Heat Units & Stage Dates on the Calendar
Predicts emergence
Crops develop on heat, not calendar days. Enter your sowing date and average local temperatures and this tool accumulates growing degree days to predict the calendar date of each stage — emergence, flowering, maturity — for 40+ crops, then checks whether maturity beats your first frost.
Plan your crop & season
Runs entirely in your browser — nothing is uploaded.
| Stage | GDD | Day | Date |
|---|---|---|---|
| Emergence (VE) | 120 | 10 | May 11, 2026 |
| 6-leaf (V6) | 475 | 40 | Jun 10, 2026 |
| Silking (R1) | 660 | 55 | Jun 25, 2026 |
| Dough (R4) | 1,050 | 88 | Jul 28, 2026 |
| Dent (R5) | 1,250 | 105 | Aug 14, 2026 |
| Black layer (R6) | 1,400 | 117 | Aug 26, 2026 |
Next: this season fits comfortably. Schedule your flowering-stage fungicide/irrigation for around Jun 25, 2026 and book harvest near Aug 26, 2026.
GDD = max(0, (min(Tmax,cap)+max(Tmin,base))/2 − base), summed from sowing. Stage thresholds: USDA-NRCS / Iowa State / OMAFRA.
Crop heat units — key facts
- Daily GDD
- max(0, (min(Tmax,cap)+max(Tmin,base))/2 − base)
- Corn base / cap
- 10 °C / 30 °C
- Corn emergence
- ≈ 120 GDD
- Corn silking (R1)
- ≈ 660 GDD
- Corn black layer (R6)
- ≈ 1400 GDD
- Wheat base
- 4.4 °C (winter)
- Stage date
- day cumulative GDD first meets threshold
- Frost check
- maturity date vs first-frost date
- Crops covered
- 40+ (cereals, oilseeds, pulses…)
- Privacy
- Runs in your browser; nothing uploaded
Why a GDD model predicts dates better than a calendar
A crop is a heat engine. It does not advance one stage every fixed number of days — it advances when it has banked enough warmth above its base temperature. Two seasons sown on the same day can flower weeks apart if one runs cool. That is why agronomists count growing degree days: a day that averages five degrees above the crop's base contributes five GDD, and each developmental stage is reached at a fairly stable GDD total regardless of how many calendar days that took.
This calculator carries the published GDD thresholds for each stage — emergence, the key vegetative marks, flowering and physiological maturity — for 40+ crops, each with its own base temperature and upper cap. It accumulates GDD from your sowing date at the temperature regime you enter and reports the predicted calendar date of every stage, the days to maturity, how that compares to a normal season, and the margin between maturity and your first frost. Most GDD tools sum degree-days and stop; the gap this tool closes is turning that accumulation into dated stages mapped against the frost window, so you can schedule sidedress, fungicide, irrigation and harvest — and catch a frost risk before you plant.
GDD stage thresholds by crop (reference)
Representative full-season values; local hybrids shift them — use the maturity slider to match a specific cultivar.
| Crop | Base °C | Cap °C | Emergence GDD | Flowering GDD | Maturity GDD | Source |
|---|---|---|---|---|---|---|
| Maize (grain corn) | 10 | 30 | 120 | 660 | 1400 | USDA-NRCS / Iowa State / OMAFRA |
| Maize (silage) | 10 | 30 | 120 | 660 | 1150 | OMAFRA CHU / Wisconsin |
| Wheat (winter) | 4.4 | 30 | 130 | 1150 | 1900 | Kansas State / Montana State |
| Wheat (spring) | 0 | 30 | 145 | 1350 | 2100 | Montana State / North Dakota State |
| Barley | 0 | 30 | 130 | 1150 | 1700 | Montana State / Univ. of Idaho |
| Rice (paddy) | 10 | 35 | 110 | 1100 | 2400 | IRRI / Univ. of Arkansas DD50 |
| Sorghum (grain) | 10 | 38 | 130 | 1130 | 1730 | Texas A&M / Kansas State |
| Soybean | 10 | 30 | 130 | 700 | 1900 | Univ. of Wisconsin / Iowa State |
| Dry bean | 10 | 30 | 130 | 650 | 1550 | NDSU / Univ. of Nebraska |
| Chickpea (gram) | 0 | 30 | 160 | 1050 | 1900 | ICRISAT / ICARDA |
| Groundnut (peanut) | 13 | 35 | 110 | 420 | 1600 | ICRISAT / Univ. of Georgia |
| Cotton | 15.6 | 37.8 | 50 | 500 | 1750 | Texas A&M DD60 / Univ. of Arkansas |
| Sunflower | 6.7 | 30 | 130 | 700 | 1700 | NDSU / USDA |
| Canola / rapeseed | 5 | 30 | 130 | 950 | 1700 | Canola Council / NDSU |
| Potato | 7 | 30 | 180 | 900 | 1600 | Oregon State / Univ. of Idaho |
| Sugar beet | 4.4 | 30 | 130 | 1400 | 2400 | USDA-ARS / Univ. of Minnesota |
| Tomato | 10 | 30 | 120 | 450 | 1250 | Univ. of California IPM |
| Onion | 4.4 | 30 | 130 | 1000 | 1900 | Oregon State / Univ. of California |
| Alfalfa (lucerne) | 5 | 30 | 0 | 550 | 900 | Univ. of Wisconsin / USDA |
| Sugarcane (formative) | 12 | 38 | 250 | 2500 | 4500 | ICAR-SBI / Univ. of Florida |
Full dataset of 40 crops drives the calculator above. Sources: USDA-NRCS, OMAFRA Corn Heat Units, Iowa State, Kansas State, Montana State, University of Wisconsin, University of California IPM, IRRI, ICRISAT and others.
