Spray Small & Win the Weed
Sizes up waterhemp
Post-emergence control follows weed size more than anything else — efficacy falls off a cliff once weeds pass label size. Enter the weed, herbicide and current size to get the expected control %, whether you're past the window, and the rate-up or switch you need.
Measure your weeds
Spray decisions follow weed SIZE more than any other factor — small is the cheapest insurance you can buy.
Next: go to the high label rate of Glufosinate (Liberty) (lifts control to ~81%) and tank-mix a partner with a different mode of action — single-pass on an oversize weed rarely finishes the job.
Control %, label-max sizes and rate-bump effects are modelled on land-grant (UNL/ISU/UW/Purdue) control-by-weed-size tables + herbicide labels on the WSSA 0–100 scale. Always confirm the rate, max size and adjuvant on your own label.
Weed-size efficacy — key facts
- Biggest driver
- weed SIZE at application
- Tough weeds (waterhemp/Palmer)
- spray by ≈ 8–10 cm / 3–4 in
- Reliable control
- ≥ 90%
- Acceptable
- 80–90% (scout escapes)
- Poor
- < 80% → rate-up or switch
- Yield loss waiting
- ≈ 0.5–1% per day late
- Efficacy shape
- flat, then a cliff past label size
- Best practice
- small + timely + overlapping residual
Maximum effective weed size by herbicide
Representative label / extension control-by-size values on the WSSA 0–100 scale. Control at the small size and right at the label maximum; the gap is the cliff. Confirm rate and max size on your own label.
| Weed | Herbicide | Group | Max size (cm / leaf) | Small control | At label max |
|---|---|---|---|---|---|
| Waterhemp | Glyphosate (Roundup) | 9 | 10 cm / 4-lf | 70% | 55% |
| Glufosinate (Liberty) | 10 | 8 cm / 3-lf | 90% | 75% | |
| Fomesafen (Flexstar) | 14 | 8 cm / 3-lf | 92% | 72% | |
| 2,4-D choline (Enlist) | 4 | 10 cm / 4-lf | 90% | 78% | |
| Dicamba (XtendiMax) | 4 | 10 cm / 4-lf | 88% | 76% | |
| Palmer amaranth | Glufosinate (Liberty) | 10 | 8 cm / 3-lf | 88% | 72% |
| Fomesafen (Flexstar) | 14 | 8 cm / 3-lf | 90% | 70% | |
| 2,4-D choline (Enlist) | 4 | 10 cm / 4-lf | 90% | 76% | |
| Dicamba (Engenia) | 4 | 10 cm / 4-lf | 90% | 76% | |
| Glyphosate (Roundup) | 9 | 8 cm / 3-lf | 60% | 45% | |
| Common lambsquarters | Glyphosate (Roundup) | 9 | 15 cm / 6-lf | 92% | 80% |
| 2,4-D | 4 | 15 cm / 6-lf | 94% | 82% | |
| Atrazine (POST) | 5 | 8 cm / 3-lf | 92% | 76% | |
| Mesotrione (Callisto) | 27 | 8 cm / 3-lf | 95% | 80% | |
| Giant ragweed | Glyphosate (Roundup) | 9 | 15 cm / 4-lf | 88% | 72% |
| 2,4-D | 4 | 20 cm / 5-lf | 92% | 80% | |
| Dicamba | 4 | 20 cm / 5-lf | 92% | 80% | |
| Fomesafen (Flexstar) | 14 | 10 cm / 3-lf | 75% | 58% | |
| Giant foxtail (grass) | Glyphosate (Roundup) | 9 | 15 cm / 5-lf | 95% | 85% |
| Clethodim (Select Max) | 1 | 15 cm / 6-lf | 96% | 86% | |
| Glufosinate (Liberty) | 10 | 10 cm / 4-lf | 86% | 70% | |
| Horseweed / marestail | 2,4-D | 4 | 10 cm / 4-lf | 90% | 72% |
| Dicamba | 4 | 10 cm / 4-lf | 92% | 74% | |
| Glufosinate (Liberty) | 10 | 10 cm / 4-lf | 80% | 62% | |
| Glyphosate (Roundup) | 9 | 8 cm / 3-lf | 45% | 35% |
Why efficacy falls off a cliff
A post-emergence herbicide has to land on the leaf, get through the cuticle, and move to the growing points faster than the plant can grow away from it. On a small seedling all three are easy: thin cuticle, little leaf area per dose, short distance to the meristems. As the weed grows it thickens its waxy layer, shades its own lower leaves, and demands far more active ingredient to kill the same proportion of tissue — so control collapses over just a few centimetres of extra height. That is the cliff this tool draws.
