Greenhouse grow lights: When to use them and how

11.08.2026.

Bloomcabin practical guide / tailored light

Greenhouse grow lights:
When to use them and how

A greenhouse already collects daylight. Supplementary lighting earns its place only when natural days are genuinely too short, weak or inconsistent for the crop and season you have chosen.

For keen home gardeners in South AfricaUse natural morning light first. Add light only where it matters.

01 / Start with the right question

Do your plants really need more light?

The most useful grow light is not the brightest one. It solves a specific limitation without creating new issues with heat, humidity, electricity use or plant development.

In a bright South African summer, a clean, unshaded greenhouse can provide more daylight than many crops require; shading and ventilation then become the real priorities. During shorter, cloudier winter days, the situation may reverse. Low sun, dense cloud, buildings, trees, frame profiles, dirty glazing and condensation reduce usable light at canopy level. A greenhouse that feels pleasantly bright to people may still be too dim for early tomato seedlings or a lower shelf.

Observe the crop over several days before buying and separate lack of light from other causes. Seedlings in weak light often lean towards the brightest pane, develop long soft stems and wider leaf spacing. But weak growth can also result from warm nights, overcrowding, constantly wet substrate, small pots, root stress or nutrient mistakes. A lamp above a cold, waterlogged root zone is not a solution; it distracts from the real problem.

Begin with a simple site check. Clean the glazing, remove unnecessary shade and note when tables or shelves fall into the structure’s shadow. Also check whether temperature, root space and air movement can make use of extra light. South Africa’s winter light varies considerably between regions and gardens. Site, orientation, weather, covering, crop and growing goal always determine actual demand.

A greenhouse is not an indoor farm with glass walls. Treat sunlight as the main resource and electric light as a precise supplement.

Likely light shortage Long, soft seedlings, uneven growth beneath frame profiles, slow winter salads or propagation started well before brighter spring conditions.

Check other causes first Cold substrate, stagnant humid air, nutrient errors, root stress, crowded plants, pests or a variety unsuited to the season.

Lighting probably unnecessary Spring and summer crops in a clear, sunny greenhouse when they grow compactly, evenly and with healthy leaf colour.

Lighting strategically useful Early propagation areas, winter leafy crops, cuttings and demanding fruiting crops where dependable timing matters.

A daylight-led greenhouse starts with orientation, clear glazed areas, roof ventilation and adaptable shading. The Classic aluminium greenhouse provides a durable, bright foundation. Glazing, roof vents and an orderly layout support seasonal climate management. Grow lights belong within this overall system: they replace neither a good site nor ventilation and sound cultivation.

02 / Terms for informed lighting decisions

Intensity × time = the crop’s daily light yield

PPFD × hours
= DLI

Plant-light marketing often makes a straightforward relationship seem unnecessarily complicated.PPFD describes the photosynthetically active photon density reaching the canopy at a particular moment.DLI or Daily Light Integral, is the photosynthetically active light accumulated over a whole day. Natural and electric light both contribute.

Put simply, moderate intensity over a longer period can deliver the same added DLI as a stronger fitting over fewer hours. The illustrative calculation is: PPFD × 3,600 × operating hours ÷ 1,000,000. It is a planning tool, not an instruction to run lights for as long as possible.

PAR: The radiation range relevant to photosynthesis. It is more meaningful for plant lighting than the brightness a lamp appears to have to people.
PPF: The total photosynthetic photon output of a fitting. Useful for comparisons, but not a measure of the light that reaches leaves.
PPFD: Photosynthetic photon flux density at canopy level, measured in µmol/m²/s. It changes with mounting height, beam spread and measurement position.
Photoperiod: The daily sequence and duration of light and darkness. It can affect flowering independently of the total light delivered.
Lumens and watts: Lumens describe visible brightness for people, while watts describe electrical power draw. Neither value alone explains light intensity at the plants.

Examples of additional DLI

100 × 10 hours 3.6 mol/m²/day

200 × 8 hours 5.76 mol/m²/day

300 × 12 hours 12.96 mol/m²/day

These figures are calculation examples, not crop recipes. Suitable DLI depends on plant species, variety, growth stage, temperature and desired quality, among other factors. Start with crop-specific guidance and establish what daylight the greenhouse already supplies before setting the electric supplement.

