Greenhouse
A transparent shelter that lets light in, holds on to warmth and keeps out wind and rain: crops are brought forward, extended or protected inside.
Explanation
Sunlight passes through the glass or plastic and warms the soil, the plants and the air; the walls keep that warmth in, and glass even reflects back inside the heat radiated within the greenhouse (RHS). Without heating, it is called a cold greenhouse: it brings spring sowings and plantings forward, takes in cold-sensitive summer crops such as tomato, aubergine or sweet pepper, and keeps hardy leafy vegetables through the winter (RHS).
Refined definition
The greenhouse is the most enclosed step in a family of shelters: the cap set over a single plant, the low tunnel covering a few rows that you cannot walk into, the high tunnel you walk through, the glass or polycarbonate greenhouse and, at the far end, the heated and lit greenhouses of professional growers (Purdue University). Fleece spread over crops and the cold frame, a low box with a glazed lid, are its cousins (RHS). Heated, a greenhouse changes in nature: it keeps cold-sensitive plants frost-free, at the cost of energy that the RHS encourages gardeners to save. In permaculture, it is an element to place like any other, close to the house and in the winter light (see permaculture zones and sectors).
Key points
What it changes
By day, the shelter warms up quickly, out of the wind and the showers: a cold greenhouse gains weeks in spring as in autumn. At night, with no sun, it cools down: when it freezes hard outside, a fleece is spread over the crops and taken off in the morning (Purdue University).
Its winter limit: light
Below about ten hours of daylight, growth becomes very slow, even in the warm: this is the 'Persephone period', in Eliot Coleman's phrase taken up by Purdue University. Hence the rule of winter growing: plant early enough for the vegetables to be close to harvest size before the shortest days.
What it asks of you
A greenhouse has to be run. It needs airing as soon as the weather is fine, because it overheats quickly from spring to autumn (RHS), watering since nothing falls from the sky inside, and watching for pests that like warm, dry air, such as red spider mite (RHS). With no rain to wash them out, salts from fertilisers and compost also build up in its soil (Purdue University).
Watch out for
- • Closed on a sunny day, a greenhouse can scorch its plants, even in spring: the RHS advises keeping it below 25 to 27 °C.
- • In France, a greenhouse more than 1.80 m high requires a prior declaration at the town hall, and a building permit beyond 4 m high or 2,000 m² of floor area (Code de l'urbanisme, articles R421-2 and R421-9).
Examples
- • A lean-to greenhouse against the south wall of a house, still warmed by the wall at night
- • A tunnel of lamb's lettuce and spinach, picked from autumn to spring
- • A cold frame on a bed of hot manure, for the year's first sowings
Sources
- RHS (Royal Horticultural Society) : « Greenhouses: choosing » (rhs.org.uk/garden-features/choosing-greenhouses)
- RHS : « Greenhouse: ventilation and shading » (rhs.org.uk/garden-features/ventilation-and-shading-greenhouses)
- RHS : « Vegetables: growing in your greenhouse » (rhs.org.uk/vegetables/growing-in-greenhouse)
- RHS : « Fleece and crop covers » (rhs.org.uk/prevention-protection/fleece-and-crop-covers)
- RHS : « Coldframes and mini-greenhouses » (rhs.org.uk/garden-features/coldframes-and-mini-greenhouses)
- RHS : « How to heat a greenhouse efficiently » (rhs.org.uk/advice/profile?pid=376)
- RHS : « Glasshouse red spider mite » (rhs.org.uk/biodiversity/glasshouse-red-spider-mite)
- RHS : « Climbers and wall shrubs for sun » (rhs.org.uk/plants/types/climbers/for-sun)
- Maynard, E. T. & O'Donnell, M. (2019). Managing the Environment in High Tunnels for Cool Season Vegetable Production. Purdue Extension, HO-297-W.
- Guan, W. (2016). « Salt Accumulation for Vegetable Production in High Tunnels ». Vegetable Crops Hotline, Purdue University.
- Smith, C. (2008). Starting Plants Early Outdoors. New Mexico State University, Cooperative Extension Service, Guide H-220.
- Code de l'urbanisme, articles R*421-2 (e) et R421-9 (g), version en vigueur depuis le 16 novembre 2024 (Légifrance).