` Selecting Herbicides for the Home Lawn Dr. Shawn Askew Virginia Tech

Select Based on Active Ingredient There are over 700 consumer products in Virginia that contain only 33 unique active ingredients of synthetic herbicides. Active ingredients are often listed by the full chemical name so a table of these chemical names is provided in the Lawn: Weeds Chapter of the PMG to help you identify the ingredient. The next step is to determine which product works best and we cover that in this presentation.

Which Product Works Best? Testing several hundred consumer products for effective weed control is not possible. At Virginia Tech, we have tested the active ingredients in consumer products and found them to be safe and effective for weed control in the lawn. These ingredients were tested using professional products at rates that are often higher than those listed on consumer product labels. When you select a consumer product from the shelf, you will need to first identify the active ingredients using Table 3 in the Lawn: Weeds Chapter of the PMG. Next, confirm that the product is approved for use on your species of lawn grass. Most lawn grasses in Virginia are tall fescue, Kentucky bluegrass, fine fescue, or perennial ryegrass and these are all called “cool-season grasses”. Eastern and southern portions of Virginia commonly have bermudagrass, zoysiasgrass, centipedegrass, or St. Augustinegrass for lawn grasses and these are called “warm-season grasses”. Some products can be used on all grasses and others will harm one type or the other. We now need to determine how the application rate specified on the consumer product compares with professional rates that have been tested at Virginia Tech and found to effectively control the weed.

Professional Equivalency Formula The professional equivalency formula was invented at Virginia Tech and is a method to compare consumer products to professional rates. In the tables of the Lawn: Weeds Chapter, you will find a column for “Professional Equivalency Constants”. These numbers are not meant to be compared to one another, rather, they are meant to be used in the Professional Equivalency Formula. The formula uses simple math to determine the fraction of the Virginia Tech Recommended Rate (A.K.A., the “Professional Rate) that is applied when using a consumer product. The formula is intended to allow consumers to quickly assess the effectiveness of multiple products and choose the product that most closely matches the professional rate recommended by Virginia Tech. The formula requires 3 things that one will easily find on the consumer product label and divides the result of these 3 by the professional equivalency constant. Before we look at the actual formula, let’s examine how to find the 3 things you will need from a selected consumer product label. You will need the percentage active ingredient, amount of product in the container, and area of your lawn treated by the product; we labeled these 3 components “A”, “B”, and “C”, respectively.

Example 1: A Crabgrass Killer In Table 5 of the Lawn: Weeds Chapter of the PMG, you will find a list of active ingredient combinations that kill crabgrass. If emerged crabgrass or other annual grass plants are your target, use this table to select and compare consumer products. We are going to use Table 5 to provide the professional equivalency constant and to give us a definition of A, B, and C; the three things we must find on the consumer product label. First we check the active ingredients list on the consumer product and find that this product contains 2,4-D; quinclorac; and dicamba (see next page for an image of the active ingredients section of this product). When we compare to Table 5, we find our product matches row #3 and has a professional equivalency constant of In the next few pages, we will demonstrate how to find A, B, and C; the three things we need to insert in the professional equivalency formula.

“A” = active ingredient % listed on the product. For crabgrass control, we are only interested in the amount of the ingredient that kills crabgrass (see Table 5 in PMG Lawns Chapter), in this case we are only interested in the amount of quinclorac. For this product, we would enter 1.61 in the formula for “A” because there is 1.61% quinclorac in this container. A

“B” = The amount of product in fluid ounces. For this product, we would enter 32 in the formula for “B” because the product contains 32 fl oz. B

“C” = The area treated in thousand square feet. For this product, we would enter 5 in the formula for “C” because the product treats 5000 sq ft. C

Using the Formula The professional equivalency formula is as follows: Fraction of professional rate = ((AxB)/C)/P Let’s break this down step by step: First multiply A x B; in our example that is 1.61 x 32 = Next divide the product of A x B by C; in our example C was found to be 6 because the product treats 6000 sq ft. So divided by 6 is equal to The last step is to divide the result of (AxB)/C by the professional equivalency constant or “P”. So we consulted Table 5 in the Lawn: Weeds chapter and found that the professional equivalency constant for a product that contained 2.4-D, dicamba, and quinclorac is So we divide 8.59 by and get That means our product will be applied at 0.69 times the professional rate.

Interpreting the Result The product in our example was 69% of the professional rate. The professional rate has been tested at Virginia Tech and found to work but the lower consumer rate has not. Since it breaks federal law to apply the consumer product in any way other than that described on the label, we can’t legally apply the product at a higher rate to make up the difference. So how do we deal with this situation? Option 1: Treat younger weeds. Smaller weeds are easier to kill and lower rates of any herbicide will be more effective than when treating mature weeds. Option 2: Apply the product multiple times. It is important to consult the fine print on the label to determine when you can legally repeat the treatment and how many times you may treat the product or you may risk breaking federal law by applying the product improperly. You can also kill your lawn grass by making repeat treatments without consulting the label, so read the label. Most products only allow for two treatments. Two to three week intervals between treatments are typical for most products. Option 3: Purchase and apply another product to make up the difference. If two products are each found to be applied at two thirds of the professional rate, apply one at its full recommended rate and apply the other immediately after at one half its’ recommended rate. You can also tank mix the products.