Post-emergent herbicide kills weeds that have already sprouted and are actively growing, applied directly to the foliage rather than to the soil to prevent germination.
If you’re staring at a lawn dotted with dandelions, crabgrass, or clover, post-emergent herbicide is the tool you need. Unlike pre-emergent products that stop seeds from sprouting, post-emergents target the weeds you can see. Applied to the leaves and stems of actively growing plants, they work fast — but only when you pick the right type, time the application correctly, and follow the label to the letter.
How Post-Emergent Herbicides Work
Post-emergents are absorbed through the weed’s foliage rather than its roots. Some are contact herbicides that kill only the tissue they touch — the plant may regrow from surviving roots. Systemic herbicides move through the plant’s vascular system to kill the roots and all, offering complete control. The active ingredient determines the uptake path, and the label tells you which weeds it stops and which turf it won’t harm.
Selective vs. Non-Selective: The Critical Difference
Using the wrong type is the most common mistake.
Selective post-emergents target specific weed types — broadleaf weeds in a lawn, for example — while leaving grass unharmed when applied per the label. Atrazine and certain grass-selective graminicides for broadleaf crops are examples.
Non-selective post-emergents like glyphosate and glufosinate kill or damage nearly every green plant they contact. Use these for spot treatment only, keeping them away from desirable turf, flowers, and shrubs. A single drift from a non-selective spray on a breezy day can scar your lawn for weeks.
When and How to Apply It
Timing and technique matter more than the product itself.
- Weed size: Treat weeds when they’re small and actively growing — ideally no more than 3 to 4 inches tall. Mature, stressed, or flowering weeds resist uptake and won’t die completely.
- Temperature: Do not apply when temperatures are above 85°F. High heat can burn or seriously damage the surrounding turf along with the weeds.
- Mixing and calibration: Follow the label rate exactly. Mix liquid herbicide with the listed water volume and stir well. Calibrate your sprayer or spreader before starting — under-applying leaves survivors, over-applying risks lawn injury.
- Application: Spray evenly for blanket coverage or directly onto individual weeds for spot treatments. With non-selective products, avoid runoff onto desirable plants. Granular post-emergents go on with a spreader; sweep any granules off hardscape back into the lawn if the label allows.
After application, wash all tools with soap and water. Treated weeds will wilt, discolor, and stop growing within days if the herbicide is working.
If you’re ready to buy, our tested roundup of the best post-emergent herbicides compares the top selective and non-selective products for different lawn situations.
Limitations You Need to Know
Post-emergents have little to no soil residual activity — they do not prevent new weeds from germinating later. That means they don’t replace a pre-emergent program for season-long control. They also require weeds to be actively growing and reasonably small; apply them at the wrong time or to the wrong weed and you’re wasting product. Always check whether the herbicide is labeled for your specific turf type or garden crop, especially with non-selective products.
FAQs
Can I use post-emergent herbicide on new grass?
Most post-emergents label a waiting period for newly seeded or sodded lawns — typically 30 days or after the third mowing. Applying too early can kill tender grass along with the weeds. Check your product’s label for the specific interval.
What happens if it rains after I apply post-emergent?
Rain within 1 to 2 hours of application can wash the herbicide off the foliage before it’s absorbed, reducing effectiveness. Most labels specify a rain-free window. If rain hits during that window, you may need to reapply once the weeds dry out.
Is post-emergent safe for vegetable gardens?
Only if the product is specifically labeled for edible crops. Most lawn-targeted post-emergents are not safe near vegetables or herbs. For garden beds, look for products with active ingredients labeled for vegetable use and apply strictly to the weeds, not the soil or crop plants.
References & Sources
- NCBI / National Library of Medicine. “Herbicide Resistance: A Growing Challenge.” Discusses post-emergent herbicide modes of action and selectivity.
- NC State Extension. “Postemergent Weed Control in Landscapes.” Practical guidance on types, timing, and application of post-emergent herbicides.
- NC State Extension. “Postemergence Non-Selective Herbicides for Landscapes and Nurseries.” Details on glyphosate, glufosinate, and safety precautions.