Building an Herbicide Program for Western Washington Potatoes

Contributed by Deniz Inci, Washington State University Mount Vernon NWREC

Potatoes are a major crop of western Washington agriculture grown on ~170,000 acres. Most potatoes grown in Washington are grown in large fields of over 100 acres (there are 640 acres in a square mile). The soil type in Skagit Valley includes silt loam, silty clay loam, and a small amount of very fine sandy loam. Almost all potatoes grown in Washington have to be watered with sprinklers. Cool and wet maritime conditions that make the region so productive for agriculture also favor a persistent and diverse weed flora. Weed interference in potato is costly on more than one front. Early-season competition for light, space, and nitrogen pulls directly against tuber bulking and lowers yield; weeds that persist into the canopy period reduce tuber size and grade, cutting into quality and packout; and heavy late-season growth fouls harvest, plugging diggers and slowing the crew at exactly the moment the operation can least afford it. A weedy field is not simply a lower-yielding field—it is a harder, slower, more expensive field to bring in.

Two features of the crop make the problem worse. First, potatoes are planted into open ground and there is a wide early-season window during which the soil surface is exposed to light and water, and weeds can establish before the crop closes over. Second, once weeds are up, the options for removing them without injuring the potato are limited. The practical consequence is that most of the weed-control battle in potato is won or lost early, on the strength of the soil-applied program applied at or before planting.

The harder, slower-moving problem sits underneath all of this. The tools we have are being asked to do too much, too often. When the same few herbicide modes of action are applied year after year—because they are the ones that work, are affordable, and are on the label—the weeds that survive each pass are the ones best equipped to survive the next one. Over enough seasons, that selection pressure shifts a population toward resistance. Across the region, several troublesome species very common and continued reliance on any single site of action raises the risk of resistance and, eventually, of outright control failures. An herbicide program, in other words, is not just a spray decision for this season. It is a resistance-management decision for the next several—and the programs that look cheapest and simplest today are often the ones that quietly narrow our options for tomorrow.

The weeds we are targeting to control

Three groups of weeds complicate western Washington potato production, and each fails a simple program in its own way.

Yellow nutsedge

Yellow nutsedge (Cyperus esculentus) is a sedge that regenerates from underground tubers as well as seed, so a single season’s escaped seed becomes the next season’s infestation from below ground where cultivation and many herbicides do not reach. It emerges over a long window, thrives in the moist soils common to the maritime valleys, and is notoriously difficult to control with the residual chemistries that handle broadleaves and annual grasses. Nutsedge is often the species that decides whether a program is truly clean or merely looks clean from the road.

Pigweeds

Pigweeds (Amaranthus species, including redroot and waterhemp) are prolific seed producers—a single large plant can set enormous numbers of seeds—and they germinate in repeated flushes through the warm part of the season. That combination of a huge seedbank and staggered emergence is exactly what erodes a one-shot program: a residual that controls the first flush may be running thin by the time the second and third emergence arrives. Pigweeds are also the group highly associated with herbicide resistance nationally, which makes mode-of-action diversity against them a stewardship priority, not just a control question.

Nightshades

Nightshades (Solanum species, such as hairy and black nightshade) are especially unwelcome in potato because they are close relatives of the crop, sharing similar insect and disease pests, and produce berries whose size and moisture can interfere with harvest and grading. They emerge alongside the crop, tolerate some of the chemistries used in potato, and are a recurring source of escapes late in the season.

Controlling all three with one product is not realistic. That is the central reason the programs in a new trial conducted by Washington State University are built as combinations rather than single active ingredients.

The herbicide trial

This spring, we established a two-year field trial (2026–2027) at the WSU Mount Vernon Northwestern Washington Research and Extension Center to answer a practical question: how do we achieve season-long weed control and good crop safety while maintaining sufficient diversity of herbicide chemistries to prevent resistance? We are evaluating 16 weed-control programs—14 herbicide treatments plus a weedy check and a weed-free check—under commercially relevant conditions.

The study is a split-plot randomized complete block design with three replications. Plots are two rows by 15 feet, with 5-foot buffers before and after each plot. Most programs are built as two-component, soil-applied treatments (pre-incorporated or pre-emergence), and the split-plot factor is incorporation depth—3 inches versus 6 inches—to see whether placing the same chemistry shallower or deeper affects control or crop response. A rimsulfuron + S-metolachlor program is also applied post-emergence at both depths, letting us compare a foliar-timed treatment against the same and other soil-applied combinations.

