RM43 kills most weeds in 1 to 2 weeks, though some take longer depending on the weed type, weather, and how you explore it

RM43 is a non-selective herbicide that works on contact—it damages the leaves and stems it touches, then moves through the plant to kill the roots. You will see the first visible damage (yellowing, wilting, browning) within 24 to 48 hours on soft-leaved weeds like clover and chickweed. Tougher, waxy-leaved weeds like dandelions and plantain take 3 to 7 days to show damage, and the roots may take another week to fully die.

The total time from spray to complete death varies. Annual weeds (those that live one season) usually die completely within 7 to 14 days. Perennial weeds (those that come back year after year) may take 2 to 3 weeks because their root systems are deeper and more established. You may see green growth stop within days, but the plant won't be fully dead until the roots have no energy left to regrow.

Key Takeaways

  • Soft-leaved weeds show damage within 24 to 48 hours; tough-leaved weeds take 3 to 7 days to start dying.
  • Annual weeds are usually completely dead within 1 to 2 weeks; perennial weeds may take 2 to 3 weeks.
  • Rain within 2 to 4 hours of spraying can wash off the herbicide before it enters the plant, so timing matters.
  • Warm, sunny weather speeds up the process; cold or cloudy conditions slow it down.
  • Spraying young, actively growing weeds works faster than spraying mature or stressed plants.

Why the timeline varies by weed type

RM43 is a contact herbicide, which means it kills what it touches—it does not move through the soil to kill seeds or roots that the spray never reaches. Soft-leaved weeds like clover, chickweed, and young dandelions have thin, waxy coatings, so the herbicide soaks in quickly and reaches the plant's vascular system (the tubes that carry water and nutrients) within hours. These weeds often look dead or dying within 2 to 3 days.

Tough-leaved weeds like mature dandelions, plantain, and thistle have thicker, waxier leaf surfaces that slow down absorption. The herbicide still enters, but it takes longer—sometimes 5 to 7 days before you see clear browning. Even after the leaves die, the roots may still have stored energy. If the root system is large or deep, it can take another 7 to 10 days for that energy to run out and the plant to stop trying to regrow.

Perennial weeds are the slowest because they have established root systems that store carbohydrates (food) underground. Killing the visible leaves is fast; starving the roots is slow. You may need to respray any regrowth that appears after 3 to 4 weeks.

How weather affects how fast RM43 works

Temperature is the biggest weather factor. RM43 works fastest when the air temperature is between 60°F and 85°F. Warm weather speeds up the herbicide's movement into the plant and the plant's own metabolism, so damage appears faster. Cold weather (below 50°F) slows everything down—the herbicide moves more slowly into the plant, and the plant's own processes are sluggish, so visible damage may take twice as long.

Rain is critical in the first few hours after spraying. If rain falls within 2 to 4 hours of process, it can wash the herbicide off the leaves before it has time to enter the plant. The exact window depends on the formulation and how much rain falls, but light rain within the first 2 hours usually means you need to respray. After 4 to 6 hours, most of the herbicide has entered the plant, and rain no longer matters.

Sunny weather helps because it dries the spray faster and allows the herbicide to absorb more completely. Cloudy, humid conditions slow drying and absorption, so the same spray may take a day or two longer to show results. Wind also matters—strong wind can cause uneven coverage and drift, so calm conditions give you better, faster results.

The difference between visible damage and complete death

A common source of confusion is the difference between a weed looking dead and a weed being dead. Within 3 to 7 days, most weeds will turn brown or yellow and look completely finished. At that point, many people assume the job is done. But "looking dead" and "actually dead" are not the same thing.

When you see browning leaves, the herbicide has killed the above-ground parts. The roots, however, may still be alive and trying to regrow. For annual weeds, this regrowth attempt usually fails within another week because the plant has no energy reserves. For perennials, the roots can push up new shoots 2 to 4 weeks after the first spray. If you see new green growth appearing from the base or from the roots, the plant is not fully dead yet.

To know for certain that a weed is dead, wait 3 to 4 weeks and see whether any new growth appears. If nothing grows back, it is dead. If green shoots emerge, the roots survived, and you will need to respray those shoots or pull them out by hand.

