Greenhouse grow lights: when and how to use them

11.08.2026.

Bloomcabin practical guide / tailored light

Grow lights in the greenhouse:
when you need them and how to use them

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

For discerning home gardeners in MaltaDaylight 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 problems with heat, humidity, electricity use or plant development.

In Malta’s bright summer, a clean, unshaded greenhouse can provide more daylight than many crops need; shading and ventilation then become the real priorities. Conditions often reverse in winter. Shorter days, a lower sun, cloud, nearby buildings, trees, frame profiles, dirty glazing and condensation all reduce useful light at canopy level. A greenhouse that seems pleasantly bright to people may still be too dim for early tomato seedlings or a lower shelf.

Observe the crop for several days before buying and distinguish lack of light from other causes. Light-starved seedlings often lean towards the brightest pane, develop long soft stems and wider leaf spacing. Yet weak growth can also result from warm nights, overcrowding, constantly wet compost, small pots, root stress or nutrient mistakes. A lamp above a cold, waterlogged root zone is not a remedy; it distracts from the real problem.

Begin with a simple survey. Clean the glazing, remove unnecessary shade and note when benches or shelves sit in the structure’s shadow. Also check whether temperature, root room and air movement allow plants to use extra light. Malta’s winter daylight varies with garden aspect, local shelter, weather, covering, crop and growing aim, so these factors determine the actual need.

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 deficit Long, soft seedlings, uneven growth below frame bars, slow winter salads or propagation begun well before brighter spring conditions.

Check other causes first Cold compost, still humid air, nutrient errors, root stress, crowding, 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 benches, winter leafy crops, cuttings and demanding fruiting crops where a reliable timetable matters.

A daylight-led greenhouse starts with orientation, unobstructed glazing, roof ventilation and flexible shading. The Classic aluminium greenhouse provides a durable, bright foundation. Glazing, roof vents and an orderly layout support seasonal climate control. Grow lights belong in this wider 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

Grow-light marketing can make a straightforward relationship appear unnecessarily complicated.PPFD describes photosynthetically active photon density reaching the canopy at a particular moment.DLI or Daily Light Integral, is the photosynthetically active light accumulated throughout the day. Natural and electric light both contribute.

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

PAR: The radiation range relevant to photosynthesis. It says more about plant lighting than a lamp’s apparent brightness to people.
PPF: The total photosynthetic photon output of a fixture. Useful for comparison, but not a measure of light actually reaching leaves.
PPFD: Photosynthetic photon flux density at canopy level, measured in µmol/m²/s. It changes with mounting height, beam spread and measuring point.
Photoperiod: The daily sequence and duration of light and darkness. It can affect flowering independently of total light supplied.
Lumens and watts: Lumens describe visible brightness for people, while watts describe electrical power draw. Neither figure alone explains light intensity at plants.

Examples of additional DLI

100 × 10 hrs 3.6 mol/m²/day

200 × 8 hrs 5.76 mol/m²/day

300 × 12 hrs 12.96 mol/m²/day

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

03 / Choose the fixture shape before the headline figure

Choose a fixture for the crop, not for the loudest claim on its packaging.

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

Compact propagation lights

Suitable for narrow propagation shelves, seed trays and cuttings when the fixture can be positioned close above small plants. Their compact form supports even, local lighting rather than an unnecessarily broad lit area.

Check: PPFD at the intended mounting height, fixture length, suitability for humidity and safe height adjustment as young plants grow.

Linear light bars

Useful over long benches, herb rows and leafy salads. Several bars can be arranged to avoid bright centres and dark edge pots. They suit greenhouses where evenness matters more than one peak reading.

Check: Light-distribution map, recommended spacing, driver position, dimming and the system’s full electrical power consumption.

High-output area modules

Appropriate for deeper crops or a clearly defined larger growing area with enough mounting height. They require particular care with load capacity, heat, glare, cabling, air movement and declining intensity at edges.

