Controlling stubborn weeds is a matter of proper identification, timing, proper application techniques, product selection, reading the pesticide label for details/adjustments and persistence.
Identification: The top 10
- Tall fescue—A cool-season perennial, the leaves are strongly veined on the upper surfaces and vary in width from 1/8 to .25 inch. A blend of improved tall fescue cultivars can make a spectacular turf stand, but can also be a weed in a Kentucky bluegrass lawn. It is objectionable due to its coarse texture and clumpy growth habit. Spot spraying with glyphosate offers some degree of control, yet requires that the killed areas be reseeded later. Sulfosulfuron (Certainty) and Corsair (chlorsulfuron) are labeled for tall fescue control in Kentucky bluegrass. Both require multiple applications, and timing is critical.
- Quackgrass—The leaves are flat, about .25 inch wide and rough on the upper surfaces. Quackgrass is a rhizomatous species, allowing it to spread vigorously. This cool-season perennial grows more rapidly than most turf species, and produces a weedy, rank appearance. Recent trials have shown sulfosulfuron to be effective on suppression of quackgrass in bluegrass stands and most warm-season grasses.
- Nimblewill—A narrow bladed, warm-season species that forms in patches in a turf stand. It tends to be thin, wiry and pale green. The leaves are characteristically joined to the stems at a 45-degree angle. Control is similar to tall fescue, with mesotrione showing potential. Mesotrione is currently labeled only for golf and sod farm turf.
- Bermudagrass—A prostrate, thin bladed, warm-season species, adapted to Tennessee, Kentucky, Georgia, southern Kansas and Missouri to the South. An entire lawn, golf green or other turf stand of bermudagrass can be a fine turf, but can also be quite objectionable and difficult to remove from a tall fescue or Kentucky bluegrass lawn. Bermudagrass spreads rapidly via both stolons and rhizomes. Suppression of bermudagrass with fenoxaprop (Acclaim) may limit its spread.
- Zoysiagrass—Adapted much further north than bermudagrass, zoysiagrass becomes objectionable when it invades a cool-season turf. Like bermudagrass, it spreads via stolons and rhizomes, and can be identified by the extensive patterns of hairs on the leaf blades, collars and sheaths. It often requires thorough and multiple nonselective herbicide treatments, followed by renovation of the infested areas.
|Products containing mecoprop provideexcellent control of white clover.||Fall treatments of combination broadleafproducts offer some control of wild violets.|
- Prostrate Knotweed—Small, oval, dark green leaves on low, wiry stems. Well-adapted to compacted soils, prostrate knotweed can grow quite well where desirable turf species will not. Some preemergence control is available, with application timing critical. Prostrate knotweed is often the first annual to emerge in the spring. It is not uncommon to see its fine grass-like seedlings soon after snow melt in mid to late March. Early season applications of combination broadleaf products such as Speedzone, Trimec and Q4 are most effective in the seedling stage. Control drops dramatically as the weeds mature.
- Wild Violets—Often mistaken for ornamental, wild violets have dark green, round leaves that grow in a tight cluster. White, purple or light blue flowers are produced from the crown of healthy plants. Fall treatments of combination broadleaf products such as Speedzone, Trimec and Q4 offer some degree of control. Repeat applications are usually necessary.
- Ground Ivy—Somewhat similar in appearance to wild violets, ground ivy leaves are usually medium green and slightly scalloped on the edges. Ground ivy spreads through growth of stolons, which spread rapidly in the spring and fall. Control is as for wild violets.
- Field Bindweed—Another stoloniferous species, bindweed is easily identified by the medium green, arrowhead-shaped leaves. Occasionally, bell-shaped, whitish flowers are produced. Excellent control has been reported with quinclorac (Drive, Q4).
- White Clover—Identification is straightforward for white clover. The dark green leaflets of three that alternate on the stems are unmistakable, and the stems are low growing and able to root at each node, providing a strong advantage for itself. Products containing mecoprop provide excellent control.
|Ground ivy spreads through the growthof stolons, which spread rapidly in thespring and fall.||Yellow nutsedge has an extensiveunderground network of smallbulb-like structures.|
A number of volunteer tree species, such as mulberry, Siberian elm, hackberry and osage orange, can become established in thin turf areas or in areas where no turf exists, such as fence rows, alleys and near patios. The best approach is to spot-treat them with a brush killer, which is made by various manufacturers.
Yellow nutsedge is a weed that seems to be in a category of its own. Not a broadleaf or a grass, this sedge has a triangular stem (in cross-section) and an extensive underground network of small, bulb-like (actually tubers) structures. Each time a stem dies, shoots arise from the nutlets to replace them. Close to 20 nutlets can be associated with each plant. Data from recent studies at the University of Nebraska-Lincoln (UNL) indicate that any removal or eradication strategy postemergence, including hand weeding and herbicide applications, is most successful if done prior to the longest day of the year (June 21). This has also been documented in studies in Louisiana and Indiana. Current recommendations from UNL include SedgeHammer for postemergence control. Dismiss (sulfentrazone), Echelon (Dismiss & prodiamine) and Tenacity, to a limited extent, have documented preemergence activity on yellow nutsedge based on results from Dr. Peter Dernoeden at the University of Maryland. Early postemergence control is excellent with SedgeHammer. However, spraying after June 21 with any systemic product often results in germination stimulation of the mature daughter tubers and will require additional applications. Contact products, such as Dismiss, do not appear to release the dormancy of the daughter tubers, but these tubers will germinate the following spring/summer.
The key to control is finding the most vulnerable stage for the weed and selecting the best product to deliver the knockout punch.
In general, fall treatments for broadleaf weeds are preferred. First, a thinner cuticle layer usually exists on the leaves in fall. Second, most are actively involved in translocating carbohydrates and sugars into the crown and roots in fall. An herbicide application is likely to be carried along with the other translocated elements. Third, if the weed doesn’t die outright from the herbicide application, it’s more likely to winterkill as a result of being weakened. Spring preemergence treatments for summer annuals, such as crabgrass and foxtail, are much more effective than postemergence applications.
When applying herbicides, it’s important to treat the areas that are already infested or are likely to become infested. For both turf and ornamentals, spot-spraying is a wise and judicious use of an herbicide. Strive to treat weeds and lawn areas uniformly, especially if they are known hot spots that are especially prone to weed invasion, such as hell strips, thin turf stands and compacted soils. Whenever possible, try to maximize the amount of surface area on the leaves of landscape weeds. For example, in a turf stand, avoid mowing the day before application. If mown three days before, sufficient time will pass to allow regrowth of the target weeds. If the weeds are drought stressed, timing post rain or irrigation often increases success.
Read the label
A thorough understanding of the label instructions is crucial to the endeavor of weed control. Typical recommendations via the label are whether a surfactant should be added when mixing, adjustments to the pH of the tank water and restrictions on the total volume that should be applied in a year’s time.
An important piece of information on the label is the rate—use the rate that is called for. Using more is illegal.
Even if all of the other factors are executed correctly, it’s pretty rare to see an entire population of a hard-to-control weed wiped out with one application.
Take careful notes about the presence of these species for each property. In addition, inform your client that more applications are likely.
John Fech is an extension educator specializing in turf and ornamentals at the University of Nebraska-Lincoln. Roch Gaussoin is a professor of horticulture and extension turf specialist at the University of Nebraska-Lincoln.