Temperature & Day Length
Most vegetables require a temperature of at least 6°C to germinate and start growing, with a higher temperature necessary for optimum growth. Vegetables are often separated into two groups:
- Cool-season crops, such as carrots, potatoes and onions, germinate and thrive in the lower temperatures of spring and autumn and can generally tolerate light frosts. Most cool-season crops can be sown directly in the garden just after the last frosts.
- Warm-season crops, which include marrows, cucumbers and tomatoes, prefer the heat of summer and should be planted after the soil has had chance to warm up. Many warm-season crops also need a long growing season and so should be started indoors in early spring or purchased as seedlings ready to be transplanted.
The list below details the optimum growing temperatures for a range of vegetables.
Cool Season Crops
- Cabbages: 15-20°C
- Carrots: 16°C
- Globe Artichokes: 13-18°C
- Lettuces: 10-20°C
- Onions: 13-24°C
- Peas: 13-18°C
- Potatoes: 16-18°C
- Spinach: 16-18°C
Temperate/Cool Season Crops
- Celery: 15-21°C
- French Beans: 16-30°C
- Tomatoes: 21-24°C
Warm Season Crops
- Cucumbers: 18-30°C
- Marrows: 18-27°C
- Peppers: 21-30°C
Some vegetables are classified by their response to the number of daylight hours (daylength) at different stages of growth. When the average daylength reaches a certain point in the late summer or early autumn, the vegetable will automatically go into a reproductive mode and try to produce seed or bulbs. Short-day varieties produce seeds or bulbs if the day length is 12 hours or less, whilst long-day varieties need over 12-18 hours.
For example, many onions are long-day plants; they stop making leafy growth and start to develop a bulb when the daylength is 16 hours or more. The more leafy growth that the plant has at this point; the bigger the bulbs. This means that the onions should be allowed to make as much leafy growth as possible before the daylength reaches 16 hours; this may be achieved by planting them as early in the season as possible.
Some cultivars, such as large seed butter (lima) beans are daylength neutral and have been adapted to long or short days.