Why incorporation depth? Soil-applied herbicides must sit in the zone where weed seeds germinate to kill emerging weeds, and how deeply a product is worked into the soil changes both where it ends up and how much reaches the crop. Incorporate too shallow and the herbicide band can dry out, break down in sunlight, or wash off the surface before it is activated; incorporate too deep, and it can be diluted through more soil, moved into the potato’s own root zone, or moved too deep from the shallow-germinating weeds. Depth is also a crop-safety lever: keeping residual concentrations near the surface, above developing tubers and roots, is one-way potatoes tolerate chemistries that would otherwise injure them. By running the same six chemistries at both 3 inches and 6 inches, the trial can separate “does this product work?” from “does it work at the depth a grower can realistically achieve with standard equipment?”—a distinction that ordinary efficacy trials often blur.

The programs deliberately draw on four different herbicide sites of action—WSSA Groups 2, 3, 8, 14, and 15 (Table 1)—combined in different pairings so that no program leans on a single mode of action. That mode-of-action diversity is the heart of the resistance-stewardship goal: a two-way combination that pairs, say, a Group 15 residual with a Group 2 or Group 3 partner gives a weed two independent hurdles to clear rather than one. A weed that happens to tolerate one of the two components is still caught by the other, which both improves the odds of clean control this season and, more importantly, keeps any single resistance trait from sweeping through the population. It is the same logic that underlies the rotation and mixing of chemistries in every well-run resistance-management program, applied here systematically across sixteen combinations.

This research was conducted under Washington State University’s Experimental Use Permit with the Washington State Department of Agriculture (WSDA). Some pesticide uses described in this article do not conform to current product labels and are not use recommendations. Always follow the pesticide label.

The chemistries evaluated in the trial

The trial builds its programs from six active ingredients spanning the four groups (Table 1). They are all registered for potatoes. Understanding what each one does explains why they are paired the way they are:

• EPTC (Group 15, a lipid-synthesis inhibitor) is a soil-incorporated carbamothioate with a long history in potato. It is volatile and must be worked into the soil promptly, which is one reason why incorporation depth remains a live question for programs that include it. It brings activity on yellow nutsedge and several annual grasses and broadleaves—notably the nutsedge activity that most other components in the trial lack.
• Pendimethalin (Group 3, a microtubule inhibitor) is a dinitroaniline that stops susceptible seedlings as they germinate, with strength on annual grasses and small-seeded broadleaves. It is a foundation residual, contributing a distinct mode of action to pair against the ALS and VLCFA chemistries.
• S-metolachlor (Group 15, a very-long-chain fatty-acid inhibitor) is a chloroacetamide residual that is a mainstay against pigweeds and annual grasses and contributes useful nutsedge suppression. It anchors several of the two-way combinations and is the residual partner in the post-emergence program.
• Rimsulfuron (Group 2, an ALS inhibitor) is a sulfonylurea with both soil and foliar activity, active on a broad range of broadleaves and some grasses at very low use rates. Because Group 2 is one of the modes of action most prone to resistance, it is deliberately never used alone here—always paired with a partner from a different group.
• Pyroxasulfone (Group 15, a very-long-chain fatty-acid inhibitor) is a newer, highly active residual that extends the length of control against pigweeds and grasses, offering a longer-lasting option within the same broad mode-of-action family as S-metolachlor.
• Carfentrazone-ethyl (Group 14, a PPO inhibitor) is a fast-acting contact chemistry that adds burndown of emerged broadleaves and a fifth distinct site of action to the program, pairing it with pyroxasulfone.

Table 1. Treatment List

WSSA Groups: 2 = ALS inhibitors; 3 = microtubule inhibitors; 14 = PPO inhibitors; 15 = very-long-chain fatty-acid (VLCFA) inhibitors. These research treatments were conducted under Washington State University’s Experimental Use Permit with the WSDA. Some treatments do not conform to current product labels and are not use recommendations.

What comes next?

This is the first season of a two-year study, so the results table is still ahead of us—and running the trial across three years is itself part of the design. A single season can be unusually wet, dry, warm, or weedy, and a program that shines in one year may stumble in the next. Repeating the trial through 2027 lets us separate programs that are genuinely robust from those that simply caught a favorable season, which is the only honest basis for a recommendation a grower can rely on.

As the ratings come in, we will report which programs held up across the full 7-to-42-DAT window, whether incorporating at 3 inches versus 6 inches changed control or crop safety, and how the post-emergence rimsulfuron + S-metolachlor program compared with the soil-applied combinations. We will be watching especially closely for treatments that hit three marks at once: strong and durable control of the difficult species, clean crop safety, and genuine mode-of-action diversity. Any two of those are relatively easy to find; the value of this trial is in identifying the programs that deliver all three.

It is worth saying plainly that no herbicide program, however well built, is a complete answer on its own. Chemical control works best as one layer within an integrated approach—sound crop rotation to break weed cycles and vary selection pressure, well-timed cultivation, attention to field history and the weeds a field is known to carry, and prevention of seed and tuber set by escapes that do get through. Rotating and mixing herbicide modes of action, as this trial does, buys time and preserves the chemistries we have; it does not replace the agronomy around them. Check back as the 2026 data come in, and we will let the numbers tell the story.