03 / Choose the form before the headline figure

Choose a fitting for the crop, not for the loudest claim on the box.

For many home greenhouses, LED fittings are a sensible starting point: they come in practical shapes, can operate efficiently and may be dimmable depending on the system. A white or balanced-spectrum grow light is usually more comfortable for everyday work than permanent purple light. It also makes leaf colour, pests, deficiencies and disease symptoms easier to inspect.

Compact propagation lights

Suitable for narrow propagation shelves, seed trays and cuttings when the light can be placed close above small plants. Their compact shape supports even, localised coverage rather than an unnecessarily large lit area.

Check: PPFD at the planned mounting height, fitting length, suitability for a humid environment and safe height adjustment for growing seedlings.

Linear light bars

Useful above long benches, herb rows and leafy crops. Several bars can be arranged to avoid bright centres and dark pots at the edges. They suit greenhouses where evenness matters more than one peak reading.

Check: Light-distribution map, recommended spacing, driver location, dimming capability and the system’s complete electrical power draw.

High-output area modules

Plausible for deeper crops or a clearly defined larger growing area with sufficient mounting height. They require particular attention to load capacity, heat, glare, cabling, air movement and reduced intensity at the edges.

Check: Measured PPFD maps, electrical requirements, permitted operating environment, minimum clearances, warranty terms and manufacturer guidance on heat dissipation.

Do not buy on wattage alone. A 100-watt light may be suitable or unsuitable depending on optics, efficiency, area, mounting height and losses. Total PPF is only a starting point. The key buying question is:What PPFD will plants receive on this bench at this exact distance?

Keep the greenhouse adaptable. A dedicated propagation bench may justify a closely mounted light in spring while the remaining space stays naturally lit. The Classic aluminium greenhouse offers room for shelving, safe walkways, ventilation and deliberately limited supplementary lighting, without filling the greenhouse with later improvisations.

04 / Mount for the current canopy, not memory

Height changes the lit area.
Evenness changes the result.

When a light is hung higher, its output usually spreads across a larger area while intensity at the canopy falls. At a shorter distance, intensity rises, but a narrow, very bright centre can develop. The useful mounting distance is therefore a changing relationship between the fitting’s optics, bench width and current plant height.

Centre Strongest light zone

Edges Lower intensity

Overlap More even area

Canopy Readjust height

Use the manufacturer’s distance recommendation as a starting point and, where possible, check distribution with a PAR meter or borrowed sensor. Measure at the centre, halfway to the edge and at the likely darkest point—not only directly below the fitting. A grid of nine readings over a bench is more useful than one spectacular value. Where differences are large, raise the light slightly, overlap edge areas, reduce the growing area or thin the crop before extending running time.

Seedlings need especially careful management. Light should be close enough to prevent excessive stretching, but not so close that leaves bleach, curl or develop dry patches. Adjustable suspension makes it easier to adapt as plants grow. The manual governs minimum clearances; crop response remains crucial for fine adjustment.

05 / Lighting time is a crop decision, not merely a timer setting

Extend the day carefully. Protect the night deliberately.

Plants need a reliable 24-hour rhythm. Supplementary light can add photosynthetic energy, but can also alter flowering if dark periods are lengthened or interrupted without understanding the crop.

Seedlings

Leafy crops

Flowering response

Seedlings: plan consistent days and a genuine night.

Leafy crops: supplement low-light hours; do not light from habit.

Flowering crops: check photoperiod before changing darkness.

Blue represents darkness, wheat colour natural daylight and clay colour supplementary hours. The graphic is only a seasonal illustration. Crop guidance, plant response and local daylight take priority over any universal timer setting. Extending light in the morning or evening is usually easier to manage than randomly interrupting the middle of the night.

May Observe

June–July Assess

August Adjust

SeptemberReduce

From OctoberReview again

Monitor the total light in cloudy or shaded locations.

