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How many hours of direct sun does my garden get?

Plant labels say “full sun”, “partial shade” or “shade”. Whether a spot qualifies depends on the season and on what stands on its sunny side: a fence, a wall or the house itself. Here are the hours, worked out for a London garden.

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The labels

What full sun and partial shade mean

Gardeners count hours of direct sun during the growing season: the sun itself shining on the plant, not bright sky.

  • Full sun: six hours or more of direct sun a day.
  • Partial sun or partial shade: roughly three to six hours.
  • Shade: less than three hours, or only dappled light through trees.

Open ground in London gets 16.6 hours of possible sun on 21 June, 12.1 at the March equinox and 7.6 on 21 December. The question is how much of that a fence, a wall or a house takes away.

The numbers

Hours of sun behind a fence or a wall

The table follows one spot on the ground at different distances north of a long east–west fence or wall, the usual situation of a garden with its sunny side to the south. The figures come from the site’s sun engine, minute by minute through each day.

Hours of direct sun north of a long east–west barrier, London, 2026
BarrierDistance21 June20 March21 December
1.8 m fence1 m16.6 h0.1 h0 h
1.8 m fence2 m16.6 h0.1 h0 h
1.8 m fence3 m16.6 h12.0 h0 h
1.8 m fence5 m16.6 h12.1 h0 h
1.8 m fence8 m16.6 h12.1 h3.5 h
2 m wall5 m16.6 h12.1 h0 h
4 m wall5 m16.6 h1.0 h0 h
6 m wall5 m16.6 h0.1 h0 h
Open ground—16.6 h12.1 h7.6 h

Three things are worth noticing. In midsummer even a six-metre wall five metres away takes no sun at all: the June sun at noon stands 62° high and the shadow of the wall reaches only 3.2 metres. In December a 1.8-metre fence keeps the ground in shade for more than six metres. And in March the spots are either in shade all day or in sun all day, with almost nothing in between.

The rule of thumb

How deep a shadow reaches at noon

At solar noon the shadow of a wall reaches a depth of its height divided by the tangent of the sun’s elevation. In London that factor is:

Depth of a noon shadow behind a wall, London
DateNoon elevationShadow depth
21 June61.9°0.53 × wall height
20 March and 23 September38.4°1.26 × wall height
21 December15.1°3.72 × wall height

So a 1.8 m fence shades about a metre of ground in June, 2.3 m at the equinoxes and 6.7 m at the winter solstice. Further north the factors grow quickly; further south they shrink. Morning and afternoon shadows are longer, but they also point away at an angle, which is why the noon figure is a good guide for a long barrier.

A curious detail

Why the equinox is all or nothing

At the two equinoxes the sun moves along the celestial equator, and the tip of any shadow then travels along a perfectly straight east–west line through the day. The depth of a wall’s shadow behind it is therefore the same in the morning, at noon and in the afternoon: a spot inside it stays in shade all day, a spot just beyond it stays in the sun. The same straight track is the basis of the shadow-tip method of finding north, which works best at the equinoxes for exactly this reason.

Gardening months

Spring and autumn decide it

The growing season runs from about March to October, so the spring and autumn figures matter more than June or December. A bed two metres from a 1.8 m fence has no direct sun in mid-March, but from the beginning of April to early September it is in the sun nearly all day; by mid-September it is down to eight hours and by October to none. Move it to three metres and it gets the sun from the March equinox to the September one.

Real gardens have shorter fences, gaps, trees and houses at odd angles. The sun position map lets you place a building or a tree of any size, and its sun-hours layer shows the hours of direct sun for every point around it on any date. See also which way a house or balcony should face.

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