What Are Growing Degree Days?
Growing degree days are a way of tracking accumulated warmth rather than simply counting calendar days. They can help gardeners understand why a crop matures earlier in one year than another, or why a northern plot lags behind a warm southern garden. They are a planning aid, not a promise of harvest.
Growing degree days add up the useful warmth a plant receives over time. Each crop has a base temperature below which it grows very slowly, and daily warmth above that point contributes to its total. You do not need to calculate every figure to benefit from the idea. The practical lesson is that a warm May followed by a cool June may produce very different results from the same dates in another year. Sweetcorn, tomatoes, courgettes and French beans respond strongly to warmth, while peas and many brassicas can grow in cooler conditions. Use weather observations alongside seed-packet timings.
Start with a simple garden record rather than a complicated formula. Note monthly weather, first sowing dates, planting dates, flowering and first harvest for a few key crops. Include whether plants were outdoors, under fleece, in a greenhouse or in a polytunnel. After two or three years, patterns will emerge. You may learn that outdoor tomatoes only ripen reliably after an early indoor start, or that your sheltered south-facing bed warms enough for an earlier sowing of dwarf French beans. These observations are your own version of growing degree day information, tailored to the microclimate where you actually grow.
Use warmth to decide when to sow tender crops, but combine it with soil condition and frost risk. Wait until after the last spring frost before planting tomatoes, courgettes and French beans outdoors. Soil should feel workable and warming, not cold and saturated. In much of southern England this may happen earlier than in Scotland, northern England or exposed upland gardens. Harden off indoor-raised plants over several days, taking them outside for gradually longer periods. A cloche or fleece can ease the transition, but remove covers on warm days to prevent overheating and allow healthy airflow.
Create modest extra warmth where it makes a genuine difference. Choose a sunny, sheltered site, use dark containers for tender crops and place them where they can absorb daytime warmth. A greenhouse or polytunnel provides a larger temperature advantage, especially for tomatoes, aubergines and peppers. Warm soil by covering it briefly with a cloche before planting, then water carefully because protected ground can dry quickly. Do not crowd plants in an attempt to create warmth: poor airflow and competition reduce growth. The aim is steady, suitable conditions, not maximum heat at every hour.
Use expected maturity dates with caution. A packet may state that a crop needs a certain number of days, but this usually assumes favourable warmth, light and moisture. In a dull, cool summer, a crop can take longer even when you sowed on time. Avoid pulling up slow plants prematurely. Instead, check whether they are healthy, receiving enough sun and being watered appropriately. If autumn approaches before tender crops ripen, remove late flowers and small, unlikely fruits so the plant can direct energy towards existing produce. Under cover, ventilate regularly to prevent excessively humid conditions.
At the end of the season, compare varieties as well as weather. A quick-maturing sweetcorn or tomato can be a sensible choice for a cooler garden, while a warm southern plot may accommodate longer-season types. Rotate crops sensibly, improve soil with garden compost and plan protected space for plants that need it most. Growing degree days will not account for every issue, including slugs, nutrient shortages, wind or poor pollination. They are most valuable when they help you replace fixed calendar expectations with careful observation and more realistic timing.
Frequently asked questions
Do I need to calculate growing degree days exactly?
No. Most home gardeners can use the principle without daily calculations. Keep notes on temperature patterns, sowing dates and first harvests, then compare seasons. Exact figures can be useful for keen record keepers, but they do not replace checking soil warmth, local frost risk, plant health and the forecast.
Which vegetables benefit most from extra warmth?
Tender crops such as tomatoes, sweetcorn, courgettes, aubergines, peppers and French beans benefit most clearly from accumulated warmth. A greenhouse, polytunnel, fleece or sheltered sunny position can improve their progress. Cool-season crops, including peas and many brassicas, need less heat and may suffer if conditions become excessively hot or dry.
Why are my plants later than the seed packet says?
Seed-packet times are guides based on favourable conditions. Cool weather, low light, cold soil, delayed germination, drought, wind, shade or transplant shock can all slow a crop. Record what happened in your garden, then adjust future sowing dates, variety choice or protection rather than assuming you made a major mistake.
General guidance for UK home gardens and allotments. Conditions vary by region, soil and season; the RHS or a local garden centre can give site-specific advice. Page updated 19 Sep 2026.