Poison hemlock

Poison hemlock continues to spread in Idaho.

Pioneer Irrigation District Superintendent Mark Zirschky said his crew kept plenty busy controlling weeds this year as the annual war against invasives has continued.

“Poison hemlock seems like it’s always an issue, and we are seeing quite a bit of Purple loosestrife,” the Caldwell, Idaho-based Zirschky said. The latter “is heavy along canals and is hard to take care of, hard to kill. We’ve noticed a fair amount of that this year.”

Weeds, including noxious invasives, have been plentiful in much of the state in 2020 due to a mild winter, rainy late spring and periods of above-normal summer temperatures. Weeds in overabundance can waste or damage resources while creating additional work for landowners.

Roger Batt, coordinator for the Idaho Weed Awareness Campaign, now in its 20th year, said weeds have been plentiful this year. “It appears that between about the end of May and the first part of July, we saw a tremendous amount of weed pressure. That continued through July.”

Some weed seeds survived the winter due to the warmer weather, which also prompted early emergence, he said.

Emergence “then just proliferated in the wet spring” from May into June," Batt said. “You would go out and spray, and then a big storm would hit and weeds would pop right up again.” Idaho has seen above-average weed pressure this year.

He said local experts recommend landowners “get on top of weeds early in the season” before they are voluminous and tall, and control them after irrigation water stops flowing. County weed departments and chemical-application providers can assist.

Applying a small amount of herbicide to biannual Poison Hemlock in fall can pay dividends if label instructions are followed, Batt said. “Typically where you have an old Poison Hemlock, you will have a new plant underneath.”

“I saw more Poison Hemlock this year than I have seen in a long time,” he said. “Not that people are not doing their jobs. It’s really gaining a foothold across Idaho.” The noxious weed is common near water.

“Because it is prevalent this year, we will need to step up efforts in the fall to help stop new plant growth, and then next year step up as well to help stop the spread of this noxious weed,” Batt said.

Canyon County Weed & Pest Control Director A.J. Mondor said 2020 noxious-weed control requests have been around the 10-year average, and approximately three-quarters of the total of 2019 — a high-volume year due to the early wet spring, late wet fall and fewer frost days overall.

Poison Hemlock was lower than during 2019 but roughly on par with the 10-year average, he said. On the non-noxious side, Kochia was up from 2019 and Miner’s Lettuce was up substantially.

Zirschky of the Pioneer District said Kochia exemplifies recently greater plant resistance to chemical treatment.

A popular herbicide’s label now recommends application no less than 200 feet from a residential area, “so we are finding we are able to use less and less as the residential areas populate along the canals,” he said. “We have had to go in with other means of weed control — primarily mechanical, so that has been a bit of a challenge.”

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