What Now?

Published online: Jun 14, 2021 Articles, Herbicide Pamela J.S. Hutchinson, UI potato cropping systems
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This article appears in the June 2021 issue of Potato Grower.

Annual grasses, such as wild oat (Avena fatua L.) and foxtail (Setaria spp.), can be a problem in potatoes. Many herbicides applied before potatoes and grasses emerge can provide good grass control. Do not rush to judgment on those herbicides, however, if grasses become a problem after potatoes have emerged. 

Deep-Seeded Issues

Wild oat germination tends to drop off once cool spring/early summer temperatures end. So, the appropriate pre-emergence herbicide tank mixture should be all that is needed, right? Most weeds in potatoes typically germinate in the top 2 inches of soil. Pre-emergence herbicides with soil activity are incorporated to that depth, and depending upon mode of action, weed germination is inhibited and/or seedlings are killed before emergence. Wild oat seed size, however, is relatively large, resulting in reserves great enough for emergence from depths down to 6 inches. Cool temperatures at that depth combined with herbicides not being in the right place at the right time leads to wild oat emerging in the summer, even though soil temperatures at and near the soil surface are too hot for the weed seed to handle.

Worse Late than Never

Green foxtail can germinate throughout the summer, so by the time soil residual herbicide concentrations are winding down, a “late flush” could occur. Residual herbicides need to last long enough to control late weeds, but not so long that it interferes with crops following application in potatoes.

Field sandbur (Cenchrus spinifex) can also germinate into late summer. Ditto for barnyardgrass (Echinochloa crus-galli [L.] Beauv.). Even though it is susceptible to shading by crops and the late-germinating plants are not as big and vigorous as those that germinate early, post-emergence barnyardgrass control might be necessary.

Now What?

There are only four potato herbicides with foliar activity on grasses: rimsulfuron (Matrix, Solida and others), metribuzin, clethodim (Select and others), and sethoxydim (Poast, Poast Plus and others). Rimsulfuron and metribuzin can perform satisfactorily, but they have follow crop restrictions (notably sugarbeet), especially with a late-season application. In addition, post-harvest interval (PHI) must not be less than 60 days for either.

“Dims” Outlook

Clethodim and sethoxydim, lovingly called “dims” by weed scientists — which have the same mode of action as “fops” such as fluazifop-p-butyl — can be very effective for controlling emerged annual grassy weeds in potatoes. These herbicides only control grasses and have no soil activity. Uptake is primarily through the leaves, and complete coverage is crucial. They can kill by contact and also by moving into and though the plant. The most effective control is achieved with application prior to tillering when grasses are small (between two- and six-leaf stage). Application should be made to actively growing grasses, not ones under stress. As such and where possible, irrigation might be needed beforehand (within one week).

Green, Green Grass of Home

Time for complete control is normally one to three weeks. As such, do not be disappointed if the plants remain green during that time. They are actually “dying green.” Check it out by pulling out the newest leaf. The base will be brown-dead. Having said that, a second application is sometimes needed during this time if the brown is not evident, especially in arid conditions or if grasses are larger than the six-leaf stage at time of application.

Do Not Antagonize

Clethodim and sethoxydim’s effectiveness is much lower if a broadleaf herbicide, such as rimsulfuron, is tank-mixed with or applied within one day of applying these grass herbicides. Referred to as antagonism — when the control of two or more herbicides  combined is less than the expected effect of each herbicide applied separately— rimsulfuron could be causing a reduction in photosynthesis and growth rate of the grass weeds. The result is a reduction of the herbicidal activity of the grass weeds. Therefore, if both herbicides are needed for post-emergence weed control, do not tank-mix, and be sure to make the applications more than one day apart.

Always use surfactants with any of these herbicides post-emergence. Specifics on herbicide rates and surfactants can be found on the herbicide labels. Always read and follow label instructions.

Resist Resistance

When using herbicides as a component of the integrated management necessary to prevent or delay the development of herbicide-resistant weed populations, tank-mix herbicides with different modes of action and make sure that more than one mode of action has activity on the same weed. If not, after repeating the mistake over the years, the “one in a million” plant in the population naturally resistant to a mode of action survives, produces seed, and eventually dominates the population.

Of note, wild oat populations resistant to the “fops” have been found in the Pacific Northwest. These have not, however, exhibited resistance to clethodim or sethoxydim.

By the way, post-emergence rimsulfuron, clethodim and sethoxydim can control perennial quackgrass when applied before the four-leaf growth stage. Conditions must be conducive, and a second application may be necessary. See the labels for further information.


Pamela J.S. Hutchinson is a weed scientist and extension specialist of potato cropping systems based at the University of Idaho’s Aberdeen Research & Extension Center. She can be contacted at phutch@uidaho.edu.