"Put up a 70% shade net — cooler and darker." But a week later: fruit isn't colouring up, growth has slowed, DLI has dropped below the crop minimum. The shade net solved overheating and created a light deficit — because the percentage was chosen on the principle of "more means better protection," not on the balance between heat load and light availability. Shading is a tool for managing two parameters at once — temperature and DLI — and it only works when both are factored in.
Quick glossary: Shading — reducing the intensity of solar radiation using nets, films, or coatings; characterised by a shading percentage indicating what fraction of incoming light is blocked. PPFD (Photosynthetic Photon Flux Density) — the flux of PAR photons at leaf level; shading reduces PPFD and, with it, DLI — the daily light integral. Reflective surfaces — materials (white polycarbonate, Mylar, reflective film) that redirect a portion of incoming or lateral light onto the plant rather than losing it to the surroundings.
Why 70% Shading Is Often Too Much
Shade nets are rated by blocking percentage: 30%, 50%, 70%, 90%. But a plant needs a specific DLI — and depending on the crop, "darker" may mean "critically insufficient."
Under natural solar conditions in Ukraine in July on a clear day — PPFD in a horizontal plane reaches 800–1,100 µmol/m²/s at peak (10:00–15:00). For leafy lettuce the optimum is 200–350 µmol/m²/s. A 70% net reduces this to 240–330 µmol/m²/s — right in the optimal range. For tomato the optimum is 600–1,000 µmol/m²/s — a 70% net delivers 240–330, well below the minimum for fruiting.
So choosing the shading percentage means calculating for the crop and season:
- Leafy crops (lettuce, greens): 40–60% in summer on clear days
- Tomato and cucumber: 20–30% maximum, only during the heat peak
- Microgreens after uncovering: 50–70% when growing under a transparent roof
- Autumn and spring: remove or switch to a lower percentage
External vs Internal Shading: What Makes the Difference
External shading (net or covering outside the greenhouse envelope) blocks solar radiation before it enters the greenhouse. Most of the heat load never enters. The most effective way to control temperature — at the same percentage, external shading reduces interior temperature far more effectively than internal.
Downside: installation and removal is more involved, especially in wind. Requires structural elements for external mounting.
Internal shading (net or curtain inside the greenhouse) — the sun's heat is already inside, and most of it converts to heat within the greenhouse even with the plant shaded. Effective for managing DLI and lighting uniformity — less effective for temperature control.
Practical approach: external shading for temperature control in summer, internal (moveable curtains) for precise DLI management and uniformity when supplementing light. For a greenhouse where overheating is the primary problem — external always takes priority.
Reflective Surfaces: Where They Return DLI
A portion of light that falls on walls, floors, and structural elements is lost. Reflective surfaces redirect this "lost" flux back onto the plants.
Side panels of shelving. In vertical racking, lower tiers receive significantly less PPFD. White or mirror-finish side panels return up to 70–80% of lateral light and floor-reflected light to the lower tiers. One of the simplest and cheapest ways to improve uniformity without increasing lamp wattage.
Floor and inter-row spaces. White plastic film or mulch on the floor between shelves returns light that has passed through the leaf canopy back upward. When growing tomato and cucumber where lower leaves and fruit are shaded by the upper canopy — bottom reflection can improve ripening of the lower tier.
Materials: Mylar (aluminised film) — reflectivity 90–95%, best for grow rooms. White matte film or paint — 70–85%, better for greenhouses where mirror reflections create uneven hot spots. Black surface — 3–5% reflectivity, absorbs almost everything and is worst for uniformity.
More on lighting uniformity and its effect on yield — in the article on PPFD uniformity.
Seasonal Logic: Shading Is Not Year-Round
Shading addresses excess sun in the warm season. In autumn and winter, the same net becomes an obstacle to an already limited DLI.
Ukraine: peak natural insolation June–August, minimum December–February. The difference in natural DLI between these months is 5–6×. A 50% shade net that saves lettuce in July will, in October, reduce an already limited DLI to below the minimum for normal growth.
Moveable shading systems (roller or folding curtains) that open in overcast conditions and close under direct sun — the optimal solution for crops that need a precise DLI. For smaller greenhouses — a removable external net taken down in September and hung back up in May.
Three Mistakes That Cost the Most
Choosing a shading percentage without calculating DLI for the crop. "70% — good protection" without knowing that the crop's target DLI is already below minimum at that shading level. The check: target PPFD ÷ (natural PPFD × (1 − shading fraction)) = whether enough remains. Different for each crop.
Leaving shading up year-round, or only removing it when symptoms appear. Shading hanging from May through November guarantees DLI deficit in autumn when natural light is already weak. Remove in September, hang in May — the minimum seasonal schedule.
Using black mulch or dark surfaces in inter-row zones. Black surfaces absorb heat and radiate it back into the air — already hot as it is. White or silver mulch and surfaces lower soil and floor temperature and return some light to the lower parts of the plant.
How to Know Shading and Reflection Are Calibrated Correctly
PPFD measured with a quantum meter at plant level on a clear midday does not exceed the crop's light saturation point by more than 20–30%. Daily DLI with shading accounted for is within the crop's target range. On a hot day, temperature inside does not exceed outdoor temperature by more than 4–5°C with ventilation open. Lower tiers in vertical growing receive at least 60–70% of the PPFD of the top tier when reflective surfaces are in use.
For deeper understanding: Lighting: DLI, PPFD, and How to Calculate the Daily Light Dose — explains how to calculate real DLI accounting for shading and cloud cover, and why using "a clear day" as the basis for choosing shading percentage is wrong without factoring in the actual annual insolation pattern.