Contributed by Doug Finkelnburg, University of Idaho
Back in January, we discussed the potential weed management implications of an unusually warm winter across northern Idaho and the Inland Pacific Northwest. At the time, volunteer canola was blooming near Lewiston in late January, annual weeds were actively growing, and many of us wondered what the coming spring would reveal.
Now that the 2026 spring herbicide season is largely behind us, we have a better idea of how those conditions affected weed control across the region.
To get an idea of how things played out, I reached out to growers and weed management professionals across northern Idaho to compare notes from the season. While experiences varied by location, cropping system, and management approach, several common themes emerged. The mild winter appeared to favor weed survival and growth, created opportunities for additional weed emergence after fall herbicide applications, and in many cases highlighted the value of residual herbicides for managing annual grasses.
The winter of 2025–2026 was characterized by extended periods of above-normal temperatures, limited snow cover at lower elevations, and relatively few prolonged freezing events. Reports from across the Northwest indicated temperatures running several degrees above normal for much of November through January, resulting in an “open winter” that allowed plants to continue growing when they would normally be dormant. NOAA data also show that much of the western United States experienced exceptionally warm winter conditions, continuing a broader trend of above-average temperatures during the period.
Not surprisingly, many growers observed greater weed pressure than normal this spring.
A recurring theme from producers and crop advisors was that annual grass weeds benefited from the mild conditions. A field representative working throughout the Camas Prairie reported substantial pressure from overwintered wild oats and Italian ryegrass, noting that weeds survived and continued developing through much of the winter. Similar observations came from University of Idaho research farms, where grassy weeds and volunteer cereals were noticeably worse than in recent years.
Several growers pointed out that the warm winter also created opportunities for new weed flushes after fall herbicide applications had already been made. In years with prolonged cold temperatures, germination and emergence are often slowed or halted for extended periods. This season, however, weeds continued emerging well after many producers considered their fall weed control programs complete. The result was larger weeds, more advanced growth stages, and in some cases narrower application windows during the spring.
As one Camas Prairie producer summarized, “Don’t look at the calendar to spray your weeds. Stay on top of it so they don’t get too big in an open winter like what we had.”
One of the more interesting observations from the season involved fall-applied Group 15 herbicides. While results were not universal, several individuals reported that fields receiving residual Group 15 products generally experienced less spring grass pressure. A Camas Prairie fieldman noted that growers who included Group 15 herbicides in their fall programs “have seen dividends in the spring grass pressure.”
Not every operation experienced the same outcome. One producer near Genesee reported that Group 15 herbicides appeared less effective than the previous year and still required spot treatments for grassy weed escapes. Nevertheless, the overall pattern suggests that residual herbicides provided valuable insurance where winter conditions favored continued weed emergence and growth.
The lesson from 2025–2026 may be less about any specific herbicide and more about flexibility. Open winters can change the timing of weed emergence, extend growth periods, and create opportunities for weeds to escape traditional management schedules. While not every year will look like this one, producers may want to consider how future mild winters could affect their weed management plans, particularly for troublesome annual grasses such as Italian ryegrass, wild oats, and volunteer cereals.
If this winter taught us anything, it is that weeds do not own a calendar. When conditions allow them to grow, they will. Our management strategies need to be prepared to respond accordingly.