Weed Emergence & Post-Spray Timing

You choose the weed species and enter the accumulated soil degree-days (the sum of daily mean-soil-temperature minus the species base temperature, since your biofix or planting). The tool maps that onto a Gompertz emergence S-curve fitted to published degree-days to 10%, 50% and 90% emergence, and reports the cumulative percentage emerged plus the degree-days remaining to the 50% and 90% points. That tells you where you are in the flush and whether to spray.

The strongest one-pass timing is the 50–90% emergence band, when over half the cohort is up but the weeds are still small and easy to kill. Spray too early and most of the flush has not emerged, so escapes follow; spray past 90% and the early weeds are large and tough. For flush-prone species like waterhemp and Palmer amaranth, tank-mix a residual at the post pass to catch the late emergers.

Soil degree-days (°C·day) are a measure of accumulated heat available for germination. Each day you add the mean soil temperature minus the species base temperature (negative days count as zero), starting from a biofix such as planting or the first emergence trigger. Different weeds have different base temperatures — kochia starts at about 5°C while Palmer amaranth and velvetleaf need about 12°C — so the same calendar date can be a different degree-day total for each species.

The base temperature is the soil temperature below which a species does not accumulate effective heat for germination. In this dataset it ranges from about 4°C for wild oat and 5°C for kochia and marestail up to 12°C for Palmer amaranth and velvetleaf and 14°C for ivyleaf morningglory. Using the wrong base shifts the whole emergence curve, so match the base temperature to the species you select.

Fifty percent emergence means half of the season's cohort of that weed has come up. It marks the practical opening of the post-emergence window because you are now controlling the bulk of the population in one pass while the plants are still small. Before 50%, a single post pass leaves too many later emergers; the tool shows the degree-days remaining until you reach it.

Species such as waterhemp, Palmer amaranth and giant ragweed have extended, flush-prone emergence, meaning the gap between 10% and 90% emergence spans many degree-days, so new cohorts keep arriving after an early spray. That is why a residual herbicide is essential for them — it controls the late emergers that a single post-emergence application would miss. The curve's spread between the 10% and 90% flags shows how drawn-out a species is.

Yes. Both accumulate heat units, but this model maps SOIL degree-days onto species-specific weed-emergence S-curves to time post-emergence weed control, whereas an insect degree-day tool maps air degree-days onto insect development stages to time insecticide applications. The base temperatures, the biofix and the biology being predicted are all different, so use the weed model for weeds and the insect model for pests.

The percentage comes from a Gompertz curve fitted through published degree-days to 10%, 50% and 90% emergence (WeedCast and Werle et al. 2014), so it is a strong field estimate rather than an exact count. Accuracy depends on using measured soil temperature, the correct base temperature and a sound biofix; a wrong base or biofix shifts the whole curve. Always pair the model with field scouting before you spray.

Usually not. At low emergence percentages most of the flush is still to come, so an early post pass controls only the first cohort and leaves later emergers to escape. The exception is when the earliest weeds are already large or competitive enough to hurt the crop — then control the escapes promptly and rely on a residual for the rest. The tool's verdict distinguishes these cases for you.

No. The model tells you where you likely are in the emergence flush so you scout at the right time, but field conditions, microclimate, soil moisture and seedbank differences mean you must confirm with your own counts. Use the predicted window to schedule a scout, then make the spray decision on what you actually see, with the herbicide label as the legal authority on rate and timing.