How do I shade a greenhouse, and when should I put shading up?

Shading is not about protecting plants from light so much as protecting them from heat. On a still, sunny June day an unshaded greenhouse can reach temperatures at which tomatoes stop setting fruit and leaves scorch. Ventilation is the first line of defence and shading is the second, and the two together are what keep a summer greenhouse usable.

Safety

Step by step

1. Ventilate properly before you shade.Shading a greenhouse with inadequate vents treats the symptom. Get the roof vents and louvres right first — shading is an addition to ventilation, not a substitute for it.
2. Time it by the season, not by a hot spell.In most of Britain shading goes on in late May and comes off in September. Putting it on too early costs you light in a month when tomatoes and cucumbers are still establishing; leaving it on too late costs you ripening.
3. Shade the south and west faces hardest.The roof and the south-facing side take the most heat. The north side rarely needs shading at all, and shading it just reduces light for no benefit.
4. Apply shade paint on a dry, still day.Dilute to the instructions — most products let you vary the strength, so mix it lighter in a dull year. Apply to the outside with a soft brush, a roller on a pole or a sprayer, working in overlapping strokes down each pane.
5. Fit netting so it cannot flap.Tension it over the roof and fix at close intervals along the eaves and ridge with the clips supplied, or with cord under the glazing bars. Loose netting rubs, tears and eventually pulls a pane out.
6. Keep shading clear of the vents.Whatever you fit must not obstruct a roof vent opening or block a louvre. Shading that stops the vents working makes the greenhouse hotter, not cooler.
7. Take it off promptly in autumn.Shade paint scrubs off with a plastic scourer and warm water, or comes away with a proprietary remover. Netting comes down and gets stored dry. Light is scarce from September and every bit of it matters.

Tips

• Shade paint is designed to become more transparent when wet, so it lets more light through on dull rainy days and blocks more in dry sunny ones. That is genuinely useful behaviour and one reason it remains popular.
• Do not paint polycarbonate. It gets into the flutes and into the surface texture and is very difficult to remove. Netting or internal blinds are the right choice on a polycarbonate house.
• If you are only trying to protect a small number of seedlings, shading one bench with fleece or netting is far better than shading the whole house.

Common mistakes

• Fitting internal blinds and expecting them to solve overheating — Once light has passed through the glass it has become heat inside the greenhouse, and a blind cannot send it back out. Internal shading protects plants from direct scorch but does much less for temperature.
• Shading before the crop has established — Tomatoes, cucumbers and peppers need all the light they can get in May and June. Shade too early and you will get a leafier, later, poorer crop.
• Leaving shade paint on into October — Autumn light is the limiting factor for ripening and for overwintering plants. Get it off as soon as the heat has gone.

Alternatives

• Shade paint or wash: A dilutable white or green wash brushed or sprayed onto the outside of the glass. Cheap, effective, self-adjusting in the sense that it becomes more translucent when wet by rain, and it washes off with a scourer in autumn. Messy to apply and not suitable for polycarbonate flutes.
• External shade netting: Woven or knitted netting in stated shade percentages, clipped or tensioned over the outside of the roof. The most effective option, because it also keeps the glass itself cooler, and it can be removed on dull weeks. Needs proper fixings or it becomes a sail.
• Internal roller blinds: Fitted inside the roof on the glazing bars. Convenient, adjustable day by day, and tidy. Substantially less effective than external shading because the heat has already passed through the glass, and they obstruct the roof vents.
• Living shade: Training a vigorous crop such as a cucumber or a vine up the sunny side. Free and productive, but uncontrollable — it shades whether you want it to or not, and it shades in September when you would rather have the light.

Keeping on top of it

• Apply shading (Late May)
• Check netting fixings after wind (Monthly through summer)
• Remove shading and wash the glass (September)

Questions people ask