What are growing degree days?
A growing degree day is a way of counting warmth that matters for growth. Each day, temperatures are compared with a base value that depends on the plant or insect, and the warmth above that base is added to a running total. The total shows how far development has progressed.
Because spring warms at different speeds each year, the calendar date is not a reliable guide. Two farms with the same planting date can be days or weeks apart in real development.
Think of it as a thermometer for development. The more warmth a crop or insect has accumulated, the further along it is. Slow, cool springs add the total slowly, and hot ones add it fast.
Why are they more useful than calendar dates?
A cool spring slows growth and a warm one speeds it up. Growing degree days adjust for this. A crop that normally reaches a stage in late spring may reach it earlier in a warm year and later in a cool one.
Using accumulated warmth helps you time scouting, planting follow-up crops, harvest and cutting. It gives you an earlier warning that a stage is approaching.
This is also why published tables of crop stages and pest emergence are given in accumulated warmth rather than dates. A table built for one region will not match another region's calendar, but the idea carries across.
- Plan scouting before pests become a problem
- Estimate crop stages
- Plan harvest and labor needs
How are they calculated?
Methods differ, and the base temperature used differs by organism. Some methods use daily highs and lows, others use hourly readings. I will not give you formulas or base numbers because they are specific to the crop or pest and the method used.
The good news is that you rarely need to do the math by hand. Many university and weather services publish accumulated totals for your area, and your Extension office can explain which method and base they use.
Where do I find local numbers?
State Extension services, university weather networks and regional climate centers often provide tools that calculate growing degree days from nearby stations. You pick a location, a start date and the crop or pest, and the tool returns the accumulated total.
If you have your own weather station, you can compare its readings with these tools. Small differences are normal, and microclimates on your farm may run warmer or cooler than the station used.
Check whether the tool starts counting on January 1 or on another date. Different tools use different start points, and comparing totals across tools can mislead you.
How do growing degree days help with pests and diseases?
Many insects develop on a predictable heat schedule. Extension specialists often publish the accumulated warmth at which certain pests emerge or become vulnerable, which helps you scout at the right time.
Use these tables as guidance, not as a replacement for looking at your plants. Always follow product labels and local recommendations for any treatment, and confirm with your Extension agent before acting.
How should I use them on a small farm?
Start simply. Pick one or two crops or pests that matter most. Note the accumulated total when key events happen, such as bloom, first pest sighting or harvest. After a few seasons you will have a personal calendar tied to warmth rather than dates.
Keep notes alongside your weather records so you can see how each year compared. That history becomes a planning tool in its own right.
If you sell at markets or to restaurants, keeping this record also helps you tell buyers when to expect first harvests, using your own past data instead of hope.
What are the limits of this method?
Growing degree days are an estimate. They do not account for moisture, soil conditions, day length, disease pressure or the genetics of the particular variety you grow. A crop with plenty of heat but too little water still struggles.
Weather stations used for calculations may be miles from your farm, so your own totals could be higher or lower. A frost pocket or a south slope can change the count by a surprising amount over a season.
Treat the numbers as one helpful signal among several. Combine them with scouting, soil checks and your own experience.
- They estimate, they do not guarantee
- Local microclimates change the totals
- Use with scouting and observation
- Confirm with Extension before any treatment
Key takeaways
- Growing degree days count useful warmth, not days on the calendar.
- Each crop or pest has its own base value, so use local guidance.
- Free tools from universities often calculate totals for your area.
- Pair the numbers with scouting and your own notes.
Sources and further reading
Last reviewed: November 12, 2026. Written by the Rural Farm Life Editorial Team. See our editorial policy.