My philodendron had been struggling for two winters. Leggy stems, pale leaves, that limp look that makes you feel guilty every time you walk past. I moved it closer to the window. I tried a south-facing spot. Nothing really fixed it. Then I put a decent LED panel above it, set a timer for 14 hours, and within six weeks it was pushing out leaves faster than I could track. That difference (between a plant surviving and a plant actually growing) is what grow lights do when you get the setup right.
This guide covers everything you need to make that call confidently: how grow lights work, the spectrum science you actually need (without the textbook overload), how to choose the right type, what PPFD and DLI mean in plain English, distance and hour targets by plant type, running costs, and how to spot when you have too much or too little light. At the end, I’ll point you to the product roundups once you know what you are shopping for.
Do Indoor Plants Actually Need Grow Lights?
Not every plant in every home needs a grow light, but more do than most people realise.
Most UK and northern European homes receive genuinely poor light, especially between October and March. A south-facing window with no obstructions might hit 2,000 lux on a clear winter day; a plant sitting one metre back from that window might receive 500 lux or less. Many popular houseplants (monsteras, pothos, snake plants) can tolerate those conditions, but tolerate is different from thrive. You will often see them grow almost nothing from November through February.
High-light plants like citrus, herbs you actually want to harvest, tomatoes, peppers, and most fruiting edibles need much more light than a British winter window can provide. Even in summer, a north-facing flat is a problem.
When a grow light makes sense:
- You want year-round growth from houseplants that go dormant in low winter light
- You are growing edibles, herbs, or seedlings indoors
- Your windows face north, are obstructed by buildings or trees, or have frosted or tinted glass
- You are setting up a grow tent or dedicated indoor growing area
- You have a plant room, basement grow, or want to grow in a room with no natural light
If you have a pothos in a well-lit south-facing kitchen and it is growing fine, you probably do not need a grow light for it. For everything else, supplemental light is one of the highest-impact upgrades you can make.
How Grow Lights Work
Plants need light to photosynthesise: they convert light energy, water, and carbon dioxide into glucose and oxygen. But not all light is equally useful. Plants absorb light most efficiently in the red (around 630-660nm) and blue (around 430-450nm) wavelengths of the visible spectrum. Green light is largely reflected, which is why leaves look green to us.
The light that plants can actually use for photosynthesis is called PAR (Photosynthetically Active Radiation) and it covers the 400-700nm range of the visible spectrum. Any grow light worth buying should emit strongly across this range.
Why Spectrum Matters
- Blue light (400-500nm) drives compact, bushy vegetative growth. It encourages short internodes (the stem length between leaves), which means stockier, sturdier plants. Strong blue is especially useful for seedlings.
- Red light (600-700nm) is the primary driver of photosynthesis and flowering. It encourages stem elongation and triggers bloom responses.
- Full-spectrum white light combines both and often includes green and far-red wavelengths that assist with deeper canopy penetration and overall plant health. Most modern LED grow lights aimed at houseplant growers use full-spectrum white LEDs because they work across the whole plant lifecycle and look more natural in a living space.
There is a lot of marketing noise around spectrum. For houseplants and general indoor growing, a full-spectrum LED rated for plant growth will cover your needs. Purple-only “blurple” LEDs (red plus blue only) can work but look unpleasant indoors and are harder to use for spotting plant health issues because everything looks pink under them.
Types of Grow Lights
LED Grow Lights
LEDs are now the dominant choice for indoor plant growing, and for good reason. They run cool relative to their light output, have long lifespans (typically 50,000+ hours), and can be tuned to emit the right PAR wavelengths efficiently.
Modern LED grow lights come in several form factors:
- Panel lights: flat boards with arrays of LEDs, good for coverage over a growing area
- Bar lights: long strips that spread light evenly; popular for shelving and grow tents
- Bulb-style / clip-on: single bulbs or small clip lights; useful for individual plants on desks or windowsills
Pros: Energy efficient, long-lasting, low heat output, suitable for all plant types, available in a wide wattage range from 5W clip lights up to 1,000W commercial panels.
Cons: Higher upfront cost than fluorescents, quality varies enormously (cheap no-brand LEDs often underperform their claimed wattage).
Typical cost: Quality LED grow lights for indoor plants range from around £15-£30 for a basic clip-on, £30-£80 for a decent bar or panel light for shelving, up to £150-£400 for full-coverage grow tent panels.
For specific product recommendations, see the guide to the best grow lights for indoor plants and the best LED grow lights for a deeper comparison.
Fluorescent Lights (T5, T8)
Fluorescent tubes, particularly T5 HO (high output), were the standard for indoor plant growing for decades and remain a viable option for some situations. T5 tubes emit a wide, even spread of light and run relatively cool.
