Put pre-emergent down a week too late in Reno and you’ve paid for a product that watches the weeds come up. Pre-emergent only stops seeds that haven’t sprouted yet, so timing is the whole game.
Northern Nevada lawns get two windows a year, one in fall and one from late winter into spring. Each one stops a different group of weeds, and most lawns do best with both.
Our weed control program is built around those two windows, and we time each application to soil temperature and the weather.
When to apply pre-emergent in Reno:
The two windows line up like this:
The forsythia are a stand-in for what actually tells weed seeds to sprout: soil temperature. Get the product down 10 to 14 days before the top inch of soil stays around 55 degrees at daybreak.
A soil thermometer pushed an inch or two into the lawn beats the date on your phone. Spring soil temperatures bounce around with the weather, so check a couple of times a week in March.
Timing cuts both ways. Too early and the product can wash out or wear off before the season is over. Too late and the weeds are already up.
The weather gets a vote, too. Some years there’s still snow on the lawn in March. Last spring, early March hit the 80s and weeds came up ahead of schedule. Our spring round takes about five weeks to reach every home, so we watch the forecast the whole way.
Some products work better split in two. Two smaller spring applications about 60 days apart give more consistent control than one big dose. The second round carries the barrier further into summer, when weeds are still sprouting. Check that the label allows a split before you plan on it.
Missed the window? Then it’s a job for post-emergent, on whatever has already come up.
Pre-emergent won’t touch perennial weeds, in any season. It works on seeds that are just sprouting, mostly annuals. Dandelions, clover, and other perennials come back from their roots, and the barrier never reaches them.
Fall is the window for the weeds that are already green when the snow melts. Winter annuals sprout with the first fall rain, sit through the winter, and take off very early in spring. By then their roots are set, and a spring application is too late for them.
Wait for the fall rains and apply in October or November. If the fall is dry, apply anyway and water it in well. Granules left sitting dry on top don’t do much. That timing stops cheatgrass, mustards, common chickweed, and annual bluegrass.
TMWA wants sprinklers off by Nevada Day, so in a dry fall, apply early enough to water it in before the shut-down. Our guide on how to winterize your sprinkler system covers the shut-down itself.
Pre-emergent doesn’t kill the seeds. It waits for them.
When a seed sprouts, its first roots and shoots grow into the treated layer of soil and take up the herbicide. Cell division in the young roots stops, and the seedling dies before it reaches the surface.
A few practical things follow from that:
Pre-emergent also can’t tell a weed seed from grass seed you want. Labels often call for a wait of several weeks to a few months between the two. Go by the label on your bag.
Rake out dead plants, leaves, and other debris first so the product reaches the soil.
Once it’s watered in, leave the soil alone. No digging, edging new beds, or other soil work in treated areas for as long as you want the barrier to hold.
Follow the label for protective gear. Some products call for gloves or long sleeves while you apply them. Keep kids and pets off the treated lawn until the product has dried or been watered in, as the label directs.
Don’t apply it near water sources, or where runoff can reach plants you want to keep.
Pre-emergent comes in two main forms: granules you spread and liquids you spray.
Granular pre-emergent comes as small pellets. If you’re doing it yourself without a sprayer, we’d go granular. A broadcast spreader does the work.
Sweep granules off the sidewalk and driveway when you’re done. Some products, such as those with pendimethalin, can leave a stain on concrete that outlasts the weeds.
Liquid pre-emergent gets mixed with water and sprayed, and it’s what we use. It can cost less per square foot, but you need a sprayer and careful measuring to get the rate right.
Either way, even coverage matters most. Calibrate the spreader or sprayer first, because any gap in the barrier lets weeds through. Walk the lawn in two directions at half the rate each time to avoid stripes and missed spots.
The active ingredient matters more than the form. Prodiamine, pendimethalin, and dithiopyr are common ones. Each label lists the weeds it prevents, so check that it covers the broadleaf weeds you’re seeing, not just grasses.
Whatever you buy, read the whole label and follow its rate.
Pre-emergent stops weeds that grow from seed. In local lawns, broadleaf weeds are the main target, along with a few weedy grasses. These are the regulars:
Not sure what’s growing in your lawn? The UNR Extension office will identify weeds if you bring in a whole plant, roots included, or email clear photos.
The best long-term defense is thick grass. Thin, patchy lawns fill with weeds, and healthy lawns have far fewer.
Mow at least 3 inches high and water deeply. Add a fertilizer schedule that fits your lawn, and there’s less bare soil for weed seeds to land on. Pre-emergent works best as one part of that routine, not a replacement for it.
Pre-emergent stops weeds before they come up. Post-emergent herbicides kill young weeds that are already growing, and they’re usually sprayed onto the leaves.
Spraying a weed that’s already dead just wastes product.
We use both in our weed control program, because hitting weeds at both stages slows their spread. Pre-emergent handles the annual weeds you can predict. Post-emergent handles whatever gets through, plus perennials like dandelions. Our guide to pre-emergent vs post-emergent explains how the two work together through the year.
In spring, before the top inch of soil reaches about 55 degrees for several mornings running. In fall, skip the thermometer and go with the rains, in October or November.
Usually not, because most products stop grass seed too. A few mesotrione products can go down at seeding on some grasses. Check the label, or schedule core aeration and overseeding for a season you’re not treating.
Usually three to six months. Prodiamine tends to outlast pendimethalin, and the label gives the number.
Cory’s Lawn Service runs a 5-step weed control program with pre-emergent and post-emergent treatments, applied by spray technicians licensed through the Nevada Department of Agriculture. If you’d rather not chase soil temperatures every March, our weed control in Reno program times both windows for you. Take your weekend back.
