Crabgrass showing up in a St. Augustine lawn is one of those problems that catches homeowners off guard because the two plants look similar enough in the early stages to create genuine confusion. You might dismiss it as just a slightly different patch of your turf, give it a few more weeks, and then realize you have a crabgrass problem that's matured and set seed. Knowing what to look for, and when, changes the outcome significantly.
Quick answer
Young crabgrass and St. Augustine can look similar to an untrained eye, but there are clear differences. St. Augustine has wide, flat blades with a boat-shaped tip and spreads via above-ground stolons with nodes spaced several inches apart. Crabgrass has narrower leaves, a pointed tip, and grows in a star-burst pattern radiating outward from a central point close to the ground. Crabgrass also feels rougher to the touch and its blades often have fine hairs. Once you know what to look for, you can tell them apart in seconds.
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What Crabgrass Looks Like
Crabgrass (Digitaria spp.) is a warm-season annual weed that germinates in spring when soil temperatures consistently reach 55 to 60°F, which in San Antonio typically happens between late February and April. It grows as a low-spreading plant with a distinctive star-burst or crab-like pattern radiating outward from a central point. The blades are narrower than St. Augustine, roughly one-quarter to one-third of an inch wide, with a pointed tip and a slightly rough, hairy texture.
As it matures through summer, crabgrass produces distinctive seed heads that look like fingers radiating from a central stalk, which is one of its most recognizable features. The plant grows flat and low early in the season, which is why it often goes unnoticed until it starts to set seed. By August, a single crabgrass plant can produce 150,000 seeds.
What St. Augustine Looks Like
St. Augustine (Stenotaphrum secundatum) has wide, flat blades with a distinctly rounded or boat-shaped tip, not pointed. The blades are typically half an inch or wider and have a smooth surface without hairs. St. Augustine spreads via thick above-ground stolons (runners) with visible nodes spaced four to six inches apart, and new blades emerge from these nodes.
When young St. Augustine growth is filling in a bare spot, the new blades come up uniformly from the stolons in an organized pattern that follows the direction of the runner. Crabgrass, by contrast, sprouts from a seed with no runner attached and grows outward in a circular pattern.
Side-by-Side Differences
Blade width is the fastest check: St. Augustine blades are wide and flat; crabgrass blades are noticeably narrower. Tip shape is the second: St. Augustine has a rounded or notched tip; crabgrass has a sharp point. Texture is the third: run your fingers along the blade surface and St. Augustine feels smooth while crabgrass feels slightly rough or hairy.
Growth pattern is the most obvious at a distance: St. Augustine spreads along runners in a single direction; crabgrass radiates in a star pattern from a central point. If you're looking at something in your lawn and it has that outward-radiating, low-lying pattern with no visible runner connecting it to the rest of the grass, it's almost certainly crabgrass or another grassy weed, not St. Augustine.
- St. Augustine: wide blade, rounded tip, smooth, spreads via stolons
- Crabgrass: narrow blade, pointed tip, slightly rough/hairy, star-burst growth
- St. Augustine: organized, directional spread from runners
- Crabgrass: radiating outward from central point, no runner
- Crabgrass: seed heads like fingers on a hand in late summer
How Crabgrass Gets Into a St. Augustine Lawn
Crabgrass seeds blow in from neighboring properties and road edges, and a substantial seed bank usually already exists in the soil of established San Antonio lawns. Seeds germinate when soil temperatures warm in spring and do particularly well in thin or bare spots where there's no grass canopy competing with them.
A dense, well-maintained St. Augustine lawn is the most effective long-term crabgrass deterrent because the thick canopy shades the soil and prevents crabgrass seeds from getting the sunlight they need to germinate. Thin lawns, overwatered lawns (which create perpetually wet conditions crabgrass loves), and areas around sprinkler heads and edges are where crabgrass typically gets its start.
What to Do About Crabgrass
The most effective tool against crabgrass is a pre-emergent herbicide applied before soil temperatures hit 55°F in spring, which typically means late February to early March in San Antonio. Pre-emergent creates a chemical barrier in the soil that prevents germination. Once crabgrass has emerged, pre-emergent does nothing to it, and you need a post-emergent product labeled for use on St. Augustine (check the label carefully, since some crabgrass killers will damage St. Augustine).
Small patches can also be physically removed before they set seed. If you pull crabgrass out in late spring before August, you prevent it from adding to the soil seed bank. Dispose of it rather than leaving it on the lawn, since even pulled plants can drop seeds.