RM43 kills most weeds in 1 to 2 weeks, though some take longer depending on the weed type and growing conditions
RM43 is a non-selective herbicide that works on contact, meaning it damages plant tissue where the spray touches. You will see the first visible damage — yellowing, browning, or wilting — within 2 to 5 days on soft-stemmed weeds like chickweed or clover. Tougher, woody plants and deep-rooted perennials like dandelions or thistles take 7 to 14 days to show serious decline, and complete death may not occur for 2 to 3 weeks.
The speed depends on several real factors: air temperature, humidity, rainfall after spraying, the age and size of the weed, and whether the plant is actively growing. RM43 works fastest when temperatures are between 60°F and 85°F and the plant is in active growth. Cold weather, drought stress, or mature woody plants slow the process considerably.
Key Takeaways
- Soft-stemmed annual weeds show damage in 2 to 5 days and die within 1 to 2 weeks of spraying.
- Perennial weeds with deep roots or woody stems take 2 to 3 weeks to die completely, even though they may look dead after 1 week.
- Rain within 2 to 4 hours of spraying can wash off the herbicide and slow or stop the kill, so timing matters.
- Warm, humid conditions and actively growing weeds speed up the kill time; cold or stressed plants slow it down.
- Dead weeds may remain standing in the soil for weeks; you may need to pull them or mow them down once they are brown and brittle.
Why the timeline varies by weed type
Annual weeds — plants that live one season and have shallow roots — die fastest. Chickweed, purslane, and pigweed typically show brown or yellow tissue within 3 to 5 days and are completely dead within 7 to 10 days. These plants have thin, tender stems and no stored energy reserves, so RM43 disrupts them quickly.
Perennial weeds — plants that return year after year from deep roots — are tougher. Dandelions, plantain, and bindweed have thick taproots or rhizomes that store energy underground. RM43 kills the above-ground leaves and stems, but the plant may try to regrow from the root. You may see no new growth for 2 to 3 weeks, but some perennials need a second process or hand-pulling to prevent regrowth.
Woody or semi-woody plants like brambles or young shrubs have waxy leaf coatings that slow herbicide absorption. These may take 3 to 4 weeks to show complete death, and larger specimens may survive a single process.
How weather and timing affect kill speed
Temperature is the biggest variable. RM43 works by disrupting plant cell membranes, a process that speeds up in warmth. At 70°F to 80°F with moderate humidity, most soft weeds die in 7 to 10 days. Below 60°F, the process slows dramatically — the same weeds may take 2 to 3 weeks or longer. Frost or freezing temperatures can stop the herbicide from working at all.
Humidity matters because RM43 needs moisture on the leaf surface to be absorbed. Dry, windy conditions slow absorption. Spraying in early morning or late afternoon when dew is present, or when humidity is high, speeds up the kill. Spraying in full sun on a dry day slows it down.
Rain within 2 to 4 hours of spraying can wash the herbicide off before it is absorbed, resetting the clock or stopping the kill entirely. If rain is forecast within 4 hours, wait to spray. Once the herbicide has dried on the leaf — usually 4 to 6 hours in good conditions — rain will not affect it.
What you will see as the weed dies
The first sign is leaf discoloration: yellowing at the edges or between the veins, or a general browning that spreads from the spray contact point. This usually appears within 2 to 5 days. The leaves may also wilt or curl inward as cell membranes break down.
By day 7 to 10, most of the above-ground plant is brown and brittle. Soft weeds may collapse and dry up completely. Perennials may still have green tissue at the base or crown, which is why they sometimes regrow — the root system is still alive.
By week 2 to 3, dead weeds are fully brown and dry. They may still be standing in the soil, held up by dead stems. You can pull them by hand, mow them down, or leave them to decompose. They will not regrow if the root is truly dead, but perennials that were only partially killed may send up new shoots weeks later.
When to expect regrowth or need a second spray
Annual weeds rarely regrow if they are completely brown and the root is dead. Once you see full browning, you can assume they are finished.
Perennial weeds are unpredictable. Some die completely from a single spray; others regrow from the root within 2 to 4 weeks. If you see new green shoots emerging from the base or crown after 3 weeks, the root survived. You can spray again, pull the plant by hand, or dig out the root. A second spray is most effective if the new growth is still young and tender — waiting until it is large and established makes the kill slower.
Mature or very large perennials often need hand-digging or a second process. RM43 is most effective on young, actively growing plants, so timing your spray to catch weeds early in the season improves your results.
Speeding up the kill: conditions that help
Spray when the air temperature is between 65°F and 85°F and the forecast shows no rain for at least 4 to 6 hours. Early morning or late afternoon, when humidity is higher and the plant is actively transpiring, gives faster absorption than midday.
Spray young weeds before they flower or set seed. A 3-inch chickweed dies much faster than a mature, flowering plant. Early-season spraying also prevents weeds from spreading.
Make sure the spray covers the entire plant, including the undersides of leaves where absorption happens. Thorough coverage speeds up the kill and reduces the chance of regrowth from missed tissue.
Do not mow or disturb the weeds for at least 3 to 5 days after spraying. The herbicide needs time to move through the plant. Mowing too soon can remove the sprayed leaves before the chemical has done its work.
What RM43 does not do
RM43 does not prevent new weeds from germinating. Once the dead weeds are gone, new seeds in the soil will sprout. You will need to spray again or use other methods like mulch, ground cover, or pre-emergent herbicides to prevent new growth.
RM43 does not always kill the root of perennial weeds on the first process. If you are trying to clear a patch of established dandelions or thistles, expect to need multiple sprays or hand-digging to fully control them.
RM43 is non-selective, meaning it kills most plants it touches, not just weeds. Avoid spraying near desirable plants, and be careful not to drift the spray onto grass, flowers, or shrubs you want to keep.
Frequently Asked Questions
Can I spray RM43 in the rain or right before rain?
No. Rain within 2 to 4 hours of spraying washes the herbicide off before it is absorbed, and you will need to respray. Wait until the forecast shows at least 4 to 6 hours of dry weather before you spray. Once the spray has dried on the leaf — usually 4 to 6 hours in normal conditions — rain will not affect it.
Why is my weed still green 2 weeks after spraying?
The most common reasons are that the spray did not fully cover the plant, the temperature was too cold, or the weed is a tough perennial with a deep root system. Check whether the entire plant was sprayed, including the base and undersides of leaves. If it was, wait another week — some perennials take 3 to 4 weeks to die completely. If new growth appears from the base, the root survived and you may need a second spray or hand-digging.
Do I need to pull the dead weeds out, or can I leave them?
You can leave them. Dead weeds will decompose over time and add organic matter to the soil. However, if the weed is large or unsightly, you can pull it by hand once it is brown and brittle, or mow it down. Pulling or mowing does not affect the herbicide's work — the plant is already dead.
Will RM43 kill grass if I spray it by accident?
Yes. RM43 is non-selective and kills most plants on contact. If you spray grass or desirable plants, they will show damage within 2 to 5 days and may die. Avoid spraying near plants you want to keep, and use a shield or careful hand-spraying to prevent drift.
How long after spraying can I plant new grass or flowers?
Once the dead weeds are removed and the soil is clear, you can plant when ready. RM43 breaks down quickly in soil and does not leave a residue that prevents new plants from growing. However, wait until the old weeds are fully dead and removed before you plant, so the new plants do not compete with dying tissue.