The short version: Use 30 per cent shade cloth over most vegetables in a heatwave, and 50 per cent over lettuce and other leafy greens in the harshest heat. Cloth of 70 to 90 per cent is shade-house cloth and cuts too much light for most food crops. Rig it above the plants on a frame with air space, not draped over them.
Which percentage
- 30 per cent: tomatoes, capsicum, chilli, eggplant, beans, cucumbers, zucchini and pumpkin. Fruiting crops need strong light to flower and set; 30 per cent takes the sting out of afternoon sun and cuts scald while leaving enough light to crop.
- 50 per cent: lettuce, rocket, spinach, Asian greens, coriander and young brassicas when the forecast is in the high thirties. These bolt and burn long before they run short of light, and heavier shade keeps them pickable for longer. It also suits new transplants of any crop for their first week.
- 70 to 90 per cent: ferns, patios and car spaces. Under it most fruiting crops grow tall and soft and set little, and leafy greens come up thin and pale.
The percentage is roughly the share of sunlight the cloth blocks, so 30 per cent cloth lets about 70 per cent through. Buy by the percentage, not the colour.
How to rig it
- Build a frame: star pickets with safety caps, poly pipe hoops, a timber frame or a wire strung between posts. Set the cloth at least 30 cm above the tallest plant.
- Cover the top and the side that takes the afternoon sun, usually the west. North of the Tropic of Capricorn the summer sun passes south of overhead, so watch where the shadow falls on your own bed. Leave the other sides open for airflow and bees.
- Fix it with clips or ties through the edge, pull it taut, and check after wind that it has not sagged onto the plants. A flapping sheet shreds leaves.
- Put it up the evening before a hot day and take it down once the heat passes, so the plants get full light. In the hottest inland districts a 30 per cent roof can stay up all summer; elsewhere treat it as a heatwave measure. Cloth also slows drying, so check the soil before you water.
Why it goes above, not on
Cloth laid straight onto plants traps heat against the leaves, rubs them raw in wind, blocks the cooling breeze, and keeps bees off the flowers. The gap does the cooling: shade above, moving air beneath. If the plants are too small for a frame, push in four stakes tall enough to hold the cloth clear of the leaves and rest it on those.
A few metres of cloth on a frame can carry a garden through its worst week.
Related reading
Lettuce holds on longer under 50 per cent cloth; choose from the lettuce seed range.