Published: April 27, 2009 11:42 am      

Controlling broadleaf weeds in home lawns

A healthy, vigorous turf is the best control for weeds. Most common broadleaf weeds are not a problem when a well adapted turfgrass is properly established, fertilized, mowed, and watered. However, a thin, weak stand of grass will be heavily invaded by weeds.

For complete information on correctly caring for your lawn, see F-6420, “Lawn Management in Oklahoma.”



Weed Identification

The first and most important step when controlling a weed problem is to properly identify the weed. Broadleaf weeds have two seedling leaves or cotyledons. The leaves of a broadleaf weed are usually wide and have net-like veins.

All broadleaf weeds have a yearly life cycle that is important to understand. Weed life cycles are divided into three general groups: annuals, biennials, and perennials. An annual life cycle is one year in duration. In other words, the weed seed emerges grows into a plant, eventually flowers, and produces seed in a single year. Biennials germinate from seed and remain in a vegetative state the entire first year. During the second year, they flower, produce seed, and die. Perennial weeds, on the other hand, can continue to grow year after year and some can increase in size by stolens or rhizomes.

Broadleaf weeds can actively grow in the cooler or warmer seasons of the year. Winter annuals, for example, germinate in the fall and die the following summer. Summer annuals germinate in the spring and die in the fall. Winter and summer perennial weeds follow a similar pattern; however, they become dormant rather than dying at the end of their respective growing seasons.



Herbicides

The most commonly used herbicides for controlling broadleaf weeds are 2,4-D, Banvel (dicamba), MCPP (mecoprop), and 2,4-DP or DPC (Dichloprop). These post-emergence herbicides selectively control most winter and summer broadleaf weeds in turf. To increase the different kinds of weeds they control, many broadleaf weed killers are a mixture of these herbicides. These herbicides are safe on established bermudagrass, Kentucky bluegrass, tall fescue, perennial ryegrass, and zoysiagrass. They are relatively immobile in the soil and pose little threat to nearby trees and shrubs from root absorption. Most chemical residue in soil is dissipated in three to four weeks. However, shrubs, trees, and vegetables can be damaged by drifting vapors or spray. Use caution when spraying them around susceptible plants and choose a time when the wind is minimal (less than 5 mph).

Banvel (dicamba) is commonly added to broadleaf weed herbicides to control tough weeds, such as henbit and knotweed. Banvel is mobile in the soil and can be absorbed by roots of ornamentals and trees, which may lead to their injury or death. Therefore, never apply Banvel or broadleaf weed herbicides containing Banvel within the drip line of trees and other ornamentals.

The amine salt formulations of these herbicides are safest around susceptible plants during hot weather. Low volatile esters can be used during the early spring and late fall when the highest day temperature does not exceed 60° to 70° F. Try not to apply any of these herbicides when the air temperature exceeds 85°F or is so low (i.e., less than 50° F) as to prevent active weed growth. These herbicides are not safe on newly established turf when applied at the recommended rate for established turf areas.



Herbicide Application Timing



Effective weed control with 2,4-D, Banvel, MCPP, and DPC combinations usually involves more than one application. Two to four applications, spaced 10 to 14 days apart, may be necessary for satisfactory results. The best time to control winter broadleaf weeds is in the fall during October and November. The plants are smaller at this time and lower rates will provide control. The next best times are during the late winter and early spring, however higher rates will be needed. Summer broadleaf weed problems are effectively controlled during the spring of each year (April and May). As the weed becomes more mature, the higher recommended rates and number of applications will be required to effectively control the weed. Always remember, small actively growing weeds are easier to control than mature weeds that are starting to produce seed.