Spraying technique and timing affect results

How thoroughly you coat the leaves makes a real difference in how fast RM43 works. A light, quick spray that misses half the leaf surface will kill that weed more slowly than a thorough spray that coats both sides of every leaf. Young, tender weeds with thin leaves absorb the herbicide faster than old, tough weeds with thick, waxy leaves. Spraying when weeds are actively growing (spring and early summer) produces faster results than spraying stressed, drought-stressed, or mature plants in late summer.

The time of day also matters slightly. Spraying in early morning or late afternoon, when the plant's stomata (tiny pores on leaves) are more open, can speed up absorption by a few hours. Spraying in the heat of midday, when stomata close to conserve water, may slow things down a little. This is a small effect—maybe 6 to 12 hours difference—but it is worth knowing if you are trying to work as efficiently as possible.

What to expect week by week

Days 1 to 2: Soft-leaved weeds may show slight yellowing or wilting. Tough-leaved weeds show no visible change yet. The herbicide is entering the plant.

Days 3 to 7: Most weeds show clear browning or yellowing. Soft-leaved weeds are usually brown throughout. Tough-leaved weeds are starting to brown at the edges and tips. The plant is dying, but roots may still be alive.

Days 8 to 14: Annual weeds are usually completely brown and dead. Perennial weeds are brown above ground, but roots are still present. New growth from perennial roots may start to appear by day 14.

Weeks 3 to 4: Any regrowth from perennial roots becomes visible. If you see new green shoots, the roots survived and you need to respray or remove them by hand.

When to respray and when to pull by hand

If a weed shows no visible damage after 10 days, something went wrong. The most common causes are that the spray did not coat the leaves evenly, rain washed it off too soon, or the weed is a particularly tough species. You can respray, but wait at least 7 days after the first spray to avoid damaging the plant further without benefit.

If you see new green growth emerging from the base or roots 2 to 4 weeks after spraying, the roots survived. At this point, you have two choices: respray the new growth (which will work faster because the shoots are young and tender), or pull the plant by hand if it is small enough. Hand-pulling is often faster and cheaper if you only have a few weeds.

For perennial weeds with deep taproots (like dandelions), pulling by hand after the first spray has weakened the plant can be effective. The herbicide will have killed the leaves and weakened the root, making it easier to pull out completely. If you leave any root fragment in the soil, it may regrow, so dig deep.

Frequently Asked Questions

Can I speed up how fast RM43 works?

Spray when the air temperature is between 60°F and 85°F, when weeds are young and actively growing, and when rain is not forecast for at least 4 to 6 hours. Coat the leaves thoroughly on both sides. Avoid spraying stressed or drought-stressed plants, which absorb herbicides more slowly. These steps will give you the fastest possible results, but the basic timeline (1 to 2 weeks for annuals, 2 to 3 weeks for perennials) is set by the plant's biology, not by how hard you spray.

Is the weed dead if it turns brown?

Browning means the above-ground parts are dead, but the roots may still be alive, especially in perennial weeds. Wait 3 to 4 weeks and watch for new growth. If nothing grows back, the weed is dead. If green shoots emerge from the base or roots, respray those shoots or pull them out by hand.

What if it rains right after I spray?

If rain falls within 2 to 4 hours of spraying, the herbicide may wash off before it enters the plant, and you will need to respray. After 4 to 6 hours, most of the herbicide has been absorbed, and rain no longer matters. Check the weather forecast before spraying and plan for a dry window.

Do I need to respray perennial weeds?

Most perennial weeds need only one spray if you coat them thoroughly and wait the full 2 to 3 weeks for the roots to die. However, if you see new growth 3 to 4 weeks later, the roots survived, and you should respray the new shoots or pull them out. Repeated spraying of the same root system over months can eventually exhaust it.

Why is my weed still green after a week?

The most common reasons are that the spray did not coat the leaves evenly, rain washed it off too soon, the temperature was too cold, or the weed is a particularly tough species with a thick waxy coating. Check your spray coverage, wait for warmer weather, and respray if needed. Some weeds straightforward take longer than others.