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Predict your vegetable harvest date using heat unit science — more accurate than seed packet days-to-maturity
📋 Daily GDD Log
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| Day | Date | High | Low | Avg Temp | Daily GDD | Accumulated |
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🌱 GDD Requirements by Vegetable
Reference guide for common garden crops. GDD calculated from transplant date using base temperatures shown.
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| Crop | Base Temp | GDD to Harvest | Growth Notes |
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Seed packets say “75 days to maturity.” But a cool, cloudy summer and a hot, sunny one are not the same 75 days. Growing Degree Days (GDD) replace calendar time with heat time — giving you a harvest prediction that actually reflects what your plants have experienced.
Days-to-maturity figures on seed packets are averages measured at specific trial locations. In a cooler-than-average year or a northern garden, the same variety can take 2–3 weeks longer. In a heat wave, it may finish early. GDD removes the guesswork by measuring actual heat accumulation.
Plants grow in response to temperature, not the calendar. Below a crop’s base temperature (the minimum for growth), development essentially stops. GDD counts how many degrees above that threshold the plant experiences each day — giving a true picture of biological time, not clock time.
Every vegetable variety has a known total GDD requirement from planting to harvest. By tracking daily GDD accumulation and comparing it to the target, you can predict your harvest date weeks in advance — and plan succession plantings, preservation schedules, and market timing accordingly.
GDD also predicts when pests emerge. Japanese beetles appear at around 950 accumulated GDD (base 50°F). Corn earworm flight peaks at 250 GDD. Knowing this lets you time row covers and applications precisely, rather than guessing by the calendar or waiting until damage appears.
The standard formula for calculating daily Growing Degree Days is straightforward:
Accumulated GDD is simply the running total of daily GDD values from your planting date onward. When accumulated GDD reaches the crop’s maturity threshold, harvest is imminent.
Instead of spacing plantings 2 weeks apart by calendar, space them by GDD. In a hot spell, 2 calendar weeks may accumulate twice the GDD — meaning your second planting matures almost simultaneously with your first. GDD spacing keeps harvests evenly timed.
If you grow for canning, freezing, or selling, GDD helps you book the processing date weeks in advance. You can accurately predict when bulk quantities of tomatoes, corn, or beans will be ready — before the first fruit is visible on the plant.
Keeping a GDD log lets you see whether this year is running warm or cool compared to previous seasons. A garden journal with GDD data is far more useful than one that records only calendar planting dates, especially as weather patterns become less predictable.
Sweet corn hits silking at about 1,000 GDD and is ready to eat at 1,400–1,600 GDD from planting. Hand-pollinate or time irrigation precisely because the silk stage lasts only 5–8 days. GDD tells you when that window is coming well in advance.
Growing Degree Days measure the amount of heat a crop accumulates each day above a minimum threshold temperature (the base temperature). Because plants grow in response to heat rather than the calendar, GDD provides a much more accurate timeline for crop development than counting calendar days. Each day’s GDD equals the average of the day’s high and low temperatures minus the base temperature, with negative values counted as zero.
Use this formula: GDD = ((High Temp + Low Temp) ÷ 2) − Base Temperature. If the result is negative, record it as 0. For most warm-season vegetables, use a base temperature of 50°F (10°C). Cap the high temperature at 86°F if it exceeds that. Add each day’s GDD together from your planting date to get accumulated GDD. The calculator above does all of this automatically once you enter your daily temperatures.
Most tomato varieties need 1,000–1,400 accumulated GDD (base 50°F) from transplanting to first ripe fruit. Early varieties like Early Girl or Stupice need around 1,000 GDD. Mid-season varieties like Celebrity need 1,100–1,200 GDD. Large beefsteak types may require 1,300–1,500 GDD. The exact number varies by variety — your seed packet’s days-to-maturity × 15–18 is a rough GDD estimate if you don’t have the specific figure.
Base temperature varies by crop. Most warm-season vegetables (tomatoes, peppers, cucumbers, squash, beans, corn, melons, eggplant) use 50°F (10°C). Cool-season crops (peas, lettuce, spinach, broccoli) use 40°F (4°C). Some specific crops have different base temperatures — sweet corn is often calculated with a 50°F base and an 86°F upper cap, while alfalfa uses 41°F. When in doubt, 50°F is the correct default for most summer vegetables.
Yes, significantly more accurate in years where temperatures deviate from average. Days-to-maturity figures are measured at specific trial locations in average-temperature years. In a cool summer, a “70-day” tomato may take 85–90 calendar days. In a hot summer, it may finish in 60. GDD accounts for actual heat received and consistently predicts harvest within a few days when the crop’s GDD requirement is known, regardless of whether the season runs hot or cold.
This free Growing Degree Days calculator is built for home gardeners who want more precision than seed packet estimates provide. Enter your daily high and low temperatures from planting day onward, and the calculator accumulates GDD using the standard average-temperature method with crop-appropriate base temperatures. It also shows crop development milestones, a daily log table, and a reference chart for common vegetable GDD requirements. For best results, use your own thermometer readings or check weather history for your ZIP code at Weather Underground or the National Weather Service.
Pair this tool with our Frost Date Calculator to plan your planting window, or use the Hardening Off Calculator to prepare transplants before they go in the ground.
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