Agronomy – Controlling Winter Weeds

Congratulations! We have officially made it to September and the worst of the Summer heat is behind us and we are all looking forward to some amazing golf course conditions this Fall. The last thing you want to think about is turf/lawn weeds but if you don’t want to stand in a sea of Chickweed, Henbit and Poa annua next spring now is the time for action. One of the most asked questions I get around the club for home lawns is when should I apply pre-emergent herbicide to control Winter weeds? For all you loyal readers, this will be a review of some things with a September spin. Let’s take a look at some of the basics to controlling weeds in your turf. 

First Things First

It is important to know what type of turf grass you have in your lawn before you start developing any weed control program. You may remember this from our February article but it bears repeating. The most common turf grass types in North Texas include Bermuda grass, Zoysia grass, Saint Augustine, centipede grass and Fescue to name a few. Each type of grass has different light, soil and fertility requirements as well as growth and environmental factors so knowing that you have picked the best suited grass for your lawn’s environment is important. Healthy turf is the first step in controlling invasive weeds as the base turf can suppress many weeds before they can establish themselves in the sward (turf stand). Assuming that you have the right grass in place we will move on to defining what a weed is and how to control weeds (focusing on winter or cool season weeds) that will eventually challenge your desire for a beautiful lawn. 

What is a Weed?

What exactly is a weed anyway? For our purposes today we will define a weed as a plant growing where it is not wanted and in competition with desired/cultivated plants. So essentially any plant that is growing where it is not wanted is a weed so Bermuda grass growing in a Saint Augustine Lawn is considered a weed. In terms of lawn or turf weeds, they come in two basic groups, winter or cool season weeds (henbit, chick weed and poa annua (annual bluegrass or common meadow grass to our friends from the UK) and summer or warm season weeds (goose grass, crab grass and nutsedge). Weeds begin to germinate at different times of year. The cool season weeds germinate generally in early September through November and the warm season grasses from mid-February through May. There are two basic approaches to weed control, physical and chemical. Physical weed control is pretty simple you literally remove the weed physically. This is often impractical depending on the number and establishment of the weed population. Chemical weed control is achieved through the use of herbicides (an agent, usually chemical used to kill plants). Herbicides also come in two primary groups, pre-emergent and post-emergent herbicides. Pre-emergent herbicides are applied BEFORE the weed plant germinates killing the plant just after seed germination before a root can establish in the soil. Post-emergent herbicide is applied to existing weeds and often require direct contact to be effective. There are a few pre-emergent herbicides that have some post emergent characteristics but by in large you will make the choice of herbicide based on the time germination of the weed. Post emergent herbicides can also be selective (kill certain plants but will not harm others) and non-selective (will kill or damage most plants that contact the product).  

Timing is Everything!

It is critical that a pre-emergent herbicide be applied properly and at the right time of year to be truly effective.  So now is the time to apply pre-emergent to control winter or cool season weeds, starting in late August through mid-September. We will not go into all the factors that affect weed seed germination just know that pre-emergent herbicides are effective if they are applied before the weed seed germinates. In fact, pre-emergent herbicides prevent the germination of weed seeds by inhibiting a key enzyme and should be applied Spring and Fall to provide year round weed protection. It is important that the herbicide make good soil contact to be effective so watering in the product is essential. Since its September and we are focusing on Winter or cool season weeds it’s important to know that the king of the cool season weeds is Poa annua, Poa as it is known in the business is a prolific seeder and highly competitive weed that often develops resistance to herbicides so changing the active ingredient of the herbicide from time to time is a good strategy. If you let Poa grab a foothold it will soon take over your lawn, increasing its numbers each year. A single Poa plant can produce 100 seeds in 8 weeks. So learn what it looks like (ID Weeds is a great App) and plan accordingly. Remember to try to have your pre-emergent down before September 15th and sooner is better than later. Most home owners will likely choose a granular pre-emergent but others prefer sprayable formulations. These can be purchased from your local nursery or garden center but remember to follow the label recommendations on any pesticide including all safety instructions. Below are some effective options. 

Prodiamine (Barricade equivalent) is the longest-lasting, and most economical due to lowest application rates.

Isoxaben (Gallery equivalent) prevents a very large list of Broadleaf Weeds. …

Oxadiazon (Ronstar equivalent) is a great granular option – safe in all turf and ornamentals.

Dithiopyr (Dimension equivalent) versatile product that at higher rates has some post emergent activity

Once you have purchased your product simply follow the instructions on the product label that is attached to the bag/container. If you have questions or want the latest tips on turf grass in Texas the Texas A&M Agrilife Extension program can help either online or by phone. To improve your results, you may consider making a spilt application of the herbicide. This involves making two applications at 50% of the recommended product rate. This allows one application early in the season and a second several weeks later to offer a larger window of control without over applying the active ingredient of the herbicide. If you are making a granular pre-emergent herbicide application here is a quick reminder of how to calibrate your rotary spreader. Note that if you intend to apply fertilizer and pre-emergent separately apply the fertilizer first water in and wait several days before applying the pre-emergent as directed by the product label. 

How to Calibrate a Rotary Spreader

Determine the amount of fertilizer/product needed for 100 sq. ft. For example, the fertilizer bag weighs 50 lbs. and covers 10,000 square feet per the label recommendation

Place this amount in the spreader.

Mark off the area. For a rotary spreader, use a 10-foot by 10-foot area (100 sq. ft.).

Apply using the suggested label setting to the test site

Adjust the setting up or down based on the actual amount of fertilizer applied, if you covered the area completely with nothing left in the spreader you are perfect! If you have product left in the hopper increase the spreader setting, if you run out of product before the area is covered then decrease the setting. Repeat this process until you find the correct setting.

Take notes for future reference

Complete the application for the rest of the lawn 

One Last Thing

Weed control is an ongoing process of controls and actions that strengthen the base turf grass and eliminate invasive plants. It is September so if you see mature weeds in your lawn they are Summer weeds (see paragraph three) and will not be killed by a pre-emergent herbicide applied now. Just remember that the Fall (September) pre-emergent application prevents Winter weeds and a Spring Pre-emergent application prevents Summer weeds. There are no 100% effective pre-emergent weed control products available so using some physical removal and post emergent herbicide applications are also required to be most effective. When it comes to weed control an ounce of prevention is worth a pound of cure. Be diligent, safe and keep reading Avid Golfer magazine and you’re your perfect lawn will soon be a reality. 

Anthony L. Williams, MG, CGCS, CGM is the Director of Golf and Landscape Operations at the TPC Four Seasons Golf and Sports Club Dallas at Las Colinas, he is an award-winning environmental author with a degree in horticulture. Anthony can be reached at anthony.williams@fourseasons.com.