“Summer is a great time to treat and tackle those stubborn weeds," shared Mary Jane Watson, Research & Development Senior Scientist – Spruce. "The heat and sun are two powerful elements that contribute to Spruce’s hardworking nature."
Spruce is a non-selective weed and grass killer that we've been testing all season. It works, I can list all the ingredients on my hands, and it's safe to use in the areas our kids and pets play.
I managed to keep up with the new weed growth all spring, but with long days and warm weather, everything exploded, and now I need to up my weed control game.
Timing, as I've learned, matters a lot. Apply too early, and you'll have to cycle back to kill more that emerged (🙋♀️); too late and you risk having to treat large weeds or heat-stressed plants don’t absorb traditional weed killers well.
The simple advice shared by Dr. Watson? Summer works in your favor. As a bonus, we're sharing smart ways to spray for better results, because at this point, we've had lots of practice.
When is the best time to spray weed killer?
Spray when weeds are actively growing—that means early summer, just after they’ve gained some size but before they seed.
Most annual weeds surge in May through mid-June, depending on your region. This active growth phase is when weed killers work best. Broadleaf weeds like dandelions and clover absorb herbicide more easily when leaves are soft and fully open. Grassy weeds such as crabgrass are more vulnerable before they start spreading aggressively.
A calm, dry day with temps between 60–85°F is best. Higher soil temperatures speed up plant metabolism, but if it’s over 90°F, weeds often shut down. Even though Spruce killed everything I sprayed within 24 hours, rain is always a factor, and spraying on a windy day can result in the mist spreading to desirable plants.
For consistent weed control, follow the product label and keep an eye on the forecast.
Why Timing Affects Performance
Weeds absorb chemicals better when they’re growing. During midsummer heat, many plants conserve energy and reduce intake. Spraying during these slow growth periods wastes time and product. Applying early in the day helps improve herbicide absorption, especially if there’s light dew to soften the plant tissue.
Some weeds like summer annuals thrive in hot weather, but even they have limits. Stick to cooler times of day and aim for active, open leaves. That’s when herbicides move into the plant and down to the plant roots.
Timing, weather, and knowing what types of weed you’re facing can make or break your results.
How late is too late to spray for weeds?
Once summer hits its peak—mid-July onward—some weeds go dormant or become too hardy. If the weed has flowered or dropped weed seeds, it’s harder to get effective control. Even strong post-emergent herbicides may only weaken the plant instead of killing it.
You’ll still get some results from spraying in late July, but expect slower progress and maybe the need for a second treatment.
You should also consider the life cycle of the type of weed you're treating. Perennial weeds like plantain or thistle keep coming back with deep roots, while annual grasses tend to die off at the season's end—but not before seeding your yard for next year. At a certain point, you’re better off waiting for a fall application window when cooler weather reactivates growth.
What if I missed the early window?
You can still treat summer weeds; just shift your strategy.
- Pick days when it’s 70–80°F and calm. No wind. No rain.
- Water the lawn lightly the night before to reduce stress.
- Spray in the morning or evening, avoiding hot midday hours.
- Use a product that targets both grassy weeds and broadleaf weeds.
- Check the herbicide labels for reapplication windows and active ingredients.
Not all weed killers work the same (for example, non-selective Spruce kills grass, too). Some selective herbicides target broadleaf weeds without harming grass, while non-selective herbicides kill everything they touch. For healthy lawns, most homeowners want to preserve turf, so know what you’re using.
