Greenhouse grow lights: when and how to use them

11.08.2026.

Bloomcabin practical guide / tailored light

Greenhouse grow lights:
when and how to use them

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

For committed home gardeners in New ZealandUse 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 New Zealand summer, a clean, unshaded greenhouse can provide more daylight than many crops need; shading and ventilation are then the priority. In the darker months, the situation often reverses. Short days, a low sun angle, heavy cloud, buildings, trees, frame profiles, dirty glazing and condensation all reduce usable light at canopy level. A greenhouse that feels pleasantly bright to people may still be too dim for early tomato raising or a lower shelf.

Observe your plants for several days before buying and distinguish low light from other causes. Seedlings short of light often lean towards the brightest pane, with long soft stems and wider leaf spacing. Yet weak growth can also result from overly warm nights, crowding, permanently wet growing medium, small pots, root stress or nutrient mistakes. A lamp above cold, waterlogged roots is not a correction; it distracts from the real issue.

Begin with a simple site review. Clean the glazing, remove unnecessary shade and note when benches or shelves fall into the structure’s shadow. Also check whether temperature, root space and air movement allow plants to use extra light. The low-light period can be longer in southern areas than in the north, but garden position, orientation, weather, cladding, crop and growing goal always determine the real 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 shortfall Long, soft seedlings, uneven growth below frame bars, slow winter greens or propagation beginning well before brighter spring conditions.

Check other causes first Cold substrate, 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 areas, winter salad greens, cuttings and demanding fruiting crops with an important schedule.

A daylight-led greenhouse starts with orientation, clear 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 management. Grow lights belong within this wider system: they replace neither a good site nor ventilation and sound crop care.

02 / Terms for better lighting decisions

Intensity × time = the crop’s daily light total

PPFD × hours
= DLI

Grow-light marketing can make a simple relationship seem unnecessarily complicated.PPFD describes the photosynthetically active photon density reaching the canopy at a given moment.DLI or Daily Light Integral is the photosynthetically active light accumulated over a full day. Natural and electric light both contribute.

Put simply, moderate intensity over longer hours can provide the same additional DLI as a stronger light over 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 is more meaningful for plant lighting than the brightness a lamp appears to have to people.
PPF: The total photosynthetic photon flux produced by a light. Useful for comparison, but not a measure of how much light reaches leaves.
PPFD: Photosynthetic photon flux density at canopy level, measured in µmol/m²/s. It changes with mounting height, beam spread and measuring position.
Photoperiod: The daily sequence and duration of light and darkness. It can influence flowering independently of total light delivered.
Lumens and watts: Lumens describe visible brightness for people and watts electrical power draw. Neither figure 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 are calculation examples only, not crop recipes. The appropriate DLI depends on plant species, variety, growth stage, temperature and desired quality, among other factors. Start with crop-specific guidance and establish how much daylight the greenhouse already supplies before deciding on the electric supplement.

03 / Choose the form before the headline figure

Select a light for the crop, not for the loudest claim on the package.

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

Compact propagation lights

Suitable for narrow propagation shelves, seed trays and cuttings where the light can be positioned close above small plants. Their compact form supports even, local coverage rather than an unnecessarily large lighted area.

Check: PPFD at the intended mounting height, fixture length, suitability for a damp environment and safe height adjustment for growing seedlings.

Linear light bars

Useful over long benches, herb rows and leafy greens. 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: coverage map, recommended spacing, power-supply position, dimming and the system’s full electrical power draw.

High-output area modules

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

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

Do not buy by wattage alone. A 100 W fixture may suit a space well or poorly depending on optics, efficiency, area, mounting height and losses. Total PPF is also only a starting point. The key buying question is:What PPFD will the plants receive on this bench at this exact distance?

Keep the greenhouse adaptable. A dedicated propagation bench can justify a closely positioned fixture in spring while the rest of the space remains naturally lit. As a structural basis for distinct crop and lighting zones, the Classic aluminium greenhouse offers room for shelving, safe walkways, ventilation and deliberately limited supplementary lighting without overloading the greenhouse with later improvisations.

04 / Mount for the current canopy, not your memory

Height changes the light footprint.
Evenness changes the result.

When a fixture is hung higher, its light usually spreads over a larger area while canopy intensity drops. At a closer distance, intensity rises but may form a narrow, very bright centre. The useful mounting distance is therefore a changing relationship between fixture optics, bench width and current plant height.

Centre Strongest light zone

Edges Lower intensity

Overlap More even area

Canopy Readjust the 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, midway to the edge and at the likely darkest point, not only directly beneath the fixture. A grid of nine readings over a bench is more informative than one impressive peak number. For major variation, slightly raise the light, overlap edge coverage, reduce the crop area or space plants out before increasing operating time.

Seedlings 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 it easier to respond during growth. Follow the manual for minimum clearances; for fine adjustment, crop response remains decisive.

05 / Lighting time is a crop decision, not just 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 it 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 dim hours; do not light from habit.

Flowering crops: check photoperiod before changing darkness.

Blue represents darkness, wheat represents natural daylight and terracotta represents supplementary hours. This illustration is only a seasonal example. Crop guidance, plant response and local daylight take priority over any universal timer setting. Extending light in the morning or evening is usually clearer than a random interruption in the middle of the night.

May Observe

Jun.–Jul.Assess

Aug.Adjust

Sept.Reduce

From OctoberReview again

Monitor total light in southern and cloud-prone locations.

The largest 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 pattern is intentionally general. A greenhouse in coastal Southland, a shaded urban site, a clear frosty inland garden and a northern subtropical garden do not follow one lighting calendar. Glass or polycarbonate, roof form, orientation, frame proportion and seasonal shade from deciduous trees all change the starting point. For a bright, flexibly zoned foundation, the Classic aluminium greenhouse provides an appropriate framework.

