Fall Panicum

Panicum dichotomiflorum

Fall panicum growing in a pot
Fall panicum growing in a pot. Photo courtesy of Bruce Ackley. Licensed under CC BY 3.0.
Fall panicum is an annual grass that tends to grow in disturbed areas with moist soil. It can grow up to 4 feet in height and has become an increasingly large problem in corn and soybean crops.
Fall panicum tend to germinate as early as late April and continue through the summer. Seedlings are often very different than mature plants, as they have many hairs on the undersides of the leaves. The leaf nodes have a ligule composed of 2 mm long hairs and no auricle.
Leaves remain rolled into the shoot and grow to be up to around 15 inches long. Leaves are alternate, generally hairless, and have a very prominent midvein. The tip or base of the leaf may be hairy on top, but the underside of each leaf is glossy and hairless. The stem is round and sometimes glossy with swollen nodes that when in contact with the ground will sprout roots. The stems may bend in any direction at the nodes, making the growth pattern of the plant as a whole quite unusual. Fall panicum may be mistaken for johnsongrass or barnyardgrass, but do not have hairs on the underside of the leaves as seedlings.
Fall panicum plant
Fall panicum plant. Photo courtesy of Bruce Ackley. Licensed under CC BY 3.0.
Flowers grow as spikelets in branches that together create a wispy, pyramidal seedhead that is 6 to 12 inches long and 3 to 6 inches wide. These flowers appear from June to October, immediately after peak growth occurs.
Fall panicum flower
Fall panicum flower. Photo courtesy of Show ryu. Licensed under CC BY 3.0.
Fall panicum is able to grow well in most areas and becomes a significant competitor for nutrients and moisture for surrounding crops.
Fall panicum seeds are only able to germinate in depths up to 2 inches. Tillage therefore does have a positive effect on eliminating many potential plants. Mowing before seeds form can also be helpful as well as cultivating crops later in the season. Establishing crop canopy before emergence is highly effective in suppressing the weed, as they tolerate shade very poorly.
Currently, atrazine has little or no effect on fall panicum. Many of the preemergence, residual herbicides used in corn will also work well in soybeans. Examples include products that contain metolachlor, alachlor, dimethenamid, and pendimethalin. Postemergence herbicides for soybeans include quizalofop, fluazifop, and clethodim. Applying postemergence herbicides for corn is difficult and must be timely in order to attack small plants. These include ALS-inhibitng herbicides. HPPD-inhibiting herbicides are generally ineffective. Please contact your local weed specialist to learn what is most effective in your area and how to use it best. Always read and follow all label instructions.