Frost & Growing Season Calculator
Southampton's mild, coastal position gives it one of the longer frost-free growing seasons in the UK. This page combines the local frost-date averages with a growing degree day calculator, so you can work out both when it's usually safe to plant out and roughly how much "growing warmth" a period of weather has actually delivered.
Southampton's Average Frost Dates
Frost risk depends on how cold it needs to get, not just whether the thermometer touches zero — a tender bedding plant can be damaged by a light ground frost that a hardy shrub shrugs off completely. These are 30-year climatological averages for Southampton (50.92°N, 1.40°W), not a forecast for any particular year.
| Frost Type | Threshold | Last in Spring | First in Autumn | Frost-Free Days |
|---|---|---|---|---|
| Air Frost | 0°C | 11 May | 20 Oct | ~162 |
| Light Ground Frost | -1°C | 24 Apr | 9 Nov | ~199 |
| Hard Ground Frost | -2°C | 12 Mar | 18 Dec | ~281 |
Averages sourced from 30-year climate records for the Southampton area. Southampton falls in USDA Hardiness Zone 8b (average annual extreme minimum -9.4 to -6.7°C), used internationally as a shorthand for which perennial plants can be expected to survive a typical winter here.
Frost Risk Through the Year
Each bar shows the period when that level of frost is possible, running from first autumn frost through to last spring frost of the following year.
Average Monthly High Temperature
30-year average daytime high for Southampton. Night-time lows range from around 3°C in mid-winter up to 14°C in July.
Growing Degree Day Calculator
Growing degree days (GDD) estimate how much "usable warmth" plants received over a period, by summing how far the average daily temperature sat above a base threshold below which growth is negligible — commonly 5°C (41°F) for cool-season crops and hardy plants, or 10°C (50°F) for warm-season vegetables like tomatoes and sweetcorn. Enter your own average daily high and low for a period (use the monthly chart above as a Southampton reference if you don't have your own readings) and a base temperature to see the total.
Using This in the Garden
Most half-hardy bedding plants and vegetable seedlings are safe to plant outdoors once the risk of air frost has passed — typically after the second week of May here — but keeping fleece or cloches handy into early June covers the years running a little colder than average. At the other end of the season, the first light ground frost in early-to-mid November is usually what finally ends the growing season for tender crops, even though the more damaging hard ground frost doesn't typically arrive until close to Christmas. Our Evapotranspiration page is a useful companion to this one for working out watering needs through the growing season itself.