Rhubarb weed control in Michigan

New herbicide registrations give rhubarb growers several options for weed control.

Several herbicides have been registered for rhubarb in recent years. These new registrations give rhubarb growers additional choices and should result in a better level of weed control.

Kerb has been labeled for weed control in rhubarb in Michigan for many years. Kerb should be applied in the fall to control quackgrass the next season. It can be applied in early spring but is less effective against quackgrass when applied in spring. Kerb also controls annual grasses for about four weeks Kerb is a restricted use pesticide (RUP) and may be applied only by licensed applicators.

Rhubarb emerges from the soil very early in the spring. It is difficult to apply herbicides before rhubarb emerges. Most fields are too wet to drive on before rhubarb emerges. Some of the pre-emergence herbicides registered for rhubarb may cause slight burn on rhubarb leaves if applied after the leaves begin to expand, but normally there is no long-term adverse effect on rhubarb. If there is substantial weed growth when the pre-emergence herbicide is applied, Gramoxone may be applied with the pre-emergence herbicide. It will burn rhubarb leaves but does no permanent crop damage. Gramoxone also is a RUP. Glyphosate should not be applied after rhubarb leaves have emerged because of potential crop injury.

Pre-emergence herbicides labeled for rhubarb include Dual Magnum, Callisto, Lorox, Caparol and Sandea. Dual Magnum is primarily an annual grass herbicide. It also controls pigweeds, nightshades and some other broadleaves. Caparol and Lorox control many annual grasses and broadleaves. Callisto and Sandea primarily control broadleaves. Sandea also controls yellow nutsedge. None of these pre-emergence herbicides will provide season-long weed control. They are effective for four to six weeks, after which annual weeds germinate. They have minimal effect on perennial weeds.

For post-emergence control, Quinstar may be applied to control Canada thistle, field bindweed and several annual broadleaves and grasses. Poast and Select Max are labeled for post-emergence grass control. At higher rates they will kill annual grasses and suppress quackgrass.

Since rhubarb is a perennial crop, many of the serious weeds are perennials. Most perennial weeds are not controlled by pre-emergence herbicides. Therefore, cultivation and hand weeding remain important weed control methods. Rhubarb should be planted on a grid so it can be cultivated in two or more directions.

Most commercial rhubarb varieties are not very competitive and suffer serious yield reduction with heavy weed competition. Growers should use all methods possible to keep their rhubarb fields weed-free during the entire growing season.

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