Pros: Cheaper upfront than LEDs, good even coverage, mature technology.
Cons: Less energy efficient than modern LEDs, tubes need replacing every 12-18 months, fluorescents are being phased out in the UK and EU under energy regulations. They are harder to source than they were.
Best for: Seedlings, propagation trays, low-to-medium light houseplants where you want broad coverage at low cost.
Typical cost: T5 fixtures start around £20-£40 for a 2-tube 2ft unit, plus replacement tubes every year or two.
HID Lights (Metal Halide and High-Pressure Sodium)
HID (High-Intensity Discharge) lights are the traditional choice for serious indoor growing. They produce extremely high light output but also significant heat and consume a lot of electricity.
- Metal Halide (MH) emits a blue-white spectrum, better for vegetative growth
- High-Pressure Sodium (HPS) emits a red-orange spectrum, better for flowering and fruiting
Pros: Unmatched raw light output for large spaces, well understood in controlled growing.
Cons: Very high heat output (requires ventilation and cooling), high electricity use, bulbs degrade over time, ballasts add cost and complexity.
Best for: Large grow tents (1.2m x 1.2m and above) where maximum yield from fruiting plants justifies the running cost. Not practical or necessary for houseplant growing.
Typical cost: A complete 600W HPS setup with ballast, hood, and bulb runs £100-£200; electricity costs are significant at scale.
The short version: For most people growing houseplants, herbs, or edibles at home, a quality LED is the right choice. The running cost savings over fluorescent or HID make LEDs pay for themselves quickly.
Understanding PPFD, DLI, and Wattage
These three terms appear on every grow light spec sheet. Here is what they actually mean and how much you need to care about each.
Wattage
Wattage tells you how much electricity a light consumes, not how much light it produces. A 50W LED grow light and a 50W incandescent bulb use the same power but produce completely different amounts of usable plant light. Wattage is useful for calculating running costs; it is not a reliable indicator of grow light performance.
Watch out for “equivalent wattage” claims: some manufacturers list their products as “1000W equivalent” when the actual power draw is 100W. Always look for the actual draw wattage.
PPFD (Photosynthetic Photon Flux Density)
PPFD measures how many photons in the PAR range land on a specific surface per second, expressed in micromoles per metre squared per second (µmol/m²/s). Think of it as the intensity of light hitting your plant canopy at a given moment, like measuring how hard it is raining, right now, at a specific spot.
Good grow lights will publish PPFD maps showing the light intensity at different distances from the fixture. Be aware that many manufacturers measure at the centre point only; actual coverage across the full area is usually lower at the edges.
Approximate PPFD targets by plant type:
| Plant Category | Target PPFD (µmol/m²/s) |
|---|---|
| Low-light foliage (pothos, peace lily, ZZ plant) | 50-150 |
| Medium-light foliage (monstera, ferns, philodendron) | 150-300 |
| Herbs and leafy greens | 200-400 |
| Fruiting edibles (tomatoes, peppers, strawberries) | 400-700 |
| High-light edibles at peak production | 700-1,000+ |
DLI (Daily Light Integral)
DLI is the total amount of PAR light a plant receives over a full day, expressed in moles per metre squared per day (mol/m²/day). Think of it as how much light rain your plants collect in a bucket over 24 hours. PPFD is the rate; DLI is the accumulated total.
DLI = PPFD x photoperiod hours x 0.0036
For example, a light delivering 200 µmol/m²/s for 14 hours gives a DLI of: 200 x 14 x 0.0036 = 10.1 mol/m²/day. That is a reasonable target for many medium-light houseplants.
You can use DLI to balance intensity and duration: if your light is weaker, run it longer. If it is powerful, you may not need to run it as many hours.
Approximate DLI targets:
| Plant Category | Target DLI (mol/m²/day) |
|---|---|
| Low-light foliage | 3-6 |
| Medium-light foliage | 6-12 |
| Herbs and leafy greens | 12-18 |
| Fruiting edibles | 18-30 |
Matching Light to Plant Type
Not all plants want the same amount of light. Running a powerful grow light at close range over a shade-loving fern will burn it just as surely as placing it under a dim desk lamp will stunt a tomato seedling.
Low-Light Foliage Plants
Snake plants, pothos, peace lilies, ZZ plants, Chinese evergreens, and cast iron plants are naturally adapted to forest floor or shaded conditions. They do not need intense light; they need consistent, moderate light.
A PPFD of 50-150 µmol/m²/s for 10-12 hours per day covers these plants well. A modest 20-30W LED positioned 40-60cm above the canopy is typically enough for a group of low-light plants on a shelf.
