7/19/2022 - By Dr. Jeff Atkinson

Spring preemergence applications are typically thought of as the start of annual weed control programs. Truth is, late summer into fall is a prime opportunity to get ahead on some of the most problematic weeds and enhance turf quality for the next growing season. Below are four ideas to consider this fall for better weed control and turf quality in 2023.

Order of Operations Matters in Late Summer Weed Control

Remember the helpful order of operation mnemonic from math class? “Please Excuse My Dear Aunt Sally.” This short phrase provides guidance on the order in which mathematical operations should be executed to solve a formula: Parentheses, Exponents, Multiplication/Division, Addition/Subtraction. For each equation, there is a proper order of operations.

The same is true for weed control in the late summer/early fall months. This is important because a single active ingredient or blended active ingredient product is unlikely to achieve all weed control objectives. Due to the variety of weeds present, budget constraints, and stakeholder expectations, the order of operations may be different from property to property, but here is an order for consideration:

First, Apply Preemergence for Winter Annuals

Applying preemergence herbicides along with fall fertility is an excellent strategy to gain an advantage on winter annual grassy weeds. Due to the slow metabolic rate of weeds in late fall through winter, many postemergence herbicides are slow to act in cooler months. For this reason, it’s important to prevent winter annual establishment.

Second, Control Perennial Weeds 

Similar to desirable turf, many problematic perennial weeds are preparing for winter dormancy by devoting products of photosynthesis to increasing root carbohydrate reserves. As products of photosynthesis flow down to the roots, it presents an excellent opportunity for applied herbicides to hitch a ride below ground. This accomplishes two things; first, a greater dosage of herbicide active ingredient reaches active sites within overwintering plant parts, such as rhizomes and tubers. This results in weakened structures that either die entirely or lack adequate vigor to survive winter and sprout anew in spring. Second, perennial weeds are growing less vigorously this time of year, which inhibits the ability of a weed to recover from herbicide injury. Bermudagrass and dallisgrass are two weeds that are more effectively controlled with late summer/fall herbicide applications than in-season applications.

Finally, Clean Up Summer Annuals

Summer annual weed control in late summer/early fall can reduce the weed-seed bank for the following year and enhance turf quality, by allowing time for turf to fill voids left by dead weeds before the onset of winter dormancy. That said, if you do not have the resources to apply a preemergence for winter annuals, control perennials, and control summer annuals, the remaining lifespan of summer annuals in late summer/early fall is short-lived. Depending on the weed species present, controlling perennial weeds and summer annual weeds may be achieved with the same application (see point 4 below).

Fertility Matters in Late Summer Weed Control

Turf 101 teaches, ‘the best weed control is a healthy turf stand.’ A healthy turf stand starts with proper fertility. Harrell’s recently conducted a research trial that evaluated the effect of fall fertility, spring fertility, and spring Dimension applications alone and in combination, on turf quality and crabgrass control throughout a growing season. Interestingly, the results indicated fall fertility not only enhanced turf quality the following spring but also enhanced crabgrass control through the growing season (Figure 1). More information on the study can be found here.

Figure 1. Left: Spring Dimension Alone; Center: Fall POLYON® + Spring Dimension; Right: Fall POLYON® + Spring POLYON® + Spring Dimension.

Rotate and Mix Modes of Action to Combat Resistance

Many of us can be classified as creatures of habit. This is true in many things, especially for herbicide active ingredient selection. Unfortunately, confirmation of weed resistance or tolerance to common active ingredients is increasing in a number of species and frequency of occurrence (Brosnan et al., 2020). Predictive models suggest consistent reliance on a single active ingredient, or mode of action, to control a specific weed can result in resistance development in as little as 10 years (Cross et al., 2017; McCarty and Yelverton 2018). The same model suggests that rotation to a unique mode of action every third year can delay resistance development to 45 years, and delay up to 90 years if unique modes of action are applied on a 3-year rotation.

To prevent herbicide resistance from developing, it is imperative to rotate and mix modes of action. Modeling by Busi et al. suggests herbicide mixtures can delay resistance up to six-fold in comparison to single-use or simple herbicide rotations (2019). Several common preemergence herbicides and their modes of action are listed below (Table 1). However, users beware: not all preemergence herbicides are equally effective against all weeds.

Similarly, the development of resistance to postemergence active ingredients can be significantly delayed by rotating and mixing modes of action. If you typically rely on the same postemergence herbicide active ingredient for control of a problematic weed species on your property, consider adding an active ingredient with a different mode of action to the mix this fall. Harrell’s representatives are an excellent resource to identify preemergence and postemergence active ingredients with activity against the most problematic weeds.

Table 1. Common preemergence active ingredients and Herbicide Resistance Action Committee (HRAC) code.

Proper Weed Identification is Key

Proper weed ID is critical to implementing an effective weed control strategy. Dalllisgrass and crabgrass is a common example. To the casual observer, both weeds are easily misidentified for the other. Pylex is an excellent control option for dallisgrass control but will not control crabgrass. Quinclorac is an excellent option for crabgrass control but will not control dallisgrass. Correct identification is the first, and most important step in ensuring the efficacy of a herbicide application.

Figure 2. Smooth crabgrass (Digitaria ischaemum) (left) and dallisgrass (Paspalum dilatatum) (right) are often misidentified for one another. Control options vary between the two species.

There is a lot to consider when formulating a holistic weed control program, let alone a full agronomic program. Each day your Harrell’s representative is working through these specific problems with customers in your area. I encourage you to reach out to your Harrell’s representative to discuss weed control priorities for the fall season and how your program may be enhanced.

References

Brosnan, JT, MT Elmore, MV Bagavathiannan. 2020. Herbicide-resistant weeds in turfgrass: current status and emerging threats. Weed Technology 34: 424-430.

Busi R, SB Powles, HJ Beckie, M Renton. 2019. Rotations and mixtures of soil-applied herbicides delay resistance. Pest Management Science 76: 487-496.

Cross, RB, WC Bridges, LB McCarty, JS McElroy. 2015. Evaluating annual bluegrass herbicide resistance evolution in golf course fairways. Weed Technology 29: 488-500.

McCarty, LB, F Yelverton. 2018. Poa annua control: A perpetually moving target. https://www.gcmonline.com/course/environment/news/poa-annua-control. Accessed 1 September 2021.