A regular LED bulb is a weak grow light, not a different kind of light. Plants count photons between 400 and 700 nanometres, an ordinary white LED emits right across that band, and the honest difference between the two is intensity and duration rather than spectrum. Up close, a regular bulb clears the floor a low-light houseplant needs. It will not carry a monstera, a herb pot, or anything you intend to eat: those plants need a daily photon total no room-lighting bulb delivers at a workable distance. So measure the light you already have before you spend anything.
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TL;DR
- The number that matters is not lumens or watts. It is PPFD, the photons reaching the leaf each second, and DLI, the daily total:
DLI = PPFD × hours × 0.0036. - Foot-candles at the leaf decide it. Below 100 fc leaves stagnate, 200 fc is where a monstera fenestrates, and up to 3,000 fc is the fast-growth band.
- “Full spectrum” is settled marketing. White LEDs already cover the photosynthetic band, and stretched internodes report that intensity is short.
- Measure with a free phone app first. A cheap lux meter is the sensible upgrade; a $300 to $500 PAR meter is not.
- A regular bulb on a timer, held close, is enough for a low-light plant. Buy a fixture for medium and high light plants, edibles, and Nordic winters.
What a plant actually counts
A grow light and a regular light are sold on different numbers, and neither is the one a plant responds to. Lumens describe human brightness, watts describe electricity. A leaf runs on photon arrivals.
PPFD is the instantaneous rate: photons in the 400 to 700 nanometre band landing on a square metre of leaf per second, in µmol/m²/s. DLI is the daily total in mol/m²/day, and DLI is what decides whether a plant grows, coasts, or declines.
DLI = PPFD × hours × 0.0036
That line is the buying decision, because it has two knobs. Raise intensity with a brighter lamp or a shorter distance, or extend the photoperiod, which costs a timer. When plants share a shelf, hours is the better lever: changing height re-measures every pot underneath, while the timer moves them together.
Cheap meters and phone apps report foot-candles or lux rather than PPFD. Convert at roughly 5 fc per 1 PPFD in daylight, or about 6.5 under a white LED, because a lux sensor over-weights green. One foot-candle is 10.8 lux either way.
Why “full spectrum” is the wrong question
Every grow light listing leads with spectrum, and spectrum is the settled part of the comparison. Chlorophyll peaks in blue and red and dips in green, but a leaf’s photosynthetic response covers the whole 400 to 700 nanometre band, with red the most efficient per photon. A modern white LED, regular or horticultural, is a blue diode behind a phosphor emitting across that range. The photons are the same photons.
Purple red and blue fixtures come from an era of less efficient diodes, when dropping the wavelengths plants use least bought real photons per watt. That trade matters less now, and it costs you easy measurement, because a lux meter underreads narrow red and blue badly.
Which leaves intensity as the binding constraint indoors, and the plant reports on it free. Too few photons trigger a gibberellin-driven stretch in the internodes: the stem elongates between leaves as it gambles on light above. Long gaps between small pale new leaves means photons are short, and nothing stretches over colour temperature. When a fixture page says household LEDs cannot grow plants, read it as a claim about intensity dressed up as a claim about spectrum.
The light levels your plants actually need
The thresholds worth planning around, in foot-candles at the leaf:
- Below 100 fc, about 1,000 lux. Leaves stagnate or shrink. The plant is alive and not growing.
- Around 200 fc. Where a monstera fenestrates: slow but steady.
- Up to 3,000 fc. Fast growth: large leaves, deep fenestration, quick pot-drying.
- Up to 6,000 fc. Direct sun through glass, the ceiling, fine only in short doses.
For how those bands map onto one species, see the light level a monstera actually needs.
Edibles sit higher and carry an upper bound: leafy greens lose quality above roughly 11 to 12 mol/m²/day, a threshold I crossed by accident. Top to bottom, a factor of sixty.
Measure before you spend anything
The cheapest useful light meter is the phone in your pocket. Free apps read foot-candles or lux off the front camera, accurately enough for houseplant decisions, and for most people that is the entire measurement budget. Do this before comparing fixtures:
- Install a light-meter app and cover the front camera with its diffuser sticker, or translucent white film.
- Hold the phone at canopy level, sensor facing the light. A reading a metre above the plant describes the room.
- Take three readings: brightest leaf, one a hand’s width aside, one at the bottom. PPFD falls rapidly toward the edges of any beam.
- Repeat on an ordinary day, at the hours the plant collects light.
- Divide foot-candles by about 5 for daylight, or about 6.5 under a white LED, then run the DLI formula.
The full walkthrough lives under measure the light you already have.
A dedicated meter is the upgrade for a whole collection, mainly for speed. The cheap option my notes point at is the Dr.meter Digital Illuminance Light Meter, listed to 200,000 lux across switchable ranges. Watch the x10 multiplier when you switch to lux: a display reading of 1,500 is 15,000 lux, and misreading it buys a fixture on a number an order of magnitude off.
Two honest notes. I have not bench-tested that meter against a reference sensor; it is here as the documented cheap upgrade from a phone, not as something I trialled. A true PAR meter, reading PPFD directly, runs $300 to $500 and is unnecessary at home.