How to use it in five steps
- 1. Pick the crop. Its base temperature, upper cap and stage GDD thresholds load automatically.
- 2. Set the sowing (or transplant) date. This anchors the calendar.
- 3. Enter season temperatures. Average daily maximum and minimum across the season.
- 4. Add your first-frost date. The tool checks the maturity margin against it.
- 5. Read the timeline. Every stage's date on the calendar Gantt, days to maturity, and the frost verdict — then schedule field operations.
Frequently Asked Questions
How does the calculator turn temperatures into stage dates?+
It accumulates growing degree days (GDD) from your sowing date. Each day adds GDD = max(0, (min(Tmax, cap) + max(Tmin, base)) / 2 − base) for your crop's base and upper-cutoff temperatures. The running total is compared to each stage's published GDD threshold — the day the total first reaches a stage's threshold becomes that stage's predicted calendar date. Maturity is simply the final threshold.
What is a growing degree day (GDD)?+
A growing degree day is a unit of accumulated heat above a crop's base temperature — the temperature below which it makes no growth. A day averaging 5 °C above the base contributes 5 GDD. Crops develop on heat, not on calendar days, so GDD predict stage timing far better than 'days after sowing,' especially across cool and warm seasons.
Why cap the maximum temperature?+
Most crops stop developing faster once it gets very hot, so the 'modified' or 'method 2' GDD model caps Tmax at an upper threshold (30 °C for corn, ~38 °C for sorghum) before averaging, and floors Tmin at the base. This is the convention used by USDA-NRCS and most state-extension phenology tables, and it is exactly what this tool implements.
How many GDD does corn need to reach black layer?+
Grain corn needs about 1400 GDD (base 10 °C) from emergence-to-black-layer in the OMAFRA/Iowa State system — roughly 120 GDD to emergence, 660 to silking, and 1400 to physiological maturity (R6). This tool carries those thresholds for grain and silage corn plus 40+ other crops, and you can scale them with the hybrid-maturity slider for early or full-season hybrids.
Will my crop mature before the first frost?+
That is the headline question this tool answers. It compares the predicted maturity date to the first-frost date you enter and reports the margin in days. If maturity falls on or after first frost it flags a frost risk; a margin under about two weeks is tight. You can then pick an earlier hybrid or move the sowing date until maturity clears frost.
What does the hybrid-maturity percent do?+
It scales every stage's GDD threshold up or down. A 90% setting models an earlier-maturing hybrid that needs 10% fewer heat units; 110% models a fuller-season hybrid. Use it to match a specific seed's relative-maturity or CHU rating, then watch the maturity date and frost margin move.
Which temperatures should I enter?+
Enter the average daily maximum and minimum you expect across the season — for example a season mean high of 29 °C and mean low of 15 °C for summer corn. The model uses a steady daily regime, so seasonal averages give the cleanest planning estimate. For a sharper forecast in-season, update the averages as real weather data comes in.
Is GDD the same as Corn Heat Units (CHU)?+
They are related but not identical. CHU (the Ontario system) uses a different two-part daytime/nighttime formula and its own scale, while GDD uses a single base-temperature average. This tool uses the GDD model with crop-specific base and cap temperatures; the corn thresholds are aligned to the widely published GDD-base-10 values that correspond to the CHU stages.
Why is my maturity date 'too cold to mature'?+
If the average temperature you entered is at or below the crop's base temperature, almost no GDD accrue and the crop never reaches its threshold within a season. That message means the thermal environment cannot finish this crop — choose a warmer window, a lower-base crop, or revisit your temperature inputs.
What does 'days ahead of normal' mean?+
It compares your predicted days-to-maturity against a reference season running at a 20 °C mean temperature. A warmer-than-reference season accumulates GDD faster, so maturity arrives earlier ('ahead of normal'); a cooler season runs behind. It is a quick gauge of whether your season is fast or slow for this crop.
Does this replace scouting the field?+
No — it is a planning and scheduling estimate. Real development depends on the cultivar, soil, moisture, photoperiod-sensitive crops, and day-to-day weather. Use the predicted dates to schedule sidedress nitrogen, fungicide at flowering, irrigation and harvest crews, then confirm by scouting growth stages in the field.
Which crops are covered?+
The dataset carries 40 crops across cereals, oilseeds, pulses, tubers, fibres, forages and vegetables — including grain and silage corn, winter and spring wheat, barley, rice, sorghum, soybean, cotton, canola, sunflower, potato, sugar beet and more, each with its own base temperature, upper cap and full stage ladder cited to USDA, OMAFRA and university-extension sources.
Can I use it for transplanted vegetables?+
Yes. For transplants, treat the transplant date as the 'sowing' date and the early threshold as establishment — the tomato, pepper, onion and sweet-corn entries are set up this way. The flowering and harvest GDD then predict first-flower, fruit-set and first-harvest dates from your transplant date.