The greatest shortfall is common, but never automatic.

Natural light returns unevenly.

Many propagation areas now need less support.

Clean glazing and ventilation often become more important.

This seasonal rhythm is intentionally general. A greenhouse in the winter-rainfall Cape, a shaded urban garden, a cold inland location and a subtropical coastal garden do not follow one lighting calendar. Glass or polycarbonate, roof shape, orientation, frame proportion and seasonal shade from deciduous trees all change the starting point. The Classic aluminium greenhouse provides a suitable framework for a bright, flexibly zoned space.

06 / One greenhouse, three different tasks

Give the light a clear job.

Seedlings

Aim: compact, resilient plants ready for planting out.

Use close, even light over a clearly defined propagation area. Check stem length, leaf spacing and leaf colour every few days. If seedlings stretch, first check mounting distance and actual intensity at canopy level. More operating hours do not reliably compensate for a poorly distributed fitting.

Leaf vegetables and herbs

Aim: consistent quality and dependable winter harvests.

Lettuces, rocket, Asian greens, parsley and basil benefit from even supply across the entire leaf area. A moderate, steady supplement is often more valuable than an intense centre over a mixed bench. More light can dry substrate faster, so reassess water needs instead of keeping the old watering routine unchanged.

Fruiting crops

Aim: strong young plants, appropriate flowering and good fruit set.

Tomatoes, peppers, cucumbers and similar crops need more than plenty of light. They also require suitable temperatures, adequate root space, balanced nutrition, air movement and appropriate conditions for pollination and fruit set.

Spectrum matters, but it replaces neither intensity nor lighting duration. A balanced white grow light suits many tasks in a home greenhouse. Special spectra and targeted far-red strategies can influence plant development, but need crop-specific knowledge. Start with enough useful light across the whole canopy, a repeatable schedule and a crop-appropriate dark period. Add technical complexity only once you can name and monitor the plant response you want.

07 / The side effects of a brighter growing area

Light changes heat, leaf temperature, humidity and watering.

Every fitting gives off heat. Efficient LED systems usually deliver less direct radiant heat to leaves than older lamp types, but lights and drivers still add energy to an enclosed greenhouse. On a cold morning that may be welcome; after a sunny afternoon, the same heat can become a burden. Measure at canopy height rather than relying only on a thermometer by the door or ridge.

More light can increase transpiration and dry pots faster. That does not mean watering should automatically become more frequent by calendar. Check substrate, pot weight and the root zone. Especially in winter, stronger top growth over cool, wet substrate can cause problems. Ventilation and gentle horizontal air movement help prevent stagnant moisture in dense foliage, but do not replace adequate spacing.

Condensation also deserves attention. A brighter, warmer microclimate changes water demand and moisture distribution. Keep leaves as dry as possible, avoid drip points above crops and give humid air a controlled route outside. The goal is not maximum ventilation at all times, but a stable usable climate around plants, especially on cool, damp transitional days.

Dry service area / diagram

Protected connection zone

Route cables high and safely. Place drivers, plug connections, timers and other connections in a dry, raised, accessible position. Cables must not be routed so water can travel along them into a connector or housing.

Keep floors and paths clear. Do not run household extension leads through puddles, wet walkways or places where pots and tools can crush cables. Use only components approved for the environment, provide strain relief and observe manufacturer clearances from plants and materials. Lights belong on load-bearing intended supports, not improvised hooks, plant stakes or unverified components.

Electrical note: A greenhouse is a damp environment with condensation, irrigation water, soil and wet shoes. Use lights, drivers, cables and connectors rated for that setting. An IP rating does not replace correct installation. Have fixed circuits, outdoor connections and installation changes completed by a competent, legally registered local electrical contractor, with applicable inspection, testing and CoC documentation. Temporary household leads are not a permanent greenhouse installation.

08 / Adjustments based on observation

Document the effect before expanding the system.

The best first installation is often deliberately limited: one bench, one crop group, a measured schedule and enough observation to establish whether the electricity used brings a horticultural benefit.

