Garden sprayers must be calibrated to deliver the proper herbicide application rate to the lawn. Spray nozzles must be clean, operating correctly, and emitting the correct volume which means the sprayer must be calibrated before applying herbicides. (image courtesy: www.lawnlove.com).
By: John Green, Certified Texas Expert Gardener, Orange County, TX
A few weeks back (beginning of September), we discussed lawn weed management. Once the soil temperature cools, gardeners can employ preemergence herbicides to edit many common cool season weeds. Turfgrass specialists recommend applying preemergence herbicides to lawns to disrupt the germination (or emergence) of many weeds in the spring. Lawn weed management and control can be easily maintained when executed properly. Fall preemergence herbicide lawn treatments will control many weeds but gardeners must read and follow herbicide product labels, otherwise the preemergence herbicide will fail! Let’s review a few reasons why a preemergence herbicide might fail.
Application Specification- Herbicide labels contain a significant amount of detailed information regarding application timing, application rate, spray volume, soil conditions, best way to incorporate, and how to activate the herbicide. Effective weed control and management strategy begins with reading the manufacturer’s product label in its entirety!
Barrier Disruption- Preemergence herbicides create a chemical barrier on the soil surface. The barrier is approximately ½ to 1-inch thick but varies depending on product, soil type, and incorporation method. Typically, most weed seeds germinate within the top ½ inch of the soil. Herbicides control weeds as they germinate within the chemical barrier, rather than preventing germination. Unfortunately, many gardeners (me included forget) disrupt the chemical barrier formed after application by moving planters, dragging pots, mowing, or walking across treated lawns. Any disruption to the chemical barrier becomes an opportunity for millions of weed seeds to germinate.
Weed Spectrum- There is no single preemergence herbicide which can provide control of all weed types. When spraying herbicides, use a product effective in controlling both broadleaf and grass weed types. Using two herbicides simultaneously may prevent an uncontrolled species. Some granular herbicides contain two active ingredients for broad-spectrum control, while others contain only one. Read the label for the granular herbicide to verify it will provide control for the prevalent weed population in the lawn.
Irrigation Volume- The herbicide label provides detailed instructions for irrigation and when to apply. There are some herbicides which state that if the product is watered within three weeks, the herbicide will be effective. Of course, the optimal condition would be in using rainfall for watering the herbicide though a single rain event which provides the complete volume of water needed. This SETX and our rainfall events may not meet the products’ watering requirements within three weeks, which will require supplemental watering! If the cumulative rainfall over 3 weeks is greater than ½ inch, the herbicide should be adequately watered.
Excessive Watering- Drastically reduces an herbicides effectiveness. Increased irrigation can rapidly leach the herbicide away from the soil surface, resulting in the soil surface retaining too much moisture (causing microbial degradation of the herbicide), and excessive water allows weeds to survive (due to herbicide dilution) which would otherwise experience the sub-lethal effects of herbicide toxicity.
Application Rate- There is a specific concentration at which an herbicide will inhibit weed growth according to the manufacturers’ precise application rate. When applying at the recommended rate, a chemical barrier forms on the soil surface, ensuring the appropriate herbicide concentration is utilized to inhibit weeds from growing for 2 to 3-months. When “less than recommended” application rates are used, the herbicide concentration in the chemical barrier is too low and quickly degrades, to where weeds will successfully germinate and grow.
Fertilizer Application- Top-dressing fertilizer is a common practice in lawn maintenance which will lead to poor weed control. Fertilizer application tends to increase microbial activity due to elevated nitrogen concentrations (caused by top-dressing), and the increased microbial activity degrades the herbicide more quickly.
Spray Volume– Sprayers must be calibrated to deliver the proper herbicide application rate to the lawn. Spray nozzles must be clean, operating correctly, and emitting the correct volume which means the garden sprayer must be calibrated (using water only)…before applying herbicides. Granular herbicide applicators are easy to calibrate, but ensuring the application is uniform is not. Gardeners, it is important to note that applying too much herbicide will cause plant injury, whereas too little will result in poor weed control. Application uniformity is paramount, so consider applying the total amount of herbicide for an area in 2 or 3 passes.
Existing Weeds- This is the number one reason herbicides fail to provide weed control, as preemergence herbicides will not kill existing weeds. Many of the herbicides used on lawns work by inhibiting root growth, but even small weeds can develop robust root systems or may have long tap roots. Weeds with existing roots, greater than one inch in length, will not be affected by preemergence herbicide. Some weeds germinate within days when provided optimal growing conditions. It is recommended to apply a preemergence herbicide in August/September (or October) for the winter growing weeds and February or March for warm season weeds. Often, another application is necessary during the summer.
Preemergence herbicide active ingredients must be considered before application! The preemergence product label will list weeds that the product will control (perennial, annual, and broadleaf). Preemergence herbicides are most effective on annual weeds, whereas other weeds must be controlled utilizing post-emergence herbicides. Identifying weeds which are to be controlled is important to ensure the correct herbicide is selected. My recommendation is to use granular herbicides for ease of application. It is important that gardeners have knowledge of seasonal and enduring weed populations to determine product selection. Selecting the correct active ingredient, then applying it properly are the most important aspects of weed management and control.
Until next week fellow gardeners, let’s go out and grow ourselves a greener, more sustainable world, one plant at a time! Gardening questions answered: jongreene57@gmail.com.
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