Where to put a greenhouse
The usual advice is to find the sunniest spot in the garden. That is the right idea for the wrong season. In summer almost anywhere open will do, and the real problem is too much heat. A greenhouse runs short of light between November and February, so site it for those months.
On this page
Site it for winter, not summer
In London the midday sun climbs to 62° in June and only 15° in December, and the day shrinks from 16½ hours to under 8. A greenhouse full of tomatoes in July has light to spare and needs shading paint and open vents. The same greenhouse in January is holding overwintering pelargoniums, and in February it is where seeds get sown. That is when every hour of direct sun counts.
Winter sun also comes from a much narrower part of the sky. At the December solstice in London the sun rises at a bearing of 128° and sets at 232°, so it spends the whole day between south-east and south-west, never more than 15° above the horizon. In June it rises at 49° and sets at 311° and swings round nearly three-quarters of the compass.
What that means on the ground. For a winter greenhouse, the view that matters is a low one, from south-east through south to south-west. A tree due north of the greenhouse is irrelevant in January. A fence to the south-south-west is not.
How far from fences and the house
An obstacle to the south casts a shadow of its height ÷ tan(sun altitude). What matters is not the full height of the fence but how far it rises above the benches, because the plants sit on staging about 0.8 m up. A 1.8 m fence stands only about 1 m above the bench top, and that is the height to feed into the sum.
| Obstacle to the south | 21 December | 21 February | 21 March |
|---|---|---|---|
| 1.8 m fence, to reach the benches | 3.7 m | 1.9 m | 1.2 m |
| 1.8 m fence, to reach the ground | 6.7 m | 3.4 m | 2.2 m |
| Two-storey house, 7 m to the ridge | 26 m | 13 m | 8.7 m |
Gap needed between the obstacle and the greenhouse for midday sun to clear it, at latitude 51.5°N (London). The midday sun is 15° high on 21 December, 28° on 21 February and 39° on 21 March.
The fence rows are the reassuring ones. Three or four metres from a 1.8 m south boundary is enough for midday sun on the benches all winter in southern England. The house row explains why a greenhouse within 8 m or so of the north side of a two-storey house sits in its midday shadow from October until March. No realistic garden is 26 m deep.
Further north the distances grow quickly. In Edinburgh the December sun peaks at under 11°, so the same fence needs about 5.3 m of clearance to reach the benches, and 2.3 m even in February. In Manchester the figures fall between the two.
Which way to run the ridge
Most guides say to run the ridge east–west, so that one long side faces south. That is right in midwinter and slightly wrong the rest of the year. We modelled a standard 3 m × 2 m greenhouse (1.5 m eaves, 2.2 m ridge) under a clear sky and summed the direct sunlight falling on its glass through the day, once with the ridge running east–west and once north–south:
| Date | Ridge east–west | Ridge north–south | East–west gains |
|---|---|---|---|
| 21 December | 23 kWh | 19 kWh | +23% |
| 21 January | 30 kWh | 26 kWh | +18% |
| 21 February | 46 kWh | 43 kWh | +5% |
| 21 March | 58 kWh | 61 kWh | −4% |
| 21 June | 83 kWh | 88 kWh | −6% |
| 21 October | 45 kWh | 42 kWh | +6% |
| 21 November | 30 kWh | 25 kWh | +18% |
Direct-beam sunlight striking the glass on a cloudless day in London, in kilowatt-hours. This measures light arriving, not heat retained, and it ignores framing, reflection off the glass and the greenhouse shading itself. The comparison between the two orientations is the useful part, not the absolute figures.
The pattern makes sense given where the winter sun is. In December the sun stays low in the southern sky all day, so a long south-facing side catches it almost square-on. In summer the sun spends hours in the east and west, and a long east- or west-facing side catches more of it.
So the choice depends on how you use the greenhouse. If it is heated, or used for overwintering and early sowing, run the ridge east–west. In Edinburgh the midwinter gain rises to about 30%. If it is really a summer tomato house, north–south gives slightly more light, and the light is more even from morning to afternoon. Either way the difference is smaller than one badly placed fence, so let the plot decide if it has to.
Lean-to greenhouses
A lean-to against a south-facing house wall is the best-lit winter structure most gardens can have. It has one glazed side facing straight at the low sun, and the brick behind it soaks up heat by day and gives it back at night. Against an east or west wall it gets a morning or an afternoon, which is fine for a porch full of seedlings in spring.
A lean-to on a north wall gets no direct sun at all from late September to late March, because the sun rises south of east and sets south of west for that whole half of the year. It works as a cool, bright shade house for ferns and cuttings, but it will not do what a greenhouse is for.
The things that are not about the sun
- Avoid the lowest point. Cold air drains downhill and pools. A greenhouse at the bottom of a sloping garden, or against a solid wall at the foot of a slope, can sit several degrees colder on a still winter night than one a few metres uphill.
- Shelter it on the north and east. Winter wind strips heat from glass quickly. A hedge or fence on the north or east side shelters the greenhouse without shading it. The same windbreak on the south side would block the light you placed it there for.
- Keep clear of trees. Bare winter branches still block a noticeable share of low sun, and overhanging trees drop leaves, sap and occasionally branches onto the roof. Distance matters more than species.
- Think about the walk. A greenhouse needs watering daily in summer and checking on frosty nights. One at the far end of a long garden, away from a tap, gets visited less often. If the sunniest spot is also the least convenient, a slightly shadier spot near the house can be the better choice.
- Plan for power. A heater, a propagator or a fan all need a safe outdoor supply, which should be installed by an electrician. It is cheaper to plan the cable run before the base goes down than after.
Check your own spot
The tables above assume a flat garden at London's latitude with one obstacle due south. Real gardens have fences on three sides, a neighbour's extension to the south-west and a tree that has grown since the house was built, and they all combine. The quickest way to find the right spot is to draw them in.
The garden tool lets you place your house, fences and trees to scale at your own address, then maps how many hours of direct sun each part of the plot gets on any date. Set it to 21 December and look for the area that still gets sun. That is where the greenhouse should go.
In the southern hemisphere, swap north and south throughout: site the greenhouse for the northern sky, and a lean-to belongs on a north-facing wall.