Medium-Light Foliage Plants
Monsteras, philodendrons, ferns, calatheas, and most popular tropical houseplants fall here. These are the plants most likely to benefit from supplemental grow lights in a poorly lit room or through winter.
Target PPFD of 150-300 µmol/m²/s for 12-14 hours per day. A 30-50W quality LED at 30-45cm distance handles this range well.
Herbs and Leafy Greens
Basil, mint, parsley, coriander, lettuce, and similar edibles need noticeably more light than foliage houseplants. They want PPFD in the 200-400 range and a DLI of at least 12-15.
These are the plants that most commonly underperform when growers assume any desk lamp will do the job. They need a proper grow light, not a repurposed reading lamp.
High-Light Edibles
Tomatoes, peppers, cucumbers, and strawberries are the most light-hungry plants you are likely to grow indoors. They want PPFD of 400-700+ µmol/m²/s and a DLI of 18-30. At this level you are looking at grow tent setups with mid-to-high-power LEDs, typically 100W+ actual draw.
For guidance on full-spectrum grow lights suited to edibles, the dedicated guide covers spectrum options in more detail.
How Far Should Grow Lights Be From Plants?
Distance is one of the most misunderstood variables in grow light setup. The relationship between distance and light intensity follows the inverse square law: double the distance, and you get roughly a quarter of the light intensity. This means a few centimetres of adjustment can make a significant difference.
The right distance depends on both the light’s power and the plant’s needs.
Reference table: Grow light distance by plant type and light power
| Plant Type | Low Power LED (<50W) | Medium Power LED (50-150W) | High Power LED (150W+) |
|---|---|---|---|
| Low-light foliage | 30-50cm | 50-70cm | 70-100cm |
| Medium-light foliage | 20-35cm | 35-55cm | 55-75cm |
| Herbs and leafy greens | 15-25cm | 25-40cm | 40-60cm |
| Seedlings | 15-20cm | 20-30cm | 30-45cm |
| Fruiting edibles | Not suitable | 20-35cm (check PPFD) | 35-60cm |
Practical tip: Start at the upper end of the distance range and lower the light (or raise the plants) incrementally. Watch for signs of bleaching or burning (see below) before making the final call.
For a more detailed breakdown by specific light model, the guide to how far grow lights should be from plants has charts for common setups.
How Many Hours Per Day?
Plants need a dark period as much as they need light. Running grow lights 24/7 is not more efficient: it stresses many plants, particularly flowering species. Getting the photoperiod right matters.
General hour targets:
| Plant Type | Daily Light Hours |
|---|---|
| Low-light foliage | 10-12 hours |
| Medium-light foliage | 12-14 hours |
| Herbs and leafy greens | 14-16 hours |
| Seedlings and cuttings | 16-18 hours |
| Fruiting edibles (vegetative) | 16-18 hours |
| Fruiting edibles (flowering) | 12 hours (to trigger bloom) |
Use a timer. This is not optional advice: it is the single most important purchase after the light itself. Plants respond to consistent photoperiods, and manual switching is unreliable. A basic plug-in mechanical timer costs £5-£8 and pays dividends immediately. For more on setting up timers, the grow light timer guide covers the options.
Seasonal adjustment: In summer, many houseplants receive natural daylight as well. You may not need to run your grow light as many hours as in winter; the two light sources combine to meet your plant’s DLI target.
Signs You Need More or Less Light
Learning to read your plants is faster than any light meter.
Signs Your Plants Need More Light
Leggy, stretched growth: Long, thin stems with wide spacing between leaves is the classic sign of light-seeking behaviour. The plant is stretching toward where it thinks the light should be.
Small, pale new leaves: New growth that is noticeably smaller than the mature leaves, or that comes in a washed-out light green, indicates the plant does not have enough energy to produce full-sized leaves.
No growth at all: If a plant that should be actively growing produces nothing for several months (accounting for normal winter dormancy), poor light is often the cause.
Variegated plants reverting: Plants with multicoloured leaves may gradually revert toward solid green as a response to low light, producing more chlorophyll to capture what little light is available.
Soil stays wet for too long: A plant in low light photosynthesises slowly, uses less water, and can be prone to overwatering problems as a downstream effect of poor light.
Signs Your Plants Are Getting Too Much Light
Bleached or yellowed patches: Leaves closest to the light source turning yellow or white (rather than the whole leaf turning yellow uniformly) suggests light bleaching or photoinhibition: the light is so intense it is breaking down chlorophyll.
Leaf scorch: Brown, crispy leaf edges or patches, particularly on the upper surface, often indicate the light is too close or too intense for that plant.
