GDD Calculation¶
Growing Degree Days (GDD) measure the accumulated heat a plant has experienced since sowing. They allow more reliable predictions of harvest time and phase transitions than calendar days alone, because plants respond to accumulated warmth — not to calendar dates.
Prerequisites¶
- A plant instance with a sowing date
- Daily temperature readings (entered manually or via sensor)
- Known base temperature for the plant species (can be stored in master data)
The GDD Formula¶
Kamerplanter uses the standard daily average method:
| Parameter | Meaning | Typical value |
|---|---|---|
T_max | Daily maximum temperature (°C) | measured or day average |
T_min | Daily minimum temperature (°C) | measured or day average |
T_base | Plant base temperature (°C) | 10 °C (most vegetables), 0 °C (wheat) |
Accumulated GDD since sowing is the sum of all daily values:
Negative values are ignored
If the daily average temperature falls below the base temperature, the result is 0 — not a negative number. The plant accumulates no heat on cold days.
Base Temperatures for Common Plants¶
| Plant | T_base (°C) | Note |
|---|---|---|
| Tomato | 10 | Harvest ~1000–1400 GDD |
| Pepper / Chili | 10 | Harvest ~1200–1600 GDD |
| Cucumber | 10 | Harvest ~600–800 GDD |
| Lettuce | 4 | Fast — ~500 GDD |
| Corn | 10 | Maturity ~1300–1600 GDD |
| Cannabis (short-day) | 10 | Flowering varies greatly by cultivar |
| Basil | 10 | First harvest after ~300 GDD |
| Carrot | 4 | Harvest after ~1000–1200 GDD |
Store base temperature in master data
Enter the base temperature for a species directly in master data under the "Growth Requirements" tab. Kamerplanter will use this value automatically in all GDD calculations for plants of that species.
Example Calculation¶
Scenario: Tomato, T_base = 10 °C, 5 days after sowing
| Day | T_max (°C) | T_min (°C) | Daily average | GDD_day | GDD accumulated |
|---|---|---|---|---|---|
| 1 | 22 | 14 | 18.0 | 8.0 | 8.0 |
| 2 | 25 | 16 | 20.5 | 10.5 | 18.5 |
| 3 | 18 | 8 | 13.0 | 3.0 | 21.5 |
| 4 | 11 | 6 | 8.5 | 0.0 | 21.5 |
| 5 | 24 | 15 | 19.5 | 9.5 | 31.0 |
On day 4, the daily average (8.5 °C) was below the base temperature (10 °C), so no GDD were accumulated.
GDD and Phase Transitions in Kamerplanter¶
Kamerplanter can evaluate GDD-based phase transition rules. When a plant reaches a defined GDD threshold, a transition prompt is automatically triggered.
Configuring a GDD Transition Rule¶
- Open the master data for the desired plant species.
- Navigate to Lifecycle > Transition Criteria.
- For the desired phase transition, select the type GDD-based.
- Enter the threshold value in GDD.
Example: Tomato Vegetative → Flowering
Enter 400 GDD as the threshold for the transition from the vegetative phase to the flowering phase. Once the plant reaches this value, a transition prompt appears in the dashboard.
GDD vs. Calendar Days¶
| Criterion | Calendar Days | GDD |
|---|---|---|
| Simplicity | Very simple | Requires temperature data |
| Accuracy during warm/cold periods | Low | High |
| Comparability across years | Limited | Comparable |
| Best for | Rough planning | Harvest and phase forecasting |
Combination recommended
For precise harvest timing, combine both methods: calendar time as a rough frame, GDD as the fine indicator for maturity.
Upper-Capped GDD (Optional Method)¶
Some plants do not accumulate additional maturity at very high temperatures — growth slows above a maximum temperature. The extended formula:
| Parameter | Meaning |
|---|---|
T_max_cap | Upper temperature cap (e.g. 30 °C) |
T_eff | Effective temperature after capping |
Simplified method in Kamerplanter
Kamerplanter uses the standard daily average method without upper temperature capping. The extended method is on the roadmap for future versions.
Background: Why GDD Are Better than Calendar Days¶
Plants are not calendars. Their development is driven by accumulated heat energy. A warm spring can advance tomato development by 2–3 weeks compared to a cold year. Expressed in GDD, those two years are directly comparable.
Practical example from outdoor growing
In a warm year (April average 16 °C), a tomato reaches the flowering phase in 4 weeks. In the cooler following year (April average 12 °C), it takes 7 weeks. In GDD, both events occur at approximately 400 GDD.
Frequently Asked Questions¶
Which base temperature should I use for cannabis?
Most cannabis strains use T_base = 10 °C. Some indoor growers use 15 °C since plants never experience temperatures below that. Consistency matters more than the absolute value — use the same T_base for all plants of one species.
Do I need to record temperatures every day?
For indoor growing with stable temperature, a single daily average is sufficient. For outdoor growing, a min/max thermometer is recommended. In future versions, Kamerplanter will be able to retrieve weather data automatically via DWD/Open-Meteo integration (REQ-005).
GDD value seems unrealistically high — what went wrong?
Check that the base temperature is correctly set in master data. If it was accidentally set to 0 °C, all ambient warmth is summed up without a meaningful floor.