If you were playing Trivial Pursuit, Jeopardy or some other similar game, and the question in front of you was: “On average, how many weed seeds does the average cubic foot of soil contain?” How would you answer? A dozen? A hundred? A thousand? Actually, weed scientists tell us that the average cubic foot of soil contains between 10,000 and 40,000 weed seeds. It’s best thought of as an endless supply.

Considering how abundant the source is, it’s no wonder how much time and effort is spent on weed control on athletic fields — not to mention how much money. Weeds are a problem for several reasons: they compete for water and nutrients, they cause turf stands to be thinned, and possibly most importantly, they are found to be objectionable because they are different from the desirable turfgrass.

Identification

Even though it may seem like a step you’d like to skip over, identification is critical to success. In short, you gotta start here. This is particularly important with weeds that look like one another, such as ground ivy and henbit or yellow woodsorrel, white clover and black medic. In some cases, product selection or timing will be dictated by the species, and it’s important to start with the correct identification.

There are many guides to assist in identification, including “Integrated Turfgrass Management for the Northern Great Plains” by Baxendale and Gaussoin. Photo guides are also available from many of the herbicide manufacturers. These are helpful when placed in maintenance carts; they can be pulled out and used quickly in the field. Also, the University of Illinois, Michigan State University, Iowa State University, Virginia Tech University, the University of California, Purdue University and others maintain websites that guide users through the identification process.

Postemergence strategies

  • Sometimes taken for granted, a thick, healthy turf stand is the best weed control, either pre or postemergence. Due to the shading that a dense canopy produces, weed seeds are less likely to germinate than in more open stands. In thick stands, competition from roots for water and nutrients is strong, and the combination of the two factors produces desirable results. The best part is that these effects are free. After all, the coaches, spectators and players want thick, healthy turf anyway, so good weed control is just a bonus.
  • Another freebie is timing. In general, it’s easier to control grassy weeds as soon as you see them and broadleaf weeds in the fall. In summer, many factors are not in your favor. During hotter periods, many weeds grow a thicker cuticle as a water conservation adaptation. This layer greatly reduces the absorption of applied herbicides. Thus, after the heat of summer is over, a well-timed application will be effective on weeds such as dandelions and plantain. Hard-to-control species, such as wild violets and white clover, often require two to three applications. Spread these out two weeks apart in fall, or as the herbicide label dictates.

  • A part of timing involves the growth stage of the turfgrass. Try to spray weeds with a little leaf area on them – this is tough to do on fields that are mowed every day or every other day, but when possible, allow the weed to gain some surface area before spraying to help with absorption of the herbicide. Mowing before application reduces the amount of weed foliage available to intercept the chemical and causes stress to the weed, which reduces herbicide uptake. Mowing after application may remove the treated portion and prevent translocation to the roots.
  • Yet another part of timing involves consideration of irrigation and rainfall events. Unlike preemergence herbicides that are only effective when they come into contact with the upper layer of soil, postemergence products are only effective if they remain on the foliage for a couple of days. Thus, avoid watering after application for at least a day. Watering can wash the herbicide off the plant. Naturally, rain can also be a diluting material, so avoid spraying if rain is expected within 24 hours. In short, try to keep it on the leaf.
  • The technique of weed mapping can be helpful, especially when many fields are involved. Many weeds that are controlled on a postemergence basis are perennials such as white clover, plantain, ground ivy, wild violets and field bindweed. Because they are perennial, they have a well-established root system and tend to persist for many years. Keeping track of the location of these weeds on a year-to-year basis can be quite helpful in controlling them. After identification, record the specific location of each weed infestation that you encounter. This will save time and effort when instructing your staff to spray weeds, as they will be able to drive directly to them rather than wasting time hunting them down.
  • In the not-so-aboveboard days of old-time Chicago politics, the phrase “vote early, vote often” was commonly overheard. Though not a good practice for public policy, early and frequent action is beneficial for weed control. Whichever term you’d like to use – observe, notice, monitor, scout – do it on a regular basis. The idea is to kill the escape before it matures, because it’s always easier to control a young weed than a mature one. If you’re on the lookout for weed infestations frequently, success is more likely. Control is easiest to achieve when weeds are small, healthy and actively growing. As the weeds age, changes in the leaf surface, growth habit and physiological function occur. These changes result in reduced herbicide uptake and movement in the vascular system of the plant.
  • In some situations, the use of an adjuvant may increase the degree of effectiveness of a postemergence herbicide application. Adjuvants are defined as anything added to the spray tank to improve pesticide performance. When applying herbicide products, the natural tendency is to try to do something special, something beyond normal considerations. The use of adjuvants falls into this category.
  • Adjuvants can be best thought of as a tool, one of many tools in the pest control arsenal. Adjuvants include surfactants, which decrease surface tension of the spray mixture providing better coverage and improved dispersion in the spray tank; stickers, which help the product stay on the target under adverse situations such as wind or rain; drift retardants; and marker dyes to name a few.
  • Investigation of the need for an adjuvant is a wise step before addition to the spray tank. Some herbicide products already have surfactants existing as part of the formulation and don’t require adjuvant addition, while some weed control efforts would benefit from their addition. Critical to the use of adjuvants is understanding that, like their herbicide counterparts, rates are critical and overapplication may actually decrease performance or increase nontarget injury.
  • A phrase too often heard in green industry circles is “when all else fails, read the label.” Of course, the phrase indicates the correct action, just in the wrong order – it should be “start by reading the label” instead. In addition to information regarding adjuvants, typical recommendations that an herbicide formulator may make via the label are the need for adjustments to the pH of the tank water, optimal water carrier volume and restrictions on the total quantity that should be applied in a year’s time. A thorough understanding of the label instructions is crucial to successful weed control.
  • A simple but important piece of information from the label is the rate. Use the rate that is called for. Millions of dollars of product testing and thousands of hours of research have gone into the recommendation rely on it. Plus, a caveat, using a higher rate than specified on the label is illegal.
  • Finally, use the strategy of persistence. Even with the utilization of the newer herbicides, if all of the other factors are executed correctly, it’s pretty rare to see an entire population of a hard-to-control perennial broadleaf or grassy weed wiped out with one application.
  • Make careful notes about the presence of each weed species for each field that you manage. In addition, success will require communication with the owners and coaches to inform them that this is not an easy endeavor, that more applications are highly likely and less than 100 percent control is not necessarily a measure of your ability so much as the nature of the beast.