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Sunday, July 8, 2018

Be Careful with Crabgrass Pre-Emergent Herbicides

Tony Koski, Extension Turf Specialist


Pre-emergent herbicide injury on a Kentucky bluegrass
home lawn. The herbicide was on a fertilizer carrier, thus
the darker appearance where a higher-than-recommended
rate was applied.
Among the mostly widely used lawn weed control products, pre-emergent herbicides (aka “crabgrass preventers") are generally very safe to use on most turf species. Safe, that is, if applied at the correct rate. Using a spreader without adjusting its setting to apply the correct rate, applying the product with a spreader that isn’t operating properly (plugged, skipping, etc.), applying with excessive overlap, or thinking that “more is better” can cause unintended damage to the desirable turf. In the past few weeks we've seen a greater than normal number of pre-emergent herbicide "oops" on home lawns.

New root growth suppressed by a 
misapplication of a pre-emergent herbicide,
displaying the symptomatic root clubbing.
Pre-emergent herbicides “prevent” annual weeds like crabgrass and foxtail from becoming a problem by killing very young, germinating seeds and seedlings; they don’t sterilize the soil, and they don’t kill seeds in the soil. These herbicides kill the young crabgrass, foxtail, and barnyardgrass seedling by stopping root formation – so the tiny plant can’t take up water and dies from drought stress. The crabgrass preventers used most often on home lawns (by lawn care companies, as well as in do-it-yourself products sold to the homeowner) include pendimethalin (Scotts crabgrass prevention products), prodiamine (sold as Barricade; also in Ferti-Lome, Monterey, Bonide, and Pennington home lawn care products), and dithiopyr (sold as Dimension; also in some Ferti-Lome, Hi-Yield, Bonide, and Pennington homeowner products).

Note the root clubbing (red arrows)
on shallower roots: deeper
roots (green oval) are more normal
in appearance because they are
growing beneath the pre-emergent
herbicide layer.
Misapplication due to poor technique (excessive overlap), using a faulty spreader, and intentional or unintentional application at excessive rates can damage desirable turf by stopping or stunting root production in the spring – when these products are typically applied AND when the grass plant is forming its root system. Damaged roots can’t take up water effectively, resulting in turf that is very drought sensitive. Severely damaged plants die – often in spots in the lawn where there was excessive overlap (as in the photos) – leaving brown strips or spots.

Diagnosing this injury can be tricky. Plants on the border of the dead turf will often display stunted root systems – with roots showing little or no branching and a distinctive “clubbing” at the end of the stunted roots. Because pre-emergent herbicides aren’t very water soluble and tend to remain near the surface of the lawn, deeper roots (maybe only an inch or so deeper; see photo) may grow more normally and not display the clubbing seen on roots growing near the surface. Above-ground, the turf may appear dead or severely thinned in stripes that follow application/overlap patterns. As the turf thins, new leaves formed by the surviving grass plants will be wider/more coarse due to the lack of crowding by adjacent plants (the more dense a turf is, the finer the leaf blades will be). The herbicide-stressed turf may also be more susceptible to diseases like leaf spot and dollar spot.

More subtle above-ground symptoms of
pre-emergent herbicide injury on a Kentucky
bluegrass lawn.
Pre-emergent herbicides can last a long time in the soil (4-6 months) – a good thing for providing season-long weed control. That long residual is a bad thing, however, when a pre-emergent causes turf death from over-application – and the residual effect can be even longer because of the very high rates of application that have killed the grass. This makes it difficult to repair areas by overseeding into them – because the herbicide will kill the young seedlings as they germinate. Creeping grasses (bluegrass, the more rhizomatous tall fescue cultivars, bermudagrass) will grow into the killed areas slowly over the summer. Repair by overseeding may have to wait until fall, when the herbicide has been degraded by soil microbes and other degradation forces in the soil. Worth trying, however, is core cultivation/aeration of the affected areas, followed by overseeding with perennial ryegrass. The seed will germinate below the herbicide “layer” on the surface and has a good chance of surviving and masking the damaged turf.

The take-home message? All herbicides have the potential to harm non-target turf, so always follow the label instructions and apply the product using good technique and well-maintained, properly-calibrated equipment.

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