P&PDL Picture of the Week for
August 13, 2007

Grassy weeds becoming obvious in dormant turf areas

Glenn Hardebeck, Turfgrass Research Agronomist, Department of Agronomy, Purdue University

As the summer heat and drought slow the growth of our cool season grasses such as Kentucky bluegrass, other drought and heat resistant grasses begin to stand out.  Three here include crabgrass, goosegrass and tall fescue.  In most situations, control of these grasses is better left until this fall or next spring depending on the species.  Crabgrass and goosegrass are annual grasses that are primarily controlled with preemergent grass herbicides applied early spring.  Tall fescue is a perennial and therefore can not be removed from a turf stand with a preemergent herbicide.  Control of tall fescue requires multiple applications of a non-selective herbicide such as Roundup or multiple applications of a selective herbicide available to professional applicators.  Additional information can be found at http://www.agry.purdue.edu/turf/pubs/AY-10.pdf    http://www.agry.purdue.edu/turf/pubs/AY-11.pdf

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Green clumps of tall fescue (left) as well as crabgrass and goosegrass (bottom right) are very conspicuous in this dormant Kentucky bluegrass lawn.

Small, round, green stems with swollen joints (nodes) can be seen at the center of this spreading crabgrass plant.  Roots can form at the nodes allowing crabgrass to spread readily.

Crabgrass seedheads are distinctive from that of goosegrass.

Goosegrass stems develop a white, flattened rosette at the plant’s center.

Goosegrass seedheads have two flattened rows of seeds on individual spiklets.

Tall fescue is wider bladed and more drought resistant than Kentucky bluegrass or perennial ryegrass forming clumps in a mixed stand.

Tall fescue is not likely to have a seedhead at this time of year and is often red to purple at the base of the stem.

Purdue Plant & Pest Diagnostic Lab Purdue Cooperative Extension Service