Pre-emergent and herbicide rate calculator

Crabgrass preventers and other lawn herbicides are dosed by area, and the label tells you the rate: pounds of granules, or fluid ounces of liquid in so many gallons of water, per 1,000 sq ft or per acre. The calculator does the multiplication for your lawn, or works from an active-ingredient rate if that is what you have. It only does the arithmetic.

The label is the law

The U.S. EPA points out that pesticide labels, unlike most product labels, are legally enforceable: every one carries a statement that using the product in a way its labeling doesn’t allow breaks federal law. So the rate, the maximum per year, the timing, the protective gear and the wait before seeding all come from your label. This calculator does one job — turning that rate into an amount for your area — and every number below is an example only, not a recommendation for any product.

Two ways to work out the amount

Most homeowners work from the label rate: an amount of product per 1,000 sq ft or per acre. Extension guides and turf professionals often work from an active-ingredient (a.i.) rate instead, because it compares products of different strengths. University of Minnesota Extension’s method for converting it: divide the a.i. wanted by the fraction of a.i. in a dry product, or by the pounds of a.i. per gallon in a liquid.

label rate:   product = label rate × area
              (per 1,000 sq ft: × area ÷ 1,000 · per acre: × acres)
granular a.i.: product per acre = lb a.i. per acre ÷ (a.i. % ÷ 100)
liquid a.i.:   gal of product per acre = lb a.i. per acre ÷ lb a.i. per gal
1 acre = 43,560 sq ft · per acre ÷ 43.56 = per 1,000 sq ft

Mixing a sprayer tank

Liquid labels give the product and the spray volume together, e.g. so many fluid ounces in so many gallons of water per 1,000 sq ft. The calculator totals the spray mix for your area, divides it into tank loads for your sprayer, and gives the product for each full tank. In the third example above, a 1-gallon sprayer needs 10 loads, each with 0.375 fl oz of product — about 2¼ teaspoons (1 fl oz = 2 tablespoons = 6 teaspoons). Amounts that small are why a measuring cup or syringe marked in millilitres, kept only for garden chemicals, is worth having.

The spray volume only works if your sprayer really puts it out at your walking pace. Fill it with plain water, spray a measured 1,000 sq ft (or 100 m²) at a steady pace, and see how much you used; adjust your pace or nozzle until it matches.

Timing and watering in

• Before the weeds come up. A pre-emergent works on seeds as they germinate. University of Minnesota Extension times crabgrass preventers for before the soil reaches about 55 °F.
• Water it in. Without water the herbicide stays on the surface and seeds germinate under it; the label says how much water and how soon.
• Not on new or thin grass. Pre-emergents stop grass seed as well as weed seed. Don’t use them on weak or recently seeded lawns, and check the label for the wait before overseeding — see the grass seed calculator for the seeding side.
• Some labels allow a split application — two smaller doses weeks apart. The label gives the maximum for the year; add both doses up against it.

Spreading granules evenly

Set the spreader from the label, then check it: weigh the product for a measured 1,000 sq ft strip into the hopper, run the strip, and adjust until it comes out right. Two half-rate passes at right angles hide small errors. Sweep granules off paving and back onto the lawn so they don’t wash into drains. The same routine is described for fertilizer in the lawn fertilizer calculator; weed-and-feed products are dosed by the herbicide, so use this calculator and the label rate for them.

Need the area first? The lawn area calculator adds up rectangles, circles and triangles.