Weed Grass Identification: Crabgrass, Goosegrass, Poa

Before naming a lawn weed, settle whether it belongs to the grass family at all. Penn State Extension's test for yellow nutsedge is physical: the stems are angular and three-sided, which you can feel by rotating a stem between your fingers, and a knife cross-section shows the triangle. True grasses have round or flattened stems.

That distinction is not academic. Penn State notes nutsedge foliage is yellow-green, coarse and shiny, that leaves come in a three-part arrangement, and that it grows fast in the summer heat exactly when cool-season turf slows down — which is why it seems to appear overnight in July. Because sedges are a different family, herbicides aimed at grasses do not control it.

So: feel the stem first. Three sides, and you are dealing with a sedge. Round, and continue into the grass key below.

These two get blamed for each other constantly. Clemson Cooperative Extension separates them cleanly.

Crabgrass has a tufted or prostrate growth habit, hairy stems, broad leaves, and flower spikes with two to nine finger-like branches. It is a summer annual that germinates in spring about the time crabapple and forsythia bloom, seeds from midsummer to fall, and is killed by the first freeze.

Goosegrass shows white to silver coloring near its base — the single easiest field mark — with flat stems, a clump-forming habit that does not root at the lower nodes, and seeds produced in two rows like a zipper on two to 13 finger-like branches. Clemson notes it germinates a few weeks after crabgrass, in late spring.

So: hairy stems and a green base, crabgrass. Flat stems and a white-silver base, goosegrass.

Annual bluegrass (Poa annua) runs on the opposite calendar from crabgrass, which is why treating for one at the wrong time does nothing for the other.

Clemson describes smooth, apple-green leaves with two clear lines, one on each side of the midrib, and leaf tips whose edges curve inward like the front of a boat. The seedhead is white and pyramid-shaped and appears in spring, often low enough to survive mowing — which is how a lawn goes pale and speckled in April.

Clemson gives germination in the Coastal Plain as September 15 to October 1; further north it comes later in the fall. The plant persists through winter, seeds in early spring and dies in late spring or early summer as the heat arrives.

If you are seeing a light-green patch in April that was not there in August, this is the first candidate.

Grasses are identified from small structures, so a photograph of a green patch will not do it. Pull one whole plant, roots and all, and shoot:

• The seedhead, if there is one, against a plain background — this is the single most informative part
• The base of the plant where the stems meet the crown, to show color and whether stems are flat or round
• The collar region where the leaf blade meets the sheath, close up
• The whole plant laid out with a ruler or a coin

Note the season as well: Clemson's germination timings mean that what you found in April and what you found in July are usually different species even if they look alike in a photo.

Photograph it and the app identifies it. For control decisions — which product, which week — take the identification to your county extension office, because the answer varies by turf type and state.