For eons, attempts at controlling weeds involved some form of cultivation, cutting, digging or burning. These practices are of little use in turf, but hoes, shovels and hands still work OK in landscape plantings for those so inclined.
Modern technologies focus on the use of biologicals and herbicides. Biological agents include insects and plant pathogens. While considerable effort is being devoted to development of biological weed control agents, as yet there is not much available for the turfgrass and ornamental markets. A similar statement can be made for naturally occurring chemicals, sometimes thought of as being safer than synthetically produced herbicides. By far, the most predictable and effective materials remain the herbicides.
Postemergence herbicides offer lawn care operators advantages in that these products can be selective (meaning they control only specific kinds of weeds) or non-selective (control almost all kinds of plants, whether weedy or desirable). Further, they may be either contact (act only where droplets are deposited) or systemic (move from point of contact throughout the plant). Although all of these products work well when used properly, the effective use of herbicides requires the simultaneous implementation of good turf management practices.
| Understand the Enemy |
If weeds are to be prevented or controlled, it is important to understand their life cycles and their means of reproduction. For this reason, weeds are divided into three groups based on their typical life spans. These are perennials, which live for more than two years, biennials, which live for two seasons, and annuals, which normally complete their life cycle in one season. As a rule, the majority of weeds are annuals. Fortunately, of the three groups of weeds, the annuals are the easiest to prevent or control, as they usually lack any vegetative reproductive structures. Biennials, like annuals, tend to reproduce from seed. However, they often possess a thickened stoarage root that allows them to survive the winter or a period of drought. Biennials occasionally cause problems, and since they possess storage roots they are often capable of reproduction by seed and by resprouting from sections of roots. Thus, their control is often more difficult than the control of annual weeds. Perennial weeds are the most difficult group to control. Most perennial weeds reproduce both by seed and by vegetative plant parts. Perennial weeds can be subdivided into two groups: the creeping perennials and the simple perennials. Creeping perennials reproduce by some type of vegetative organ, which may be rhizomes, stolons, tubers, bulbs or similar organs. Simple perennials do no normally reproduce vegetatively, but may do so if they are disturbed. The common dandelion is a good example. Its normal method of reproduction is by seed, however, it the taproot is disturbed it is capable of sprouting from sections of the root. Excerpted from Nursery & Landscape Weed Control Manual. |
THE BEGINNINGS. Phenoxy herbicides ushered in the era of true, selective postemergence weed control and the foundations of modern weed control. Developed during World War II in the United States and Great Britain as a potential anti-crop warfare agent, 2,4-D and several close relatives have provided economical, dependable and (considering the huge amounts used by novice and professional applicators) relatively safe weed control, however they have been linked to numerous incidents over the 50 or so years of use.
Despite all of the purported problems, the extensive use of 2,4-D and its relatives as tools for selective broadleaf weed control in turf continues. Most use of 2,4-D is in combination with another phenoxy or a different type of broadleaf herbicide. Such combinations broaden the spectrum of weed species controlled and provide good control at reduced rates – akin to a synergistic response. Accordingly, formulators have created and marketed dozens of products that are combinations of two or three herbicides in various proportions.
Such numerous formulations are difficult to compare because some products try to target certain weed species while minimizing damage to particular turfgrasses, and many labels don’t list the species controlled. As a result, users rely on three-component products that will control many species but may be overkill for some.
HIT THE BOOKS. Given today’s environmental concerns and concerns relating to pesticides in general, we as professionals should make every attempt to minimize herbicide inputs into our control strategies. Part of doing so involves knowing the least impactful product and least amount of herbicide required to control the problem weeds in each situation. This also requires competence in weed identification since a weed name on a label is only useful to one who can identify the weed.
Weed identification is really only part of knowing a weed. Other important considerations involve knowing whether the weed is an annual, biennial or perennial weed. If it is an annual, is it a winter annual that blooms by late spring, or a summer annual that reproduces and dies by fall? If it is a perennial, how does it increase? Does it have rhizomes or other subterranean organs that are hard to kill, or does it spread by stolons, tillers or seeds?
While answers to these questions might not seem necessary, knowing something of the biology of the weeds one attempts to control will certainly lead to more successful control efforts. This expertise will also give clients more confidence in the professionals they hire.
Similar basic knowledge about how a particular postemergence herbicide works is also important. Such questions as timing of herbicide application relative to a weed’s developmental stage, gallonage required for effective coverage, necessity of adjuvants for optimal effects, interval required for rain-fastness, dangers to nearby desirable species and other off-target hazards are some considerations. And, always remember that “the dose makes the poison.” As with table salt, sugar or even water, an excess can be lethal. Thus, while an herbicide can selectively control weeds at the label rate with minimal side effects, applying too much product can negate selectivity and cause severe environmental damage.
Instances of herbicide overdose and the accompanying disastrous effects often form an image of all herbicide use in the eyes of many persons. It is essential that lawn care professionals commit themselves to proper training of staff and providing clean, well-calibrated equipment.