This calculator gives the expected control %, the label maximum size, the rate-bump effect, the best alternative herbicide, and the yield at risk if you wait. Use it to make the spray-now-versus-wait call, to decide between rate-up and a product switch, and to justify staying ahead on size. Pair it with the Pre-Emergence Activation and Fungicide Timing tools for a full crop-protection program.
How to use it — 5 steps
- 1
Identify the weed
Pick the species you are targeting — the window is far tighter for waterhemp and Palmer than for easy broadleaves.
- 2
Pick the herbicide
Choose the product you plan to spray; its label max size sets the cliff.
- 3
Measure the largest weeds
Enter height in cm or the leaf stage of your biggest weeds, not the average.
- 4
Read the control %
See expected control and whether you are inside or past the label window.
- 5
Act on the verdict
Spray now, bump to the high rate, or switch products and add an overlapping residual.
Frequently Asked Questions
How does weed size affect herbicide efficacy?+
Weed size is the single biggest controllable driver of post-emergence success. While weeds are small, more spray reaches the growing points relative to leaf area and the plant translocates the active ingredient efficiently, so control is high. As the weed grows it puts on waxy cuticle, the canopy shades lower leaves, and the same dose now has to kill far more tissue — so control does not fade gently, it falls off a cliff once the weed passes the label's maximum effective size.
What does the control percent mean?+
The control percent is the expected proportion of the weed population killed at the size you entered, on the standard 0–100 scale used in land-grant efficacy tables. Roughly, 90%+ is reliable, 80–90% is acceptable but you should scout for escapes, and below 80% is poor — you will be left with survivors that set seed. The tool reads this off an efficacy-vs-size curve anchored on each herbicide's small-weed performance and its label maximum size.
What is the maximum effective weed size?+
Most post-emergence labels state a maximum weed height or leaf stage at application — for many summer annuals that is around 8–10 cm (3–4 inches) for tough species like waterhemp and Palmer amaranth, and larger for easier broadleaves. Spraying beyond that size is not just less effective; it is also off-label for products that set a maximum size. The red line on the chart marks that ceiling.
Should I spray now, bump the rate, or switch products?+
If expected control is 80% or higher, spray now while you are still in the window. If control has slipped but the high label rate (and a tank-mix partner) brings it back above 80%, bump the rate. If even the high rate cannot recover acceptable control and a different mode of action does much better at this size, switch products and add a residual. The tool makes that call for you and names the best alternative.
Why is small-and-timely better than big-and-strong?+
Because efficacy falls off a cliff with size, a moderately effective herbicide on a small weed almost always beats a strong herbicide on an oversize weed. Spraying small also means lower rates work, you avoid resistance pressure from sub-lethal doses on survivors, and you cut the early-season yield loss from weed competition. The cheapest insurance in weed control is simply spraying on time.
How much yield do I lose by waiting?+
Early-season weeds compete for light, water and nitrogen, and the critical period studies show roughly half to one percent of yield lost per day that control is delayed in a competitive crop. The tool estimates the days until your weed outgrows the label window and the approximate yield at risk if you wait that long — usually far more than the cost of spraying promptly.
Does glyphosate still work on waterhemp and marestail?+
Often not. Glyphosate-resistant waterhemp, Palmer amaranth and marestail (horseweed) are widespread, so the modelled control for glyphosate on those species is deliberately low even on small plants. On resistant weeds the tool will steer you to glufosinate, a PPO, or an auxin product depending on your trait package — the size curve alone will not save a herbicide the weed is resistant to.
Can I measure by leaf stage instead of height?+
Yes. Switch the size input to leaf stage and enter the number of true leaves (or collars for grasses) on your largest weeds. The tool converts leaf stage to an equivalent height using each weed's height-to-leaf relationship so the two measures agree at the label ceiling. Leaf stage is often the more reliable field measure because it tracks the plant's physiology rather than a stretched, etiolated height.
Does a rate bump always fix an oversize weed?+
No. A high label rate buys a little extra size — a few centimetres — and improves uptake, but it cannot resurrect control on a weed that is well past the window, and over-relying on rate increases selects for resistance. The tool shows the control you would get at the high rate; if that is still below acceptable, it tells you to switch rather than throw more chemistry at a lost cause.
Why add a residual herbicide with my post pass?+
Weeds like waterhemp and Palmer emerge in repeated flushes all season, so even a perfect post-emergence kill today does nothing for the seedlings emerging next week. Tank-mixing a soil-residual (pre-emergence) herbicide with your post pass — an 'overlapping residual' — keeps the next flush from ever reaching sprayable size, which is the whole point of staying ahead on size.
Is this a substitute for the product label?+
No. The control figures and label maximum sizes are modelled from published land-grant efficacy-by-weed-size tables and labels on the standard scale, and are meant for planning the spray-now-versus-wait decision. The legal rate, the maximum weed size, the required adjuvant and the crop registration always come from your own current product label — confirm them before you spray.