Leaves curling inward: Leaves that curl toward their underside are trying to reduce their exposed surface area in response to excessive light.
Rapid soil drying: Intense grow light heat can dry soil quickly. If the medium is drying out faster than expected and the leaves look stressed, excessive light is worth checking.
The fix for too much light: Raise the light, reduce the hours, or use a dimmer if your fixture has one. Do not simply move the plant to a darker spot and leave the light running.
How Much Do Grow Lights Cost to Run?
Running costs for grow lights are often lower than people expect, particularly with modern LEDs. Here is how to calculate yours.
The formula:
Daily cost = (Watts x Hours) / 1,000 x unit rate per kWh
UK electricity rate (April 2026): The Ofgem price cap sets a typical unit rate of around 25p/kWh (your tariff may vary slightly).
Example calculations:
| Light Setup | Wattage | Hours/Day | Daily Cost (25p/kWh) | Monthly Cost |
|---|---|---|---|---|
| Small clip-on LED (1 plant) | 10W | 12 hrs | 3p | ~90p |
| Bar light (shelf of plants) | 30W | 14 hrs | 10.5p | ~£3.15 |
| Mid-range panel (grow tent) | 100W | 16 hrs | 40p | ~£12 |
| High-power LED (large tent) | 300W | 16 hrs | £1.20 | ~£36 |
For most houseplant setups (a bar light or panel over a shelf), you are looking at £3-£8 per month. That is not nothing, but it is well within reason for the growing results you get in return.
LEDs offer the lowest running costs of any grow light type. A comparable HID setup (400-600W) running the same hours would cost three to five times more in electricity and would need bulb replacements every 12-18 months on top.
Frequently Asked Questions
Can Normal LED Bulbs Work as Grow Lights?
Standard white LED bulbs emit some PAR light but are not optimised for plant growth. They typically produce insufficient intensity across the right wavelengths and are inefficient compared to dedicated grow lights. For seedlings or a single low-light plant, a bright daylight-spectrum (6500K) bulb positioned very close to the plant can work in a pinch. For anything more demanding, a proper grow light is worth the relatively modest cost.
How Do I Know if My Grow Light Is Strong Enough?
The simplest check is plant response over four to six weeks. If new growth is coming in healthy, full-sized, and well-coloured, your light is working. If growth is slow, leggy, or pale, you need more intensity (lower the light, increase hours, or upgrade to a more powerful fixture). A dedicated PAR meter or a smartphone app like Photone can give you PPFD readings if you want to dial in precisely.
Do Grow Lights Work for All Plants?
Grow lights work for any plant that photosynthesises, which is all of them. The question is whether the right type of light, at the right intensity and distance, is being used for that particular plant’s needs. A cactus and a fern have very different requirements; applying the wrong light setup for a species will still underperform.
Should I Leave Grow Lights on Overnight?
No. Plants benefit from a dark period for respiration and normal growth cycles. Most plants do best with 10-18 hours of light and the remainder in darkness. Running lights 24/7 does not accelerate growth and can stress some species, particularly those that rely on photoperiod signals for flowering.
Are Grow Lights Safe for My Home?
Quality grow lights from reputable manufacturers carry CE certification (for UK/EU compliance) and are safe for home use. Keep them away from flammable materials, make sure any timer or smart plug is rated for the load, and do not use a fixture that is not designed for its environment (some grow lights are not splash-proof and should not be used where water is regularly misted).
Do Grow Lights Help Succulents?
Yes. Succulents are actually quite light-hungry. Most need at least 6 hours of bright light per day, and without it they etiolate (stretch toward the light source, losing their compact form). A full-spectrum LED at moderate intensity and a 12-hour photoperiod significantly improves succulent growth and colouration compared to a shaded windowsill.
Getting Started
A grow light does not have to mean a full grow tent setup with extraction fans and carbon filters. For most people with indoor plants, it means a decent LED bar or clip-on light, a cheap plug-in timer, and paying attention to your plant’s response over the first month.
Start with the light positioned at the upper end of the recommended distance range for your plant type. Set the timer for 12-14 hours. Watch the new growth. Adjust from there.
Once you know what you are buying for, the buying guides will help you narrow down to specific models and brands. For the best options on the market right now, see the best grow lights for indoor plants buyer guide, or if you know you want LEDs specifically, the best LED grow lights review is the place to start. For plants that specifically benefit from broad-spectrum output, the full-spectrum grow lights guide covers the key options.
If you are growing a range of indoor plants and want to understand more about what each species needs, the indoor plants hub is a good starting point for species-specific light requirements.