BROADLEAF CONTROL. As indicated earlier, many broadleaf species can be controlled by combination products. The basic components are 2,4-D, MCPP (mecoprop), MCPA or 2,4-DP (dichlorprop). Another common component of many combinations is dicamba, which is a benzoic acid derivative rather than a phenoxy. Dicamba is effective at very low rates and is frequently used by itself. In 2,4-D combinations, dicamba broadens the weed control spectrum. A downside risk is that dicamba can be quite mobile in soils and could have a negative effect on nearby trees and shrubs if it is absorbed by their shallow roots.
All of these materials produce formative effects on broadleaf weeds, which include twisting, curling, cupping and other malformations. Also, while grasses are seldom killed, some temporary yellowing can occur. Some grasses, e. g. closely mowed bentgrass and fine fescues, can be severely injured. Generally, one wants to avoid applications when temperatures are in the mid-80s or higher. Also, drought stress tends to reduce control effectiveness.
Formulations of the phenoxy (and other) herbicides can be either of the amine or ester type. Basically, the amines are water-soluble while the esters are oil soluble. The esters are formulated as emulsifiable concentrates that form milky suspensions while amines form clear solutions, and the formulations act differently as well. Amines are non-volatile, which means that they don’t form vapors that can move to other plants as can occur with esters.
However, most ester products are of the low-volatile type, which reduce but don’t eliminate the problem. High air temperatures increase volatility considerably. Generally, esters are thought to be the more potent form, mainly because they penetrate the waxy coating on leaves better. Both the amine and ester are converted to an acid form once in the plant. This conversion gives rise to use of the term ‘acid equivalents’ on most labels.
Beginning in the 1980s, a new class of broadleaf herbicides having a pyridine base was introduced. The most useful compounds have the common names triclopyr and clopyralid. They were the first of the non-phenoxy herbicides developed in part to replace 2,4-D should its registration be cancelled, as was once feared.
Triclopyr products in the amine formulation are a combination of 2,4-D and triclopyr amines, while other formulation features both 2,4-D and triclopyr esters. However, the ester formulation of triclopyr isn’t produced in combination with other products.
Basically, triclopyr is a broad-spectrum herbicide that controls some of the species that 2,4-D is weak on. It is also quite safe on cool-season turfgrasses. The ester formulation can control tough weeds like violets, ground-ivy and yarrow. These compounds degrade fairly quickly in soils and have limited mobility so groundwater problems are not a concern.
The sister compound, clopyralid, is available to the turf market in combination with triclopyr. It is also marketed in some three-way combinations containing 2,4-D. These combination products are as effective as the various other 2,4-D products on many weed species and more effective on some of the species difficult to control with the phenoxy materials.
Clopyralid by itself tends to be specific in that it mostly controls many species within the sunflower, smartweed, potato and legume families. But, combining it with the broader spectrum triclopyr yields a synergy which actually makes both more effective. As with dicamba, clopyralid is fairly mobile in soils so it can be harmful to shallowly rooted ornamental species and can reach groundwater on coarse soils with a shallow water table.
Questions frequently arise concerning when it is safe to apply broadleaf herbicides after seeding turfgrass. There is no simple answer to this since it depends on the turfgrass species, weed species present (this is a good time to know your weeds and their biology ), drought stress and the herbicide used. A good rule of thumb, however, suggests waiting until the second or third mowing. Since an application made too early can readily injure the young turfgrass, try to wait until tillering commences unless the weeds are threatening to crowd-out the turfgrass.
Another question concerns timing reseeding application after an application of a broadleaf herbicide has been made to an area. All of the postemergence herbicide products have some preemergence activity on grasses, albeit slight, but their residual is generally short. So reseeding one month after the postemergence application should be relatively safe, but always check the label for specifics. Rate of application, soil type and environmental conditions can alter this timing.
Other selective postemergence broadleaf herbicides that are available lack the broad-spectrum control typical of the phenoxy and pyridine types. In other words, they control fewer species. Some, however, control both grasses and broadleaves. A few have very limited activity and will not be discussed.
Bromoxynil is primarily a contact herbicide that works best on small, broadleaf seedlings. It is mainly used in new seedings where there are heavy infestations of broadleaf weeds. The arsenical MSMA, sold under numerous product names, is used in turf to control annual grasses and nutsedge. It also controls some broadleaf weeds, notably yellow woodsorrel.
The newest turf herbicide on the market is quinclorac, which is chiefly a postemergence annual grass herbicide but controls a fair number of broadleaf species as well. Among those species are dandelion, white clover and some speedwells.
The author is adjunct professor of Turf Weed Management at the University of Rhode Island, Kingston, R.I.
EDITOR'S NOTE: Mention of a tradename is for identification only and does not imply any endorsement or recommendation of this product by the author, the University of Rhode Island or Lawn & Landscape.
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