How to Prevent Weeds Before They Start (The Long Island Pre-Emergent Program)

Here are the exact amounts per 1,000 square feet, which work for almost every Long Island lawn (Kentucky bluegrass, ryegrass, tall fescue). The three prodiamine rounds, which you spread: Round 2 in early spring is 1.8 lbs of granular (or 0.16 oz liquid), Round 3 in late spring is 2.0 lbs granular (or 0.18 oz liquid), and Round 5 in the fall is 2.4 lbs granular (or 0.21 oz liquid). The two pendimethalin rounds, which you spray, are Round 1 in late winter and Round 4 in mid-July, both at the bag rate, about 1.6 fl oz liquid per 1,000. If you ever miss the early-spring Round 2, the reserve is Dimension at 0.37 fl oz liquid per 1,000. The one exception: a shady fine-fescue lawn is more sensitive, so drop every prodiamine round about 25 percent. Always measure your square footage first (length times width) so you apply the exact amount, since too little leaves gaps and too much can thin the grass.

Five times a year, and you start in late winter. Round 1 is late winter (late February to early March), a pendimethalin spray that catches early-spring weeds and the spring POA flush. Round 2 is early spring (around April 6), a prodiamine barrier down before crabgrass sprouts, this is your main crabgrass round. Round 3 is late spring (late May into June) to reinforce it through summer. Round 4 is mid-July to hold coverage into fall. And Round 5 is the fall round (Aug 25 to Sep 5), a prodiamine barrier that stops annual bluegrass (POA), the single most important round of the year, since most POA sprouts in the fall. Time each round by soil temperature and the calendar together, and use the Weather Hub to catch each window. One firm rule: there is no November round. POA sprouts in late summer, so a November application is too late, the fall round in late August is what handles it.

You only need two products, and both are green-tier, meaning any homeowner can buy and apply them: granular prodiamine (a 0-0-7 product with prodiamine is ideal, because it also skips Suffolk's winter fertilizer ban) and pendimethalin (sold as Pendulum AquaCap). Prodiamine covers the three main barrier rounds and pendimethalin covers the two bridge rounds, so those two run the entire program with no license needed. Two other products are amber-tier, legal but restricted, and completely optional: Dimension (dithiopyr), which is commercial-applicator-only in New York and only used as a backup if you miss the spring round, and prodiamine 65 WDG, a New York restricted-use product for people who want to mix a precise liquid. Finally, one thing is red-tier and banned here for everyone regardless of license: oxadiazon on lawns. Bottom line, your kit is two products and two tools: granular prodiamine with a broadcast spreader (for Rounds 2, 3, 5), and Pendulum AquaCap, a liquid, with a sprayer (for Rounds 1, 4). That covers the whole program.

Not on the same area, because a pre-emergent barrier blocks every seed from sprouting and cannot tell your good grass seed from weed seed. If you barrier an area and then seed it, you kill your new grass along with the weeds. So you pick one per area each year: prevent weeds, or plant grass. If you need to overseed this fall, use the workaround: skip Round 1 on that area, use Tenacity (the one pre-emergent that is labeled safe to use with new grass seed) when you plant, then put prodiamine down in early November after you have mowed the new grass twice. A lot of people split their lawn, running the full prevention program on the established areas and reseeding only the thin spots. That way you get both, just not stacked on the same ground the same season.

Because Long Island soil is sandy and drains fast, so a pre-emergent barrier only lasts about 8 weeks here before it breaks down, much shorter than the 3 to 4 months you might read about for heavier soils. One spring application wears off by early summer and leaves the rest of the year unprotected, which is exactly why people who apply once still fight crabgrass in July and POA in the fall. The 5-round program keeps a fresh barrier in the ground across every window when weeds try to sprout: late summer and late winter for annual bluegrass, and spring through mid-summer for crabgrass. It uses two products on purpose, prodiamine for three rounds and pendimethalin for two, so that each stays within its yearly legal limit while together they cover the whole calendar. It is more work than a single application, but it is the difference between mostly-weed-free and constantly playing catch-up.