Check: Measured PPFD maps, electrical requirements, permitted operating environment, minimum clearances, guarantee terms and cooling guidance.

Do not buy on wattage alone. A 100-watt fixture may suit a space well or badly 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 close fixture in spring while the rest remains naturally lit. For orderly crop and light zones, 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 today’s canopy, not yesterday’s

Height changes the light footprint.
Uniformity changes the result.

When a fixture is raised, its light generally spreads across a larger area while intensity at canopy level falls. At a shorter distance intensity increases, but a narrow, overly bright centre can develop. The useful mounting distance is therefore a changing relationship between fixture optics, bench width and the plants’ current height.

Centre Strongest light zone

Edges Lower intensity

Overlap More even area

Canopy Readjust height

Use the manufacturer’s spacing 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 beneath the fixture. A grid of nine readings over one bench is more useful than a dramatic single value. If variation is high, raise the fixture slightly, overlap edge zones, reduce the crop area or space plants out before extending operating time.

Young plants need close attention. Light should be near enough to prevent excessive stretching, but not so close that leaves bleach, curl or develop dry patches. Adjustable suspension makes changes easier as plants grow. Follow the manual for minimum clearances; crop response remains vital 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 dependable 24-hour rhythm. Supplementary light can add photosynthetic energy but can also alter flowering when dark periods are extended or interrupted without regard to the crop.

Young plants

Leafy crops

Flowering response

Young plants: plan consistent days and a proper night.

Leafy crops: supplement dim hours rather than lighting by habit.

Flowering crops: check photoperiod before changing the dark period.

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

Nov.Observe

Dec.–Jan.Assess

Feb.Adjust

Mar.Reduce

From AprilReview again

Watch total daylight in shaded or cloudy locations.

The greatest deficit is possible, never automatic.

Natural light returns unevenly.

Many propagation areas now need less support.

Clean glazing and ventilation often matter more.

This seasonal rhythm is intentionally general. A sheltered Maltese coastal garden, an exposed rooftop, a shaded urban plot and a warm inland location do not share one lighting calendar. Glass or polycarbonate, roof shape, orientation, frame proportion and seasonal shade from trees all alter the starting point. The Classic aluminium greenhouse provides a suitable framework for a bright, flexibly zoned growing space.

06 / One greenhouse, three different tasks

Give the light a clear purpose.

Young plants

Goal: compact, sturdy plants ready for planting out.

Use close, even light over a clearly defined propagation area. Check stem length, leaf spacing and colour every few days. If seedlings stretch, first check mounting distance and actual canopy intensity. Extra hours do not reliably correct a poorly distributed fixture.

Leaf vegetables and herbs

Goal: consistent quality and dependable winter harvests.

Lettuce, rocket, Asian greens, parsley and basil benefit from even provision over the whole leaf area. A moderate, steady supplement is often more valuable than an intense centre above a mixed bench. More light can dry compost faster, so reassess water needs rather than keeping the previous watering routine unchanged.

Fruiting crops

Goal: strong young plants, suitable flowering and good fruit set.

Tomatoes, peppers, cucumbers and similar crops need more than ample light. They also require suitable temperatures, enough 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. Specialist spectra and targeted far-red strategies can influence plant development, but require crop-specific knowledge. Start with enough useful light across the complete canopy, a repeatable schedule and a dark period suited to the crop. Add technical complexity only when you can define and monitor the plant response you want.

07 / The accompanying costs of a brighter growing area

Light changes heat, leaf temperature, humidity and watering.

Every fixture releases heat. Efficient LED systems usually deliver less direct radiant heat to leaves than older lamp types, but the fixture and driver still add energy to an enclosed greenhouse. This may be welcome on a cool morning and burdensome after a sunny afternoon. 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. This does not mean watering more often automatically by calendar. Check compost, pot weight and the root zone. In winter, stronger growth above cold wet compost can cause problems. Ventilation and gentle horizontal air movement help prevent stagnant humidity in dense foliage, but do not replace adequate plant spacing.

