Growing Degree Units Calculator
Calculate growing degree units (GDUs) per day for corn, soybeans, wheat, and other crops.
Includes modified corn GDU method and crop stage reference table.
Growing Degree Units (GDUs), also called Growing Degree Days (GDDs), are a measure of heat accumulation used to predict plant development, crop maturity timing, and insect pest emergence. The concept is based on the observation that biological development is strongly correlated with accumulated heat above a minimum threshold temperature — not just calendar days.
History: The concept dates to René de Réaumur’s 1735 observation that plants complete their life cycles after accumulating a fixed amount of warmth. Modern agricultural GDU systems were standardized in the mid-20th century to help farmers predict harvest timing and variety selection.
Standard GDU Formula: GDU (per day) = ((T_max + T_min) / 2) − T_base
Where T_max and T_min are the daily high and low temperatures, and T_base is the biological threshold below which no development occurs. If the result is negative, use zero — there is no negative GDU accumulation.
Modified Corn GDU Formula (most common in the US Corn Belt): This version accounts for the fact that corn growth stops at extreme temperatures:
- If T_max > 86°F (30°C), use 86°F (corn’s upper threshold)
- If T_min < 50°F (10°C), use 50°F (corn’s lower threshold)
- Apply standard formula to these capped values
Common Base Temperatures:
| Crop | Base Temperature | Notes |
|---|---|---|
| Corn (modified) | 50°F / 10°C | Upper cap of 86°F (30°C) |
| Soybeans | 50°F / 10°C | No upper cap standard |
| Small Grains (wheat, barley) | 32°F / 0°C | Cold-tolerant crops |
| Cotton | 60°F / 15.6°C | Warm-season crop |
| Tomatoes | 50°F / 10°C | — |
| Alfalfa | 41°F / 5°C | — |
Corn Crop Stage Reference (50°F base, from planting):
| GDU Accumulated | Corn Stage |
|---|---|
| 100–120 | Emergence (VE) |
| 210 | V2 (2 leaves) |
| 475 | V6 (6 leaves) |
| 700 | V10 (10 leaves) |
| 1,135 | VT (tassel emergence) |
| 1,400 | R1 (silk / pollination) |
| 1,750 | R3 (milk stage) |
| 2,100 | R4 (dough stage) |
| 2,700 | R6 (black layer / physiological maturity) |
Wheat (32°F base, from green-up in spring): Winter wheat typically needs ~1,700 GDUs (base 32°F) from green-up to harvest.
Insect Pest Management: GDUs are used to time pesticide applications based on insect life cycles rather than calendar dates, which vary by location and year.
- Western corn rootworm egg hatch: begins at ~684 GDUs (base 52°F from January 1)
- Codling moth (apple pest) first flight: ~250 GDUs (base 50°F from biofix date)
- Alfalfa weevil first instar: ~200 GDUs (base 48°F from January 1)
Practical Application: A corn farmer in central Minnesota planting a 95-day hybrid typically expects about 2,700 GDUs to accumulate between planting (early May) and first killing frost (late September). This confirms the hybrid will reach black layer — physiological maturity — before the growing season ends. In a cool year, the same hybrid may not mature, meaning the farmer should have planted a shorter-season (85-day) variety instead.
Limitations of GDU Models: GDUs model heat accumulation only. They do not account for drought stress (which can accelerate or delay development unpredictably), soil temperature (which lags air temperature), photoperiod sensitivity (important for soybeans), or excessive rainfall. Use GDU models as guides, not absolutes.