When a regular bulb is genuinely enough
Here is the branch the fixture industry skips. If your plant is a genuine low-light species, a pothos, a ZZ, a snake plant, and the leaf clears 100 fc for ten to twelve hours a day, buy nothing. You own a grow light. It is the lamp on your ceiling. The 200 fc in the list above is a monstera’s fenestration floor, not a universal one, and my notes document no floor at all for these three.
Distance is why. Output collapses with distance, so an ordinary bulb 20 cm above a shelf plant beats a proper fixture a metre away. The falloff that makes a weak lamp adequate up close is what makes a strong lamp useless mounted politely out of the way.
Everything worth trying first is free: move the plant nearer the window, clean the glass, open the sheer curtain through the dark months, lower the lamp you own. Then add a cheap mechanical timer, because consistency is the one thing a room light lacks: a plant wants the same ten to twelve hours daily and a real dark period, and human evenings do not supply that.
Below 100 fc at every hour you measure, the branch closes: no household bulb fixes it, because the shortfall is output rather than placement.
When you need a real fixture
A grow light stops being optional in three cases: plants needing the upper part of the 200 to 3,000 fc band, anything edible, and a Nordic winter. At 57 to 60 degrees north, solar noon sinks to between 6.5 and 10 degrees around the December solstice and stays under 15 degrees for most of November through January, and a good south window can sit near that 200 fc mark for weeks. See also winter light for a monstera.
What separates a grow bulb is photon flux and beam shape. The spec to hunt for is PPF, total photons emitted per second in µmol/s, not lumens. Watts is the crude tell: 36 watts into a reflector aimed at one canopy is a different proposition from 9 watts spread around a room.
My workhorse is the SANSI 36W full-spectrum bulb. I run four, three at 36W and one at 24W, about 132 watts of the roughly 198 watts of grow-light capacity here, alongside Nelson Garden LED strips. Measured at the leaf, it gives over 200 PPFD at 10 to 18 cm. Past that the curve is steep: inverse-square from the middle of that band, not a second reading, puts 30 cm near 45 PPFD.
Be clear about the base before you click. My bulbs are the E27 screw base sold in my market; the US listing I link, the SANSI 36W LED Grow Light Bulb, is the E26 / BR30 form of the same lamp, E26 being the US screw fitting and E27 the European one. Both listings quote PPF within a rounding error, 65.6 µmol/s on the US unit against 65 µmol/s on the E27 one I buy from, but the 200 PPFD figure above was measured on my E27 bulbs, not the E26 version. Treat it as a close indication, not a reading from the SKU in the link. I only link the US store.
Beam shape is what no listing tells you. The Nelson Garden strips throw a narrower spread than the SANSI globes, and under either, PPFD falls off fast toward the edges. A fixed inch recommendation is meaningless across fixtures: measure at several points, and again after any change in height or placement.
The two ways people overdo it
More light is not strictly better, and neither failure below is the lamp’s fault.
Too many hours. My grow-light mistake was a photoperiod, not a purchase. The 36W bulb at close range, over 200 PPFD, on a 16 hour timer works out at 11.5 mol/m²/day. That is at or above the safe daily maximum for lettuce, and the crop said so: bitter leaves and premature bolting. The fix was cutting the photoperiod to 14 hours rather than moving the lamp, since other plants shared the shelf at that height.
Too much lamp in a living room. Reaching 3,000 fc means high-wattage lamps, and in a shared living space the real cost is not electricity. It is living with a bright 4000K bulb where you eat dinner. Kill This Plant flags it too. Decide where the light will point before deciding what to buy.
FAQ
Can you use a regular LED light as a grow light?
Yes, within limits. A regular white LED emits across the 400 to 700 nanometre band plants photosynthesise in, so it works as a weak grow light. Held within 20 to 30 cm of a low-light houseplant and timed for ten to twelve hours, an ordinary bulb can clear the 200 foot-candle floor, though whether yours does is a measurement. It will not reach the band that medium and high light houseplants and edibles need.
What color grow light is best for indoor plants?
Full-spectrum white is the practical choice, and grow light colour matters far less than the marketing implies. A leaf’s photosynthetic response is broad across the visible range, so a white LED covers what an indoor plant needs. Purple red and blue fixtures buy a little efficiency and cost you easy measurement. Choose a grow light on PPF, not spectrum charts.
How far should a grow light be from my plants?
No single grow light distance transfers between fixtures, because PPFD depends on the lamp’s output and beam shape. My SANSI 36W bulb gives over 200 PPFD at 10 to 18 cm above the canopy and falls off steeply past that; a panel has a different curve. Set the height so the canopy reads in the band the plant needs, and re-measure after every move.
How long should grow lights be on for houseplants?
Ten to twelve hours a day suits most houseplants under a grow light, and a dark period is not optional. Photoperiod is the knob to reach for first because it changes DLI without disturbing lamp height. Run the numbers with DLI equals PPFD times hours times 0.0036, and aim to clear 200 fc at the leaf rather than a DLI target.
Do I need a grow light for my houseplants in winter?
At Nordic latitudes, usually yes for anything above low light. Solar noon sinks under 10 degrees around