Shade cloth percentage calculator

What shade cloth is actually for

Shade cloth does two things and growers usually think about only one of them.

It reduces heat load. This is the reason most people fit it, and it is genuinely the cheapest cooling in a greenhouse: it removes solar energy before that energy enters the structure, which no fan, cooler or vent can do. It also has no running cost and no moving parts.

It reduces light to a level the crop can use. This is the part that gets overlooked, and it is why the correct percentage is a crop question rather than a temperature question. Photosynthesis saturates: above a certain intensity a leaf cannot use more light, and the additional photons become heat and, at extremes, cause photoinhibition and leaf damage. For tomatoes and peppers that saturation point is well below full summer sun, which is why removing 40 to 50 percent of it costs almost nothing in yield.

For shade-adapted plants such as orchids and ferns the second consideration dominates completely. These are understory species whose leaves burn in direct sun regardless of air temperature, so they need 70 to 80 percent shade even in a cool climate.

Setting the percentage

The method is to compare what the crop needs with what the season delivers, and shade the difference.

Outdoor daily light integral at temperate latitudes runs roughly 20 to 30 mol per square meter per day at midsummer, 10 to 20 in spring and fall, and 5 to 15 in winter, varying with latitude and cloud. Greenhouse glazing transmits around 78 to 90 percent depending on material, so the figure inside is proportionally lower before any shade is applied.

Against that, lettuce wants 12 to 17, seedlings 10 to 14, tomatoes 22 to 30 and orchids 4 to 6. In midsummer, an unshaded greenhouse is delivering more than a lettuce crop can use and roughly what a tomato crop wants. In midwinter it is delivering less than either. That seasonal inversion is the whole shade strategy in one sentence.

The practical consequences: shade cloth is a summer tool, taking it off in fall matters as much as putting it on in spring, and in a mixed house the correct percentage is set by the most light-sensitive crop unless you can shade zones separately. The percentages are tabulated in the shade cloth chart and the light targets in the DLI chart.

Outside the glazing, always

Where the cloth hangs determines how much cooling it delivers, and the difference is large.

Cloth outside the glazing intercepts solar radiation before it enters. The energy the fabric absorbs is dissipated to the outside air and never becomes heat inside the greenhouse.

Cloth inside the glazing intercepts radiation that has already passed through the panel. The fabric absorbs it, warms up, and re-radiates into the very air volume you are trying to cool. Interior cloth still reduces light on the plants and still stops direct sun scorching foliage, so it is not useless, but as a cooling measure it delivers a small fraction of what the same cloth achieves outside.

The air gap matters too. Cloth held a few inches clear of the glazing allows air to move between the two and carry heat away. Cloth laid directly on the panel conducts heat straight into it. Simple standoffs or a batten frame make a measurable difference for almost no cost.

Alternatives and combinations

Shade cloth is not the only way to reduce light, and the alternatives have genuine advantages in specific situations.

Shading paint or whitewash is applied to the glazing and washed off at the end of the season. It costs less per square foot, needs no fixings, cannot flap or tear, and gives completely even diffuse shading with no stripes. Its drawback is that it is a commitment: it cannot be removed for a cool cloudy week and it must be reapplied annually. The comparison is worked through in shade cloth versus whitewash.

Diffusing glazing achieves something related but different. Twin-wall polycarbonate scatters light rather than transmitting a direct beam, and diffuse light penetrates a canopy better than direct light does, illuminating lower leaves that a beam never reaches. A house with diffusing glazing can produce more total photosynthesis than a clearer one despite transmitting less light overall, which is a genuine and underappreciated argument for twin-wall.

Zoned shading is the refinement worth knowing about. A propagation area under 70 percent cloth inside a house otherwise at 40 percent lets one building serve two very different light requirements. It costs a second piece of cloth and some hanging, and it is what makes a mixed house work properly.

Reading the crop rather than the thermometer

Plants tell you when the shade percentage is wrong, and the signs differ in each direction.

Too little shade: pale or bleached patches on leaves exposed to direct sun, leaf edges scorching and going brown and crisp, wilting in the middle of the day despite moist medium, flower drop on tomatoes and peppers, and lettuce bolting or turning bitter. The wilting sign is the diagnostic one: a plant wilting at midday in wet soil is losing water faster than roots can supply it, which is a light and heat problem rather than a watering problem.

Too much shade: stretched, elongated stems with long gaps between leaves, pale green foliage, thin weak growth, poor flowering and fruit set, and in herbs a noticeably weaker flavour, because essential oil content rises with light. Seedlings leaning toward the brightest side of the house is another clear signal.

Both patterns can appear in the same greenhouse at the same time if it holds crops with different requirements, which is the argument for zoning rather than for compromising on a single percentage.

Working out the figures for your own house and season? The Greenhouse Build & Growing Planner is the paid version of these pages: 8 printable worksheets you fill in with your own numbers, plus the full PDF, $29.