Winning The War
By Matthew Bertucci
A weed is most commonly defined as “a plant out of place.” It can more formally be defined as a plant that interferes with the welfare of humans or livestock or is otherwise objectionable. In the case of recreation turf sites or sports fields, any species that differs from the originally planted turfgrass is considered a weed. These species disrupt the uniformity of the turf, which reduces aesthetic quality and functionality.
Species diversity may be desirable in landscaped areas or in pollinator gardens, but guests ready for a picnic or a soccer match will expect a uniform, healthy stand of turfgrass. Large patches of clover are not only unsightly, but they also affect ball-speed and footing for sports fields.
Much like turfgrass species, weed species vary by geographic region, and it is beyond the scope of this article to offer management strategies for all potential weeds. Instead, it will offer general guidelines to prevent weeds from becoming an issue and, when weeds are present, what resources are available to develop a management strategy.
Warding Off The Beasts
When it comes to weed control, there is no substitute for a healthy stand of turfgrass. Herbicide applications and hand weeding are secondary measures and reactive. Instead, it is important to take a proactive approach to prevent the invasion of weeds to a managed turf. The best method is implementing cultural practices to keep a thick, healthy turf without patchiness or thin spots. A dense turf creates a physical barrier that will prevent sunlight from reaching the soil. Many problematic weed species—like large crabgrass—cannot germinate without light.
Here some tips to aid in the fight against weeds:
Prevent. The most basic form of weed prevention is to keep weeds out of the site at the time of seeding, sprigging, or sodding. Thus, managers should buy certified seed, sometimes referred to as “blue tag seed.” Certified seed is inspected for weed seed, ensuring that contaminants are not present or kept below an acceptable threshold. In addition, the germination rates are tested for certified seed. When it comes to seed and sod, you get what you pay for. Trying to save a few dollars on seed or sod will result in increased management costs for years to come.
Select the proper turfgrass. Choose species and varieties adapted to the specific geographic region, according to temperature and rainfall. Turfgrass species will perform best in climatic zones to which they are adapted. For example, bermudagrass and zoysiagrass will perform well in transition zones, while St. augustinegrass and centipedegrass will perform better in warmer zones. A turgrass grown in the wrong environment will perform poorly, despite a manager’s best efforts.
Irrigate. Be sure to supplement rainfall with irrigation when necessary. It is advisable to irrigate deeply and infrequently. Irrigating in this manner will encourage turf root systems to grow deeper into the soil and allow for rapid nutrient-uptake, healthier turf, and fewer weeds. Irrigation should be timed early in the morning to allow deep penetration of water into the soil and also to avoid prolonged leaf-wetness, which favors turf diseases.
Test soil and fertilize appropriately. Conduct annual soil tests of representative turf sites and sample according to the soil-testing facility’s instructions. The test results typically come with recommendations of which soil nutrients and pH need to be amended. Properly fertilized turf will grow thick enough that very few weeds can emerge.
Use proper mowing techniques. It is advisable to mow at the highest acceptable height for the turf’s intended purpose. A general rule is to never cut more than 1/3 of the grass blade in a single mowing event. A higher mowing height will reduce mowing frequency and also allow the turfgrass to develop a deeper root system. Cleaning mowing equipment between sites will prevent spread of weed seed from one site to another.
When The Weeds Show Up
Even the most meticulous and diligent preventative measures cannot keep weeds from invading a property at some point. Many weeds have evolved specialized forms of transport to travel from one site to another: burs may stick to a dog’s fur then fall onto your property, bird droppings can contain viable weed seeds, and many small seeds are carried in the wind. Next time you see a child making a wish by blowing on a dandelion, think of all the seeds that were just dispersed across the property!
However, not all weeds are created equal, so proper identification of weed species is critically important for developing a management strategy. The weed species will determine what chemistries will be effective and may be indicative of what conditions or cultural practices allowed them to occur. The most common types of weeds that arise in turf include grasses, broadleaves, sedges, and rushes. Grass weeds have a similar appearance to many turfgrasses with parallel veins and long-leaf blades. Common grass weeds include goosegrass, crabgrass, and annual bluegrass. Broadleaves typically have netted veination and conspicuous, colorful flowers; problematic broadleaf weeds include plantains, clover, henbit, and dandelion. Sedges and rushes may appear superficially similar to grasses with parallel veins along narrow leaf blades, but they differ physiologically and can be identified by a few distinguishing characteristics. Sedges have solid, triangular stems with leaves extending in three directions, and rushes have solid, round stems.
Weeds also have different life cycles: annual, biennial, or perennial. Annual weeds germinate, produce flowers and set seeds in a single year. Biennials grow vegetative for one year, then produce flowers and seed the second year. Perennials include species that persist for several years and are capable of reproducing by seed and sometimes by vegetative structures, such as tubers or rhizomes. An understanding of the weed life-cycle will inform management decisions. Any weed-management strategy—whether herbicides or hand-weeding—should minimize weed seed production. Killing weeds before seed production will reduce weed populations on your property in subsequent years.
Weed identification comes with experience and practice, so it may be a good idea to buy regional or local resources, depending on the location of your managed site. Weeds of the South, Weeds of the Northeast, and Weeds of the Midwestern United States and Central Canada are all useful books with pictures of weeds at multiple growth stages. Many university websites have weed ID webpages, databases, and smartphone apps that will also aid in identification.
Some things to remember:
Select the proper herbicide. Herbicides must be selected based on their ability to kill the weed species without injuring the turfgrass species. While many chemistries exist with diverse spectrums of weed control, herbicides can be categorized into two general types: pre-emergent and post-emergent. Pre-emergent herbicides are applied before weeds emerge and are only effective as weed seeds are germinating. Post-emergent herbicides are effective on actively growing weeds, but are typically most effective on smaller weeds. Pre-emergent herbicides require knowledge of weed species anticipated at the site because applications must be made prior to weed emergence, while post-emergent herbicides require diligent scouting to catch the weeds when they are small and susceptible to treatment.
Keep records. A final and often undervalued aspect of weed management is diligent record-keeping. If a manager has records of which species were present over the past several years, extension specialists and county agents will be able to give much more helpful advice about which herbicide regimes or management strategies will be most effective and appropriate. Good records will indicate which pre-emergent herbicides to use or determine which weeds to scout for when a post-emergent herbicide is used.
Get help. If you are developing an herbicide program for the first time or encounter an unfamiliar weed species, contact the local county agent or extension specialist. Remember to take several pictures of the weeds and the site. If you are struggling with a weed species, the county agent has probably spoken with several neighbors dealing with the same issue!
Herbicides are labeled for specific uses, and some restricted-use herbicides may only be sprayed by a licensed applicator. Keep in mind: it is the legal responsibility of the applicator and manager to be familiar with the herbicide label before making any application. The label provides all information to safely and effectively make an application. Always read and follow label instructions!
Ideally, your healthy turf will minimize the need for herbicide applications and ensure that, when applied the herbicides kill weeds and allow the turf to quickly fill the gaps!
Matthew Bertucci, Ph.D., is a Research Scientist in Turf, Pasture, & Specialty Crop Weed Management for the Department of Crop, Soil, & Environmental Sciences at the University of Arkansas System Division of Agriculture. Reach him at Bertucci@uark.edu.