By Scout Nelson
Fall is an important season for controlling perennial weeds in agricultural fields, pastures, and other areas. However, changing weather conditions can make it difficult to decide whether herbicide applications are still effective. As temperatures drop and frost become more common, farmers may wonder if spraying weeds late in the season is worthwhile.
Graig Reicks, SDSU Extension Weed Ecology Field Specialist, explained that fall herbicide applications can help control difficult perennial weeds, such as Canada thistle. However, the success of these treatments depends on plant health, temperature, soil moisture, and the timing of application.
Understanding the Best Time for Fall Spraying
The ideal fall spraying period generally occurs when daily high temperatures remain above 60˚F and nighttime lows stay above 40˚F. During this period, perennial weeds actively move nutrients, and other resources toward their roots to prepare for winter.
Many farmers focus on harvesting crops and completing other field activities during this time. As a result, weed control may receive less attention. However, missing the ideal spraying window does not always mean that herbicide applications are no longer useful.
Perennial weeds are difficult to control because their roots can survive even when the leaves and stems above the ground die. Effective treatment must reach the roots rather than only damaging the visible plant parts.
During late summer and early fall, many perennial weeds move stored resources from their leaves toward their roots. Herbicides that move through plants can follow this process and reach underground structures, helping improve long-term weed control.
Herbicides Used for Perennial Weed Control
Synthetic auxin herbicides, also known as Group 4 herbicides, are commonly used to manage perennial weeds. These include 2,4-D products such as 2,4-D amine, LV4, LV6 and Freelexx; dicamba products such as Clarity and Overdrive; picloram (Tordon); aminopyralid (Milestone); and clopyralid (Stinger and Transline).
Group 2 herbicides, known as ALS inhibitors, can also move toward plant roots during fall. Examples include chlorsulfuron (Telar XP), metsulfuron (Escort XP) and imazapic (Plateau and Panoramic).
Choosing the right herbicide and application timing is important for achieving effective weed control.
How Frost and Soil Moisture Affect Herbicides
Soil moisture plays an important role in herbicide performance. Adequate moisture supports healthy plant growth and helps weeds absorb and move herbicides through their systems. September rainfall provides helpful soil moisture in many areas.
Light frost before or after spraying generally does not greatly reduce herbicide performance. However, a hard frost can damage plant tissues and limit the movement of herbicides toward the roots.
Before applying herbicides, farmers should inspect weed leaves and overall plant health. If leaves show severe frost damage, spraying may not provide effective results.
What Research Shows About Mid-Fall Spraying
Research on herbicide applications during mid-fall remains limited. A western Nebraska study by Wilson and Michiels (2003) examined dicamba applications on Canada thistle and dandelion.
The researchers applied dicamba at 0.5 pounds per acre, equal to 16 ounces of Clarity per acre. Results showed significantly better control when applications occurred one to 10 days after the first frost, which was below 27˚F. This was compared with applications made five to 11 days before the first frost.
The first frost in the three-year study occurred in late September or early October. However, it remains uncertain whether the same results apply in years when the first hard freeze arrives much later.
Is Late-Fall Spraying Still Worthwhile?
Sunny conditions and daytime temperatures mostly above 50˚F for several days before and after spraying may support better results.
Some relatively inexpensive herbicide treatments may still be worth considering under these conditions. Cooler weather and reduced foliage on nearby sensitive plants and trees may also lower the risk of off-target movement from volatile herbicides such as dicamba and 2,4-D ester.
However, applying expensive herbicides across large areas can be risky this late in the season. Farmers should carefully consider weather forecasts, weed health and treatment costs before making application decisions.
For additional information, farmers can visit SDSU Extension's Weed Control in Pasture and Range and Noxious Weed Control resources.
Photo Credit: Graig Reicks, SDSU Extension
Categories: South Dakota, Crops, Sustainable Agriculture