotted with tiny black flowers.
- Short, thick underground stems that make each clump tough to pull.
What works
According to LSU AgCenter, the selective sprays used on dallisgrass in other lawns are not safe on St. Augustine. That leaves:
- Spot-kill each clump with glyphosate, painted or sprayed only on the dallisgrass. It kills St. Augustine too, so be precise.
- Dig out small clumps, getting the whole crown.
- Replug the holes once it's dead. See how to plug bare spots.
- For a badly infested area, kill everything and re-sod, waiting at least two weeks after spraying.
- Prevent new seedlings with a pre-emergent labeled for St. Augustine.
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Virginia buttonweed
How to identify it
- Low, spreading plant with opposite, lance-shaped leaves on long, trailing stems.
- Small white, four-petaled, star-shaped flowers in summer.
- Often in wet, low areas of the lawn. It can regrow from roots and stem pieces, so mowing spreads it.
What works
LSU AgCenter recommends a program of several applications. A single late-season spray rarely works on mature plants.
- Spring (April–May): spot-spray as it comes out of dormancy with a three-way broadleaf product (2,4-D, dicamba, and mecoprop) labeled for St. Augustine.
- Once it's above about 85°F: stop using 2,4-D products, which can injure St. Augustine in heat.
- Summer: switch to a product like Celsius WG or metsulfuron, and repeat about 4 to 6 weeks later.
Pre-emergent does not control Virginia buttonweed. Fixing drainage and watering less often does help.
For both: fix the wet spots
Both weeds thrive where the soil stays wet. Check your sprinkler run times with the tuna can test, core aerate compacted areas, and keep St. Augustine thick and tall so it crowds them out.
Everything in my St. Augustine program
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