Populations of winter annual weeds will become more prevalent in early April and can compete with wheat and barley and slow the rate of crop development potentially reducing yield. Extension agronomist Bill Curran reminds us if winter annual weeds like common chickweed, henbit, purple deadnettle, horseweed, and others emerge with the small grain and are left unchecked, the potential impact on yield could be great.
Harmony Extra is still probably the most broad spectrum herbicide for broadleaf control. Harmony SG contains only one of the active ingredients (thifensulfuron) in Harmony Extra and thus does not have the same weed control spectrum. Clarity, Banvel, 2,4-D, or MCPA can improve the control of some winter annuals and perennials and Stinger is the most effective small grain herbicide for thistles. Harmony SG or Harmony Extra should be included where control of chickweed is desired, since these are the only herbicides that control this weed but if it is ALS-resistant than you will need to include Glory or Starane. Consider using 2,4-D and Clarity or Huskie if horseweed/marestail is a problem in small grains. Prowl H2O is also labeled for use up to growth stage 7 but it must be applied before weeds germinate or tank mixed with effective post herbicides. Refer to the Agronomy Guide table for additional details:
(http://extension.psu.edu/agronomy-guide/pm/tables/table-2-5-5).
Herbicides applied in early spring can be slow under the typically cool conditions in March and early April. Remember that cool (less than 50 F) cloudy days can reduce herbicide activity. Applications this early are not likely to effectively control dandelions or Canada thistle. These weeds would be more effectively controlled with a later spring application.
Weedy grasses, such as downy brome, cheat, annual bluegrass, annual ryegrass, and roughstalk bluegrass are also becoming more of a problem in Pennsylvania. The wheat herbicides available for control of grasses, Axial, Maverick, Osprey, PowerFlex, and Prowl H2O are most effective when applied in the fall. Some of these products can work in the spring, but to be effective, the weeds must be small. Or in the case of Prowl, it must be applied before weeds germinate.
Once wheat has passed Feeke’s Stage 6 (i.e., when the first node of stem is visible), the risk of herbicide injury from 2,4-D, MCPA, Banvel/Clarity, or Curtail increases and application of these herbicides is not recommended. In this situation, the remaining herbicide options for broadleaf weed control are Harmony Extra (similar products: Edition, Treaty Extra, Nimble, others), Harmony SG (similar products: Treaty, Harass, Volta), Buctril, Stinger and Starane. Harmony Extra and Harmony SG (and the similar products listed above) can be applied to wheat until the flag leaf is visible (before Feeke’s Stage 8). Buctril, Huskie, Stinger and Starane can be applied to wheat up to the boot stage (before Feeke’s Stage 9).
Each spring there are questions about the risks associated with 2,4-D or MCPA application to wheat past Feeke’s Stage 6. Wheat tolerance of 2,4-D is highest between Feeke’s stages 3 and 6 and is lowest in Feeke’s Stages 9 and 10. Between stages 6 and 9, sensitivity to 2,4-D gradually increases as wheat growth stage advances. Thus, the risk of injury increases as wheat growth stage advances between stages 6 and 9. Severe injury is highly probable when 2,4-D is applied at Feeke’s stages 9 and 10.