Weed Control: It's Only Natural
Science has done a great job in discovering chemicals to kill or suppress weeds in turfgrass. Conventional wisdom also indicates that cultural methods can help a turf stand grow dense enough to compete with weeds, without using chemical herbicides. But the scientific proof of this is hard to find.
Cultural management of weeds is the use of mowing, fertilization, irrigation, cultivation, planting and turfgrass selection to affect weed populations. There is some evidence that cultural management of weeds can be an effective alternative to herbicides. Probably more often than we can prove, appropriate cultural management already reduces herbicide use. But research is needed to show us the details. How can cultural management replace herbicides in practice, and most importantly, how can the two approaches work together?
HISTORY OF CULTURAL MANAGEMENT
Before the introduction of synthetic pesticides (including herbicides) in the 1940s, scientists attempted to control weeds in turfgrass by adjusting the soil and other environmental factors. In 1921, at the Arlington Turf Garden, a long-term study was conducted on the effects of different fertilizer sources, pH values and weed populations. In the study, sodium nitrate reduced soil acidity, thus favoring weeds such as goosegrass.
In an even longer-term study, the 140-plus year Park Grass Experiment at Rothamsted, United Kingdom, dandelion populations were shown to increase up to 20-fold as a result of continuous potassium fertilization. Lengthy ecological studies such as these may depend on the carryover weed seed bank in the soil, so a gradual improvement or worsening would not have been detectable in a single season, the way an herbicide effect is noticed.
Even simply pulling weeds up by hand is an ancient form of cultural management. Growing up in Colorado, my father paid us 5 cents for every cigar box full of dandelion flowers and flower buds that we brought him from the lawn. Our proposal to him was that by removing flowers, the dandelion plants would not go to seed and spread. This long-term strategy might reduce the weed seed bank in the soil, lessening next year's dandelion invasion. But the strategy was not economically viable, as my fellow 10-year-old contractor and I realized that the big money in those days was in recycling bottles, not dandelion removal. Such may become the fate of other attempts at cultural management of weeds, unless their costs and benefits are well understood.
SCIENCE OF TURFGRASSES
To find facts, I conducted a review of the expanding scientific literature on turfgrass. I was pleased to find over 700 scientific publications on the use of herbicides to control turfgrass weeds. But only 25 articles emphasized cultural management of weeds.
A few dozen articles reported on the biology of weeds, and this helps explain how cultural management might work. Most turf weeds, such as goosegrass, are “gap colonists,” that is, they germinate and grow quickly in openings in the turf canopy. While no one has shown the relationship of cultural management and light gaps in the turf, there is a causal connection between light gaps and weed infestation. Roy Nishimoto, Ph.D., professor in tropical plant and soil science at the University of Hawaii, and Bert McCarty, Ph.D., professor of horticulture at Clemson University, showed that goosegrass germinates best under fluctuating temperatures, the same conditions that we would expect from a damaged area of turfgrass, where the ground is warmed by the sun during the day and cools off at night.
Close mowing is a large contributor to crabgrass populations in both Kentucky bluegrass and tall fescue (see Figure 1, page 36). Based on studies at multiple universities, mowing Kentucky bluegrass at least 2 inches high, and mowing tall fescue at least 3.5 inches, resulted in essentially zero crabgrass infestation in the lawn. The relationship of scalping to crabgrass infestation was consistent with the idea that exposure of the soil to sunlight allows weeds to germinate. Peter Dernoeden, Ph.D., professor of natural resource sciences and landscape architecture at the University of Maryland, and fellow scientists showed that smooth crabgrass cover in tall fescue was negligible when mown tall (3.5 inches) but that under close mowing (2 inches) the crabgrass population was more severe, even when pre-emergence herbicide was applied.
The practical problem with this is that many clients will not automatically accept a taller mown lawn. But if clients are reminded that, depending on the species of turfgrass, there is an optimum mowing height, and that weeds can be reasonably expected from inappropriate cutting practices, they should understand the consequences and decide accordingly. Depending on the species and situation, excessively tall mowing may also contribute to weed problems. It is therefore important for scientists to repeat the mowing height research on different turfgrasses and find the optimum mowing height for each.
