Where do weeds on sports fields come from and why are they a problem?

Weeds are spread by birds, wind, water and people, and they interfere with not just aesthetics, but also playability and possibly safety. Sports fields are a prime candidate for weed infestations primarily because the desirable grasses are constantly under stress from foot traffic as well as environmental stress like heat, drought and cold. In most cases, weedy fields are not the cause of poor fields, but rather the result of poor fields. For example, it is far more likely for fields to become infested with crabgrass if the turf is thin and there is bare soil. A dense, healthy sward of turfgrass is the best defense for weed control.

Prostrate knotweed between hash marks from the 30-30 yard lines on this high school football field are a symptom of a bigger problem: severe soil compaction.

Weeds are successful at invading sports fields because they are highly effective at seed production and dispersal, and they have rapid growth. The problem with weeds is that they compete for nutrients, water and sunlight and detract from surface uniformity, thereby affecting athlete performance. Some weeds are also not wear-tolerant, like creeping bentgrass and annual bluegrass, and can cause surface stability issues during games. Weeds are also indicators of other, more pressing, issues. For example, a field loaded with prostrate knotweed generally has severe soil compaction. Weeds like annual bluegrass and yellow nutsedge are typically found on fields that are irrigated too much, and low-maintenance fields are usually infested with white clover and dandelions. The fact that these weeds are in such abundance sends a clear message that the underlying problem needs fixing before a weed control plan can take place.

Tall fescue is a better fit for low-maintenance and nonirrigated athletic fields in the transition and coolseason zones. Kentucky bluegrass gets stressed during summer and needs moderately high cultural input to thrive.

Setting thresholds and choosing control strategy

Setting a threshold for the amount of weeds allowed on a field is a difficult call to make. At the pro and NCAA levels there may be a zero-tolerance approach, but on schools and parks and recreation fields some weeds may be more acceptable. The Ohio State University’s Field Evaluation Document (FED) Guidelines state that more than 40 percent weed coverage is unacceptable and zero to 5 percent is best. The STMA Playing Conditions Index (PCI) also states that more than 40 percent weed cover is unacceptable, but zero to 9 percent is best. In addition, they consider dandelions “moderate” and crabgrass “severe” weeds. For the sake of getting down to numbers, I calculated it out: there are about six dandelions per square foot. A standard football field is 54,000 square feet. If just 1 percent of the field surface was covered in dandelions, there would be 3,240 dandelion plants on the field, which seems like a huge number of weeds. Maybe weed thresholds shouldn’t be determined by percent cover, but by numbers instead. The field manager needs to determine what that number is, dependent upon the expectations of the end user.

As mentioned earlier, the most important weed deterrent is dense, healthy turf. I am frequently asked, “When is the best time to seed?” My response is always, “Whenever you see bare soil or thin turf,” because a dense stand of turf will be better able to deter weeds and fight off insect and disease damage. Using a fast-germinating grass like perennial ryegrass also boosts the field’s chances of preventing weeds. An additional approach is to have the right grass on the field for the sport and location. This is especially important in the cool-season zone where fields are typically made up of tall fescue, perennial ryegrass, Kentucky bluegrass or a mix of all three. High-maintenance turf like Kentucky bluegrass is much more prone to weeds than tall fescue on a low-input, non irrigated field, because it gets very stressed in the hot, dry weather. Likewise, perennial ryegrass is much better than tall fescue on fields mowed short for games like hockey and soccer because tall fescue doesn’t tolerate low mowing heights very well (though dwarf varieties are becoming more available). In addition to aggressive seeding and correct grass selection, management practices have a major bearing on weed populations. Mowing height, timing of coring and verticutting, along with applications of water and fertilizer are all key practices that can affect the turf’s health. There are best management practices (BMPs) or integrated pest management (IPM) programs in each state via the university extension service, turf association or national STMA. Other cultural approaches to weed control have shown varying degrees of success and have included hand-picking or hoeing, heat sterilization, topdressing with compost and mulching maple leaves (this is more relevant to homeowners who mulch their tree leaves).

If weed prevention via overseeding has not been successful or a mature weed-ridden field needs renovating, then it is time to look at weed control options. Postemergence weed control describes the process of applying an herbicide (natural or synthetic) to the weed when it is visible and actively growing. In order for the right herbicide product to be selected, the common rules of control should be followed. These rules can apply to just about any pest, disease or weed that we see on sports fields:

  • Know your enemy
  • Identify its strong and weak points
  • Use a variety of tactics

Natural herbicides are becoming more popular, and in some cases are being mandated by school and park districts. The most common of these is corn gluten meal (CGM), which is a product of the corn milling industry. It is a preemergent weed seed suppressant that is applied heavily as topdressing in spring to prevent crabgrass and in the fall to prevent broadleaf weeds (while this is an article about postemergent control options, CGM deserves a mention). The advantage to CGM is that is also contains 10 percent nitrogen so it has a positive effect on turf health and density. Research has shown that it is not very effective in the first year, but 80 percent control can be achieved in subsequent years. It is an expensive product, but it is worth noting that it prevents seed germination, so sports field managers that have a long-term and aggressive seeding program should use caution. Other natural herbicides include oils, acids, vinegars and soaps. There is also a new natural weed killer called Fiesta, which is iron-based. All of these natural herbicides act as a burn-down product in that they burn weed tissue that they touch, and they do not have soil residual activity. They are best used when the weed is a seedling/very young, and they are more effective on broadleaf weeds than grassy weeds. High spray volumes are needed, an adjuvant helps, and they are more expensive than their synthetic counterparts.

White clover is a leguminous plant that fixes its own nitrogen and survives in low-maintenance situations. Many athletic fields contain clover, and OSU is researching the use of microclovers for low-input turf use.

There are a plethora of postemergent synthetic herbicides available to the sports field manager (Tables 1 and 2). If your budget is tight, a broad-spectrum herbicide will offer the best results for the least money. The efficacy of the herbicide relies upon the four Rs: right product, right rate, right time and right place. The four Rs were originally created for fertilizer recommendations, but they apply to any nutrient or pest control product. In addition, it is important that the sports field manager has a good grasp of herbicide mode of action (systemic versus contact, selective versus nonselective) and also amine versus ester formulations. It is also better to spot-spray rather than blanket spray if at all possible, which is not only more eco-friendly, but also saves money. Lastly, keep in mind that most postemergence herbicides have soil residual activity, which means that there is a waiting period between applying the herbicide and being able to seed into that area.

Pam Sherratt is a sports turf specialist at Ohio State University and served on the STMA board of directors from 2010-2011.