Before changing the light

Crop and stage: Record species, variety, sowing date, development stage and plant size.
Natural conditions: Note daylight, shading, glazing cleanliness, weather and bench position.
Symptoms: Describe stretching, uneven growth, pale leaves, slow development or delayed harvest.
Growing climate: Document canopy temperature, substrate moisture, condensation and air movement.

After two weeks

Light setting: Record mounting height, operating hours, dimming level and, where available, measured PPFD.
Plant response: Compare leaf colour, internodes, branching, uniformity, strength and harvest development.
Water response: Note altered drying and watering frequency for pots.
Energy use: Multiply power in kilowatts by operating hours and compare the result with electricity cost and crop value.

Keep adjustments calm and reversible. If leaves bleach, curl or develop dry patches in the bright centre, dim or raise the fitting and check temperature. Where coverage is uneven, improve distance and overlap before extending run time. If plants develop healthily and natural spring light increases, shorten the supplement gradually and eventually switch it off. If flowering changes unexpectedly, check the photoperiod and protect an uninterrupted night.

As technical equipment increases, the space should still work as a whole. Shelving, shading, ventilation, walkways and irrigation must support the lighting plan. The Classic aluminium greenhouse offers a durable basis. A good greenhouse remains pleasant, orderly and practical to cultivate even when the lights are off.

FAQ / South Africa

Questions before using greenhouse grow lights

Do I need grow lights in my greenhouse during winter?

Possibly—but not automatically. They are particularly useful where an actively growing crop has an identifiable daylight shortfall: very early propagation, winter leafy vegetables, cuttings or a schedule requiring strong plants before brighter spring conditions. Check your garden site, weather, shading, glazing, crop goal and the light already available in the greenhouse first.

How many hours should greenhouse grow lights run?

Use total daily light and plant response rather than one universal number. Seedlings, leafy vegetables and photoperiod-sensitive species may need different schedules. A timer should preserve a regular, continuous dark period. Start with crop-specific advice and the light’s manual, then adjust duration using readings, daylight and plant development.

Are white full-spectrum LEDs suitable for a greenhouse?

Yes. A balanced white plant LED is practical for many home-greenhouse uses because it can provide a broad spectrum while making leaf colour, pests and disease easier to inspect. Still check PPFD distribution, intended mounting height, power draw, approved operating environment and real area coverage. The term full spectrum alone is not enough to choose a light.

Should grow lights be on during the night?

It is usually easier to extend the natural day in the morning or evening while keeping an uninterrupted night. Intentional night interruption is a specialised photoperiod technique for particular crops and should not be used casually. Plants use darkness as part of their development rhythm; flower initiation in some ornamental and flowering plants is especially sensitive to day length.

Can I connect grow lights to an extension lead?

An ordinary household extension lead should not serve as a permanent greenhouse installation, especially where condensation, irrigation water and foot traffic are present. Use components whose intended environment, protection rating and installation suit the task, and keep connections dry and raised. Fixed wiring, new circuits, outdoor connections or uncertainty require a competent, legally registered local electrical contractor. Follow the manufacturer’s instructions and use a properly designed installation.

Sources and further reading

Plan further with measurements and crop-specific knowledge.

Julius Kühn Institute – LED assimilation lighting in greenhouses. Research on the effects and application of LED systems under controlled greenhouse conditions.
LVG Heidelberg – Red leaf lettuces under different lighting treatments. Trial report on hydroponically grown red lettuces and different lighting treatments.
Saxony State Institute of Agriculture – Greenhouse trials on assimilation lighting. Trial findings and technical context on the effects of supplementary greenhouse lighting.
German Environment Agency – Site-appropriate, resource-conscious gardening. Guidance on site choice, plant spacing, cultivation and regular ventilation to consider before adding technology.
BAuA – Protection classes and IP ratings for electrical lights. Background on protection classes, markings and protection of lights against contact, foreign bodies and moisture.
BAuA – Use and inspect electrical products as intended. Guidance on safe condition, intended use and regular checks of electrical products.
VDE consumer protection – Guidance on outdoor lights and cables. Safety information on suitable outdoor lights, protection ratings, connectors and extension leads in damp conditions.