The average homeowner primarily uses herbicides when establishing or maintaining a lawn. Herbicides are broadly classified as either selective or non-selective. They are further classified according to their predominant chemical ingredient and the effect these chemicals have on plants. Before selecting an herbicide to control lawn or other weeds, determine the results you desire and whether you wish to selectively kill weeds or simply eradicate all vegetation in a specific area.
The phenoxy acid class of herbicide is systemic and selective. They injure and control broadleaf plants without injury to grass-type plants. Acting as a synthetic growth hormone, they regulate growth by causing the plants' leaves and stems to twist and curve downward. Easily absorbed by foliage, they oftentimes translocate into the plants' root system. Examples of this class of herbicide include 2,4-D, mecoprop and 2,4-D plus 2,4-DP.
Similar to the phenoxy class of herbicides, benzioc acid herbicides are also systemic and selective in nature and are also used to control broadleaf weeds. Benzioc acid herbicides are sprayed on the foliage, where they invade the plants' system. One example is dicamba, which is considered quite mobile in soil. Some sensitive trees and ornamental plants can take up this herbicide through their roots and exhibit symptoms of treated plants, which are similar to the phenoxy class and include downward curving and twisting of leaves and stems.
A pre-emergent class of herbicides, dinitroaniline-types are used primarily to control annual grass-type weeds in established lawns. They inhibit the growth of the roots of newly germinated seeds, preventing them from becoming established. Some examples of dinitroaniline herbicides include pendimethalin, trifluralin, benefin and prodiamine. Rainfall or irrigation after application activates these herbicides and some may persist in the soil for several months.
A non-selective class of herbicide, bipyridylium types are sprayed onto the foliage. They begin to kill on contact with plants but have no biological activity in the soil, thus becoming inert. Dependent on light to work, they interrupt the plants' photosynthesis, causing damage to cell membranes. An example of bipyridylium herbicides is diquat. Symptoms of plants affected by this herbicide include quick wilting and desiccation.
As a selective herbicide, the substituted urea class of herbicides such as siduron and limuron, among others, are applied to the soil. They are primarily used to control annual grass weeds or crabgrass in newly seeded lawns. These herbicides are absorbed by the roots of plants, where they inhibit the growth of roots. Affected plants--primarily young seedlings--exhibit a slow death.
Primarily contact herbicides, examples of the arsenical class include MSMA, Weed-Hoe and DSMA. They are only applied as a foliage spray. Recommended as a control in established lawns for already-growing crabgrass or nutsedge, symptoms include yellowing foliage seven days after initial treatment. They are tightly bound to the soil and do not affect plants unless their foliage comes in contact with them.
Glyphosate (commonly known as Roundup) is a non-selective herbicide. It is sprayed on the foliage and interferes with aromatic amino acid synthesis. Symptoms include yellowing and wilting of leaves and shoots, progressing from newest to oldest and generally appearing within two to 10 days of initial application. Gylphosate becomes inert in the soil.
Dithiopyr (commonly known as Dimension) is a new class of herbicides called pyridines. Selective in nature, it is used to prevent the emergence of annual grass weeds in established lawns, although it can also be used to kill young grass seedlings with three or fewer leaves. Dithiopyr is lost from soil through chemical and microbial processes.
Glufosinate (commonly known as Finale) is non-selective and applied by spraying the foliage. It limits glutamine synthesis once absorbed into the plants' systems, although it eventually degrades into carbon dioxide, nitrogen and water. Because it shows no residual activity in the soil, it is often used to remove weeds before starting a new lawn, as seeding or sodding can be done soon after its application.