Lower rates of nitrogen fertilization, within the range of rates used, tend to result in larger weed populations (see Figure 2, at right), and larger rates of fertilization may be the first defense against weeds. The cost of additional fertilization for weed prevention should be balanced against the cost of chemical weed controls. We cannot jump to the conclusion that either herbicides or fertilizers are more cost effective in weed control or more environmentally sound, without actual data. In a study by B. J. Johnson, Ph.D., and Bob Burns, Ph.D., professors emeriti of agronomy at the University of Georgia, it was shown that high rate of fertilization, approximately 4 pounds of N per thousand square feet per year, was partially effective at dandelion suppression. But they also found that lowering of soil pH was comparable to the only effective herbicide in that study, Ronstar, in managing dandelion. Repeatedly in other studies, including pasture investigations, fertilization with adequate nitrogen lessened white clover populations.
The prescription for other practices, such as irrigation, is even harder to write, because it varies depending on the situation. In Florida, high rates of sprinkler irrigation are associated with dollarweed infestation. In my research, which confirmed practical experience, daily irrigation sustained a year-round average 30 percent dollarweed cover. Conservative weekly irrigation, which was skipped following rain, sustained negligible dollarweed: less than 10 percent. Other studies at multiple universities have shown that irrigation rates can have a positive or a negative effect on different weed species. Roch Gaussoin, Ph.D., professor and extension turfgrass specialist at the University of Nebraska, and Bruce Branham, Ph.D., associate professor of turfgrass science at the University of Illinois, showed that a lower total rate of irrigation applied daily resulted in more annual bluegrass than a higher rate of irrigation applied three times per week. This might have been because annual bluegrass seed germination could have been enhanced by the high irrigation frequency and the constantly damp soil, even though there was less total irrigation amount.
Theirs was also one of the few studies in which weed seed bank was measured in turf. It was shown that more annual bluegrass seed accumulated in the soil where clippings had been returned, which suggested that clippings return may have been an unintentional form of annual bluegrass overseeding. But other studies have shown the opposite. Ali Harvandi, Ph.D., extension horticulture specialist at the University of California Extension Service, reported that return of clippings reduced dandelion population in one of two years.
Besides mowing, fertilization and irrigation, weed populations can also be reduced by choosing a turfgrass species and cultivar that is adapted to the situation. Turfgrasses that are attacked by insects, diseases, nematodes and other stresses are also susceptible to subsequent weed infestation. I have reported how differences in nematode susceptibility among zoysiagrasses help explain differences in weed infestation. Dernoeden and fellow workers showed that hard and blue fescue cultivars were more resistant to smooth crabgrass infestation, compared with tall fescue. Drought and the absence of fertilization were the main factors in that experiment.
Weed problems can be avoided by using proper planting methods, which include appropriate timing and seedbed preparation and the use of mulches, in some cases. I discovered that timing of fertilization was important. The same amount of fertilizer applied at the time of seeding different bahiagrass cultivars resulted in more weeds than fertilization five weeks after planting. My interpretation was that before the bahiagrass roots had become established, the early fertilizer was not available to the bahiagrass, but was available to weeds.
CULTURAL MANAGEMENT OF WEEDS IN PRACTICE
There is much that we do not know about the biology of weeds and their control using alternative technology. The turf manager's job is made easier in knowing that if a certain herbicide is sprayed at rate X, then Y percent of the weeds will be killed. For cultural management, there is little in the way of hard quantitative information. It is therefore difficult to make the right choices. While it is reasonable to generalize about “dense healthy turf warding off weeds,” no one prescription can be written for all grasses.
Another part of the managers' problem is economics, e.g., “Should we spend the money on fertilizer, or herbicides, or (most likely) a little of both?” The biggest argument for cultural management of weeds is economic. Mowing, fertilization and other cultural practices are normally performed anyway, to maintain appearance and function of turf. Therefore, adjusting cultural management practices for maximum weed control effect may not greatly increase the cost of weed control.
Another force driving the need for cultural management alternatives for weed control is that herbicides are not always effective. Some tough weeds have no known selective chemical control. Other weed species are sometimes selected for resistance to herbicides, which shortens the life of the herbicide. In such cases, cultural management of weeds is not an alternative, but a necessity.
With continuing concerns about possible non-point pollution from turfgrass practices, it is time to look more closely at cultural management to determine not if, but how, we can do a better job of managing weeds.
Philip Busey, Ph.D., is associate professor of environmental horticulture at the University of Florida (Fort Lauderdale, Fla.).
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