Lesson 1
What these structures actually do
A greenhouse and a cold frame do not keep plants warm in the way a house does. They trap solar heat during the day and release it slowly at night. On a February morning, the air outside might be 2°C while inside an unheated greenhouse it is 8°C. That six-degree margin is transformative. It is the difference between a tomato seedling that sulks and one that puts on real growth.
The cold frame is the greenhouse's modest cousin. It sits low to the ground, captures warmth close to the soil, and asks almost nothing of you. You lift the lid on mild days, close it at dusk, and in return it extends your season by four to six weeks at each end. If you have space for only one structure, start with a cold frame. If you have both, they work as a relay: seedlings begin in the greenhouse, harden off in the cold frame, then graduate to open ground.
Neither structure is about exotica. The real prize is ordinary crops grown out of season. Salad in January. Ripe tomatoes in May. Courgettes that fruit until November. You are not defying nature, just giving it a head start and a kinder microclimate in which to finish.
What you'll learn
- How much warmth each structure provides and when that matters
- The difference between season extension and true year-round growing
- Why a cold frame often delivers better value than you expect
- What crops genuinely need protection and which are just tolerant of it
Numbers that matter
- Unheated greenhouse in winter: typically 4–8°C warmer than outside air
- Cold frame on a sunny March day: can hit 20°C inside while outside is 10°C
- Frost protection: both structures keep temperatures above –2°C on most UK nights
- Season extension: 4–6 weeks earlier in spring, 4–6 weeks later in autumn
If you can only afford one, buy the cold frame first — it pays back faster and asks less of your attention.