How to Choose Greenhouse Shade Netting and Measure Light
Shade netting can reduce the solar heat load in a greenhouse. It also takes light away from the crop, even on days when light is limited. Before choosing a percentage, find out how much light reaches the plants and when temperatures peak.
Measure before you install the net
Record temperature and light just above the crop canopy. A quantum sensor measures the flux of photosynthetically active photons. Log the readings over a day to calculate the daily light integral, or DLI, in moles per square metre per day. A noon reading helps identify the peak. The daily total tells you how much light the crop received, and the target depends on crop and growth stage.
Where possible, compare two similar positions at the same time, one with a sample of the net and one without. Keep both sensors level and clear of shadows from the frame or equipment. Repeat the check in full sun, variable cloud and during the hours when your greenhouse tends to overheat.
Test a net against a specific target
Suppose the crop receives 20 mol/m² per day and the net transmits 70% of PAR under your test conditions. Leaving the net closed all day would give a rough estimate of 14 mol/m² per day. Glazing, sun angle, diffuse light and the hours when the net is closed can change the result. Measure again at crop height after installation.
If the greenhouse overheats for a few hours, consider a retractable screen that closes during that period. An external net intercepts some sunlight before it reaches the glazing. An internal screen may be easier to operate, depending on the structure. Keep vents clear and follow the installation and wind limits for the system.
Account for insect screens and airflow
Choose an insect screen with openings suitable for the pest you need to exclude and assess its effect on ventilation. A finer screen can restrict airflow during the hottest part of the day. If you use both a shade net and an insect screen, measure temperature and light with both in place.
Bring your readings and the hours of peak heat when you ask for quotes. Our guide to reading a shade net specification explains which ratings can be compared directly.
Sources: Purdue Extension, Measuring Daily Light Integral in a Greenhouse and FAO, Greenhouse Vegetable Crops.