Will a Second Crop Fit & Before the First Frost?

It depends on the heat units left, not just the calendar days. The tool multiplies the days from your second-crop planting date to the first frost by your area's late-season growing-degree-day (GDD) accrual to get the heat units available, then compares that to the GDD the second crop needs to mature. If at least 100% of the needed GDD are available it returns feasible (115%+ is comfortable), 100–115% is tight, and below 100% is infeasible — the crop will not finish before frost.

A growing degree-day (GDD) is a unit of accumulated heat: each day contributes the amount its mean temperature exceeds the crop's base temperature (about 10°C for warm-season crops). Crops mature on banked heat, not elapsed days, so a late-summer planting that has the calendar days but cool, short days may still not bank enough GDD. That is why this tool works in heat units and flags windows that look long enough on a calendar but are not warm enough to finish a crop.

It is the last date you can plant and still bank the crop's required GDD before frost, working back from the frost date at your daily heat-unit rate. The tool reports it for the crop you select; planting after it means the crop runs out of heat and is caught immature by frost. Use the latest plant-by as your hard deadline for clearing crop 1 and getting crop 2 in the ground.

Use the average late-season GDD per day for your location — typically about 14–22 °C-days/day (base 10°C) through the back half of summer, falling as autumn approaches. Your state extension service or a GDD map publishes monthly normals; take the mean for the months between your harvest and your frost date. A lower rate shortens the window in heat-unit terms even if the calendar window looks the same.

Sixteen common double-crop and catch-crop options: ultra-early to mid soybean maturity groups, 80- and 95-day maize, grain and forage sorghum, early sunflower, pearl millet, cowpea, mung bean, snap bean, sweet corn, buckwheat, forage turnip and forage oats. Each carries its GDD-to-maturity and base temperature, and the tool ranks them by how well they fit your window so you can see which maturity group is short enough.

Tight means the crop has 100–115% of the heat units it needs — it should just finish in an average year, but an early frost or a cool spell could leave it immature. If you get a tight result, plant your earliest-maturing hybrid, plant by the latest plant-by date the tool shows, and watch the autumn forecast. If you want safety, drop to the next earlier maturity group, which the fit table highlights.

Every day between harvesting crop 1 and planting crop 2 is a day of heat units lost to the second crop. The tool adds your turnaround days (combine, clear residue, prepare seedbed, plant) to push the crop-2 planting date later, shrinking the GDD window. Tightening turnaround — for example planting green or no-tilling directly into stubble — is often the single biggest lever to make a marginal double crop feasible.

No. A double crop is a second harvestable cash or forage crop grown in the same season after the first; a cover crop is grown for soil benefit and usually not harvested. When the tool returns infeasible for every cash crop, a quick cover crop is the sensible fallback — it uses the short window to build soil and suppress weeds rather than chasing a harvest the season cannot finish.

Because heat-unit accumulation plateaus at high temperatures — once the mean exceeds the crop's upper threshold, extra heat stresses the plant rather than speeding it, so most GDD methods cap the daily contribution (commonly around 30°C-days). The tool applies this cap so very hot inputs do not overstate the heat available, keeping the feasibility verdict realistic.

Yes. Although the framing is harvest-to-frost, the method works for any 'window between clearing one crop and a hard season-end' — frost, monsoon withdrawal, the start of the next main crop, or a contracted field-vacate date. Set the season-end date to whatever ends your window and the GDD math gives the same go/no-go answer and latest plant-by date for the catch crop.