Winter in British glasshouses brings a persistent problem: insufficient light. Between November and February, the daily light integral—the amount of light energy a plant receives per day—often falls to 5–15 mol m⁻² d⁻¹, far below the 20–30 mol m⁻² d⁻¹ required for optimal growth of high-value crops such as tomatoes and cucumbers. This winter light deficit directly suppresses yields and extends production cycles, eroding profitability. Supplementary lighting has long been recognised as a solution, but the question facing UK growers is whether LED technology now makes economic sense compared to traditional high-pressure sodium (HPS) systems.
Understanding the winter light deficit
Crops do not yield uniformly across the year in the UK. During summer, natural daylight intensity and duration are sufficient for rapid growth. But as autumn progresses and the sun’s path across the sky lowers, light availability drops dramatically. The ‘1% rule’ observed across cucumber, pepper, and leafy greens shows that a 1% reduction in daily light integral produces approximately a 1% reduction in yield—a direct relationship that quantifies the value of light during winter months.
High-value crops such as tomato and cucumber are particularly light-demanding. A 26% reduction in winter light translates to an 18.5% yield loss; a 40% deficit causes 23.3% yield loss. For growers of premium winter crops targeting urban markets, this calculation makes supplementary lighting economically compelling: lost yield directly equals lost revenue.
How LED inter-lighting improves winter performance
Supplementary LED lighting installed between plant canopy layers—a technique called inter-lighting—addresses winter light deficits by delivering photosynthetically active radiation directly to leaves throughout the canopy, rather than relying solely on overhead natural light.
Published research demonstrates substantial yield improvements under winter conditions. Tomato yields increased by 27% with daytime LED inter-lighting and by 24% with nighttime supplementary lighting during winter months. Beyond yield, nutritional quality improved: nighttime inter-lighting increased total soluble solids by 20% and ascorbic acid content by 25%. Cucumber and other high-value crops show similarly robust responses.
The mechanism is straightforward: LED photons extend the duration and intensity of photosynthesis, allowing plants to accumulate more biomass during the energy-limited winter season. Inter-lighting proves particularly effective because overhead light during short winter days becomes limiting; additional light between plant layers captures previously unused canopy volume.
LED versus HPS: the energy comparison
High-pressure sodium lamps, installed in many UK glasshouses from the 1990s onward, remain common but increasingly less competitive as LED technology matures. The critical advantage of LED systems lies in photon efficacy: LED fixtures deliver 2.5–3.0 μmol photons per joule of electrical input, compared to 1.7–1.9 μmol/J for HPS lamps. This superior conversion of electricity to usable light means LED systems require substantially lower electrical energy input to achieve equivalent light output at the crop.
Beyond energy consumption, LED and HPS systems differ in operational costs. HPS bulbs require replacement every 18–24 months; LEDs typically operate for 50,000 hours or longer, effectively eliminating bulb replacement across a decade or more of continuous use. LEDs also generate considerably less waste heat than HPS, reducing cooling demands during warm months and improving overall facility energy balance.
Economics favour LEDs when electricity costs are high—a factor entirely within the control of local utility supply and national energy policy—and when crops command sufficient market value to justify investment. The longer operational life of LEDs and lower heat output provide structural cost advantages independent of electricity pricing.
Economics: when supplementary lighting makes sense
The profitability of supplementary LED lighting depends on several interlocking factors. First, crop value matters fundamentally: high-value winter crops justify greater capital investment than marginal-value commodities. Second, electricity costs in your region determine operating expenses; areas with lower-cost renewable electricity or off-peak tariffs improve LED economics substantially. Third, whether production operates year-round or seasonally affects the return on investment: continuous operation spreads capital costs across more sales, whilst seasonal production concentrates costs into shorter payback windows.
The yield response is consistent across research: supplementary lighting increases tomato and cucumber production by 20–27% during winter months. This yield uplift generates additional revenue per square metre. That revenue must exceed the fixed capital cost of installation and the variable operating costs of electricity, labour, and maintenance. For high-value winter crops—tomatoes and cucumbers sold into premium markets—the yield increase typically justifies investment.
Marginal-value crops such as lettuce or herbs may not generate sufficient additional revenue to offset supplementary lighting costs within acceptable timeframes, particularly if operated seasonally. Propagation nurseries raising seedlings represent a special case: the high per-unit value of transplants and the labour intensity of the operation mean that even modest improvements in cycle time or uniformity can justify substantial investment per square metre.
Local electricity tariffs, available market prices for winter crops, and production intensity are the key variables growers should assess when evaluating supplementary lighting projects. AHDB guidance on lighting in horticulture provides technical frameworks for assessing these trade-offs in context.
Design pitfalls and practical implementation
Common errors include incorrect fixture spacing leaving canopy under-lit, misalignment with crop architecture, and inadequate cooling in high-intensity systems. Successful implementation requires matching photosynthetic photon flux density (PPFD) to crop demand: typical inter-lighting delivers 150–200 μmol m⁻² s⁻¹ PPFD with 15–16 hour photoperiods, accumulating 8–12 mol m⁻² d⁻¹ of supplementary light.
Growers shifting from HPS should re-optimise fixture placement to exploit LED’s lower heat output and flexible positioning, improving uniformity and cost-effectiveness compared to replicated HPS configurations.
Choosing crops and timing for LED investment
High-value winter crops—tomato, cucumber, and leafy greens—justify supplementary LED investment, particularly in October–March when premium market prices peak. Summer crops rarely justify lighting in the UK, where abundant natural light makes cooling the limiting factor. A phased approach—starting with high-value crop areas, gathering data, then evaluating expansion—reduces financial risk.
Key points
- Winter daily light integral (5–15 mol m⁻² d⁻¹) falls 60–75% below optimal levels (20–30 mol m⁻² d⁻¹) for UK glasshouses; supplementary lighting directly addresses this deficit.
- LED inter-lighting increases winter tomato and cucumber yields by 20–27%, with improved fruit quality (soluble solids and ascorbic acid); the ‘1% rule’ quantifies yield loss from light deficits.
- LED fixtures deliver superior photon efficacy (2.5–3.0 μmol/J) compared to HPS (1.7–1.9 μmol/J), and operate for 50,000+ hours, eliminating frequent bulb replacement required for HPS systems.
- Economics depend on crop value, local electricity costs, and production intensity; high-value winter crops and propagation nurseries typically justify supplementary lighting investment.
- Common design errors include incorrect fixture spacing, inadequate intensity, and failure to adjust setpoints during crop growth; matching PPFD to crop demand and total DLI to crop capacity is essential.
Related service: Agricultural consulting services