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

Dry service area / diagram

Protected connection zone

Route cables high and safely. Position drivers, plugs, timers and other connections dry, raised and accessible. Cables must not run where water can track along them into a connection or housing.

Keep floors and paths clear. Do not run household extension leads through puddles, wet paths or areas 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. Fixtures belong on designed, load-bearing mounts, not improvised hooks, plant supports or unverified components.

Electrical note: A greenhouse is a damp environment with condensation, watering, soil and wet footwear. Use only fixtures, drivers, cables and connectors approved for their intended use. An IP rating does not replace correct installation. Fixed circuits, outdoor connections and alterations to an installation should be handled by a REWS-authorised electrical professional with the appropriate scope. Temporary household cables are not a permanent greenhouse installation.

08 / Adjust using observations

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 produces a gardening benefit.

Before changing the light

Crop and stage: Record species, variety, sowing date, growth stage and plant size.
Natural conditions: Record 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, compost moisture, condensation and air movement.

After two weeks

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

Adjust calmly and reversibly. If leaves bleach, curl or develop dry patches in the bright centre, dim or raise the fixture and check temperature. If coverage is uneven, improve spacing and overlap before extending run time. When plants grow well and natural spring light increases, reduce the supplement gradually and finally switch it off. If flowering develops unexpectedly, review the photoperiod and protect the uninterrupted night.

As technical equipment increases, the space should still work as a whole. Shelving, shading, ventilation, walkways and watering 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 / Malta

Questions before using greenhouse grow lights

Do I need grow lights in my greenhouse in winter?

Possibly, but not automatically. They are most useful where an actively growing crop has a clear daylight deficit: very early propagation, winter leafy vegetables, cuttings or a schedule requiring sturdy plants before brighter spring conditions. First assess your garden setting, weather, shade, glazing, crop aim and the light already available in the greenhouse.

How many hours should greenhouse grow lights run?

Use total daily light and plant response, not one universal figure. Young plants, leafy vegetables and photoperiod-sensitive species may need different schedules. A timer should preserve a regular, continuous dark period. Start with crop guidance and the fixture manual, then adapt duration using measurements, daylight and plant development.

Are white full-spectrum LEDs suitable for a greenhouse?

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

Should grow lights be on during the night?

Usually it is clearer to extend the natural day in the morning or evening and preserve an uninterrupted night. Deliberate night interruption is a specialist photoperiod method for certain crops and should not be used casually. Plants use darkness as part of their development; flower initiation in some ornamental and flowering plants is particularly sensitive to day length.

Can I connect grow lights to an extension lead?

An ordinary household extension lead should not be a permanent greenhouse installation, especially around condensation, watering and foot traffic. Use components whose intended environment, protection rating and installation suit the job, and keep connections dry and raised. Fixed wiring, new circuits, outdoor outlets or uncertainty should be referred to a REWS-authorised electrical professional. Follow the manufacturer’s instructions and proper installation practice.

Sources and further reading

Plan further with measurements and crop-specific knowledge.

Julius Kühn Institute – LED assimilation lighting in greenhouses. Research into the effects and use of LED systems under controlled greenhouse conditions.
LVG Heidelberg – red leafy salads under lighting variants. Trial report on hydroponically grown red salads under different lighting arrangements.
Saxon State Office for Agriculture – greenhouse assimilation-lighting trials. Trial results and technical context on supplementary greenhouse lighting.
German Environment Agency – gardening appropriately and resource-efficiently. Advice on siting, plant spacing, crop care and regular ventilation before adding technology.
BAuA – protection classes and IP ratings for electrical lights. Background on classifications, markings and protection against contact, particles and moisture.
BAuA – use and inspect electrical products correctly. Guidance on safe condition, intended use and regular inspection of electrical products.
VDE consumer protection – outdoor lights and cable guidance. Safety information on suitable outdoor lighting, protection ratings, connectors and extension cables in damp areas.