06 / One greenhouse, three distinct jobs

Give light a clear purpose.

Seedlings

Goal: compact, sturdy plants ready at the right planting time.

Work with close, even lighting above 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 canopy intensity. Extra hours do not reliably compensate for a poorly distributed fixture.

Leaf vegetables and herbs

Goal: consistent quality and dependable cool-season harvests.

Lettuce, rocket, Asian greens, parsley and basil benefit from even coverage across the full leaf area. A moderate, steady supplement is often more valuable than an intense centre over a mixed bench. More light can dry growing media faster, so reassess water needs rather than keeping the same watering calendar unchanged.

Fruiting crops

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

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

Spectrum matters, but 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 steer plant development, but require crop-specific knowledge. Begin with enough usable light across the entire canopy, a repeatable schedule and a crop-appropriate dark period. Add technical complexity only when you can name the desired plant response precisely and then monitor it.

07 / The accompanying costs of a brighter crop area

Light changes heat, leaf temperature, humidity and watering.

Every fixture gives off heat. Efficient LED systems usually produce less direct radiant heat at leaves than older lamp types, but the fixture and power supply still add energy to an enclosed greenhouse. That can be welcome on a cold morning and burdensome after a sunny afternoon. Measure at canopy height rather than relying only on a thermometer by the door or roof ridge.

More light can increase transpiration and make pots dry faster. This does not mean watering automatically more often by the calendar. Check the growing medium, pot weight and root zone. Especially in winter, stronger top growth above cool, wet media can cause problems. Ventilation and gentle horizontal air movement help prevent stagnant moisture 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 drip points above crops and provide humid air with a controlled route outdoors. The aim is not maximum ventilation at all times, but a stable, useful climate around plants, especially on cool, damp changeable days.

Dry service area / diagram

Protected connection zone

Route cables high and safely. Place power supplies, plugs, timers and other connections somewhere dry, elevated and readily accessible. Cables must not be routed so that water can track along them into a connector or enclosure.

Keep floors and walkways clear. Do not run household extension leads through puddles, wet paths 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. Fixtures belong on strong, intended supports, not improvised hooks, plant stakes or unverified parts.

Electrical note: A greenhouse is a damp environment with condensation, watering, soil and wet footwear. Use only lights, power supplies, cables and connectors rated for their intended conditions. An IP rating does not replace correct installation. Consult a licensed electrical professional about fixed circuits, outdoor connections and alterations. Temporary household leads are not a permanent greenhouse installation.

08 / Adjusting through observation

Record 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 delivers a gardening benefit.

Before changing the light

Crop and stage: Record species, variety, sowing date, growth 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 measured PPFD where available.
Plant response: Compare leaf colour, internodes, branching, consistency, strength and harvest development.
Water response: Note changed drying rates and watering frequency for pots.
Energy use: Multiply power in kW by operating hours and compare the result with electricity cost and crop value.

Adjustments should remain calm and reversible. If leaves bleach, curl or develop dry areas in a bright centre, dim or raise the fixture and check temperature. With uneven coverage, improve spacing and overlap before extending run time. If plants develop healthily and natural spring light increases, gradually shorten the supplement and eventually switch it off. For unexpected flowering, review photoperiod and protect an uninterrupted night.

As technical equipment increases, the space should still work as one whole. Shelves, 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 manage even when the lights are off.

FAQ / New Zealand

Questions before using greenhouse grow lights

Do I need grow lights in my greenhouse during winter?

Possibly, but not automatically. They are most useful where an actively growing crop has a clear daylight shortfall: very early propagation, cool-season leafy vegetables, cuttings or a schedule requiring sturdy plants before brighter spring conditions. First consider garden location, weather, shading, glazing, crop goal and the light already available in the greenhouse.

How many hours should greenhouse grow lights run?

Base this on total daily light and plant response, not a universal number. Seedlings, leafy vegetables and photoperiod-sensitive species may need different schedules. A timer should preserve a regular, continuous dark period. Begin with crop-specific guidance and the fixture manual, then adjust duration using measurements, 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 coverage, intended mounting height, power draw, permitted operating environment and actual area covered. The term full spectrum alone is not enough to choose a light.

Should grow lights be switched on during the night?

It is usually clearer to extend the natural day in the morning or evening and retain an uninterrupted night. A deliberate night interruption is a specialised photoperiod technique for particular crops and should not be used casually. Plants use darkness as part of their growth 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 there is condensation, watering and foot traffic. Use components whose intended environment, protection rating and installation suit the task, and keep connections dry and elevated. Consult a licensed electrical professional about fixed wiring, new circuits, outdoor connections or any uncertainty. Follow manufacturer instructions and correct installation practice.

Sources and further reading

Plan further with measurements and crop-specific knowledge.

Julius Kühn Institute – LED assimilation lighting in greenhouses. German research on the effects and use of LED systems under controlled greenhouse conditions.
LVG Heidelberg – red leafy salads under lighting variations. German trial report on hydroponically grown red salads under different lighting treatments.
Saxon State Institute for Agriculture – greenhouse trials on assimilation lighting. German trial results and technical context on the effects of supplementary greenhouse lighting.
German Environment Agency – garden in a site-appropriate, resource-conscious way. German guidance on site selection, spacing, crop care and regular ventilation before adding technology.
BAuA – protection classes and IP ratings for electrical lights. German background on classifications and protection from contact, foreign objects and moisture.
BAuA – use and inspect electrical products as intended. German guidance on condition, intended use and regular inspection of electrical products.
VDE consumer protection – outdoor lights and cables. German safety information on suitable outdoor lights, protection ratings, connectors and extension leads in damp areas.