Large & Smooth Crabgrass
Tim Trower* and Chris Boerboom
Family: Poaceae
Name: Large- Digitaria sanguinalis (L.), Smooth- Digitaria ischaemum.
Life Span: Annual
Reproduction: Seeds
Description:
Large: Leaves are flat, measuring 0.75-6 inches long and up to 0.5 inches wide, with a prominent mid-rib and pubescent on both sides. Large crabgrass has a membranous ligule that is truncate and has a jagged appearance. The sheath (area where the leaf and stem join) is covered with straight, long hairs. The culm (stem) is rather stout, may exhibit a decumbent (reclining) growth pattern, and additional roots may form at the soil/node region. The inflorescent consists of a panicle of branches with spikelets arranged in two rows of a flattened rachis. Large crabgrass may grow up to 3 1/2 feet in height.
Smooth: Leaves are flat, measuring 0.75-6 inches long and up to 0.3 inches wide, with mid-rib or pubescence on either side. Smooth crabgrass has a membranous ligule that is truncate and often has a smooth appearance. The sheath has no hairs. The culm is rather slender, may exhibit a decumbent growth pattern, and additional roots may form at the soil/node region. The inflorescent consists of a panicle of digitate branches with spikelets arranged in two rows of a flattened rachis. Smooth crabgrass may grow up to 2 1/2 feet in height.
Overall, smooth crabgrass is similar to large crabgrass with the exception that smooth crabgrass is not as coarse or tall, has no hairs around the sheath or on the leaves, and is more reddish or purple.
Importance:
Large and smooth crabgrass can present significant challenges to corn growers if not managed properly. Both germinate at higher soil temperatures, usually later in the season, when the residual activity of some soil-applied herbicides may have dissipated to the point that the grasses may escape. Poor crop canopy (stress, wide rows) allows sunlight penetration which aides crabgrass germination and development. Finally, reliance on ALS herbicides has increased crabgrass problems as these herbicides have no activity on smooth crabgrass and only fair activity when applied to small large crabgrass.
Control Methods:
Soil-applied grass herbicides should be the cornerstone treatment for any field with a large or smooth crabgrass infestation when the field is to be planted with conventional corn hybrid. All of the acetamides herbicides (Axiom, Dual II Magnum, Frontier, Harness/Surpass, and Lasso/Micro-Tech/Partner, and their premixes with atrazine), DoublePlay, and Prowl provide good to excellent crabgrass control when applied prior to the crabgrass emergence. Although many of the previously listed herbicides have postemergence labels, none will consistently control grasses that have emerged. All of the preemergence-applied herbicides need rainfall for activation. Generally, 0.5-0.75 inches of rainfall are need for herbicide activation; however, some are activated by less. In the absence of rainfall, rotary hoeing is recommended at five to seven days after a preemergence application two reasons: it helps activate the herbicide with a shallow incorporation, and it mechanically removes small emerged weeds that the herbicide may miss due to delayed activation.
Control options are extremely limited in conventional corn hybrids once crabgrass has emerged. Postemergence applications of Bladex to small crabgrass (less than 1.5 inches) may provide acceptable control; however, Bladex is being phased out and the maximum rate of 1 lb ai/A may give inconsistent control. A residual grass herbicide should be tank mixed with Bladex to control later flushes. Postemergence applications of Bladex may cause crop injury under cool and wet growing conditions. Postemergence applications of atrazine will miss crabgrass with more than two leaves and will not control subsequent flushes.
ALS herbicides (Accent, Accent Gold, Celebrity, Basis, and Basis Gold) generally provide poor postemergence control of crabgrass, unlike the control of foxtails. Smooth crabgrass is tolerant to these ALS herbicides and will not be controlled. Large crabgrass can be suppressed or controlled by some of the ALS herbicides, but plant height at application will determine the level of control. Basis Gold (Accent/rimsulfuron/atrazine) is the best postemergence option of the ALS chemistry if not in an atrazine prohibition area. Basis Gold should be applied early when the large crabgrass is small, less than one inch, and tank mixed with a residual grass material to control later flushes as the ALS herbicides have very short residual activity. It is very important to scout fields infested with crabgrass and plan on cultivation, if needed, to controlled escapes.
Herbicide resistant corn hybrids are an option in fields with serious crabgrass problems. Roundup Ultra is labeled to control crabgrass up to eighteen inches in height, but should be applied sooner to minimize yield loss due to weed competition. Liberty applied at 28 fl oz/A is labeled to control three-inch crabgrass. Both Roundup Ultra and Liberty lack residual control and should be tank mixed with a residual grass herbicide and/or a later cultivation to control late germinating plants. Lightning is labeled for control of three-inch crabgrass and has provided reasonable residual control over the years, but a cultivation may be needed to control escapes. All three of the above herbicides must be applied to the respective herbicide resistant hybrids, which eliminate these herbicides as rescue options in conventional hybrids.
Crabgrass control is less of an issue in soybeans due to several excellent postemergence options. The acetamides herbicides (Dual II Magnum, Frontier, Lasso) and Prowl are good choices if a soil-applied herbicide is planned. Otherwise, postemergence applications of ACCase herbicides such as Fusion, Assure II, Select, or Poast Plus provide application flexibility and excellent control. Postemergence application of glyphosate on Roundup Ready soybeans provides another excellent tool in controlling crabgrass and other emerged weeds. Competition from narrow-row soybeans should shade out late germinating crabgrass.
Crabgrass is seldom a problem in established alfalfa unless the stand is in decline. Postemergence applications of Poast Plus and Select are both labeled for alfalfa, but will provide little economic return unless other grass species, such as quackgrass, are also providing competition.
Other Information:
Both species of crabgrass are native to Europe. Large and smooth crabgrass has no forage quality. The seeds and leaves may be an important food source for wild turkeys, and may be utilized to a lesser degree by upland gamebirds and waterfowl. The seeds are an important food source of ground-foraging songbirds.
Sources:
Weeds of Nebraska And The Great Plains, Nebraska Department of Agriculture
2000 Pest Management in Wisconsin Field Crops
Crabgrass Management in Corn, Chris Boerboom, University of Wisconsin
Please read and follow the label.
*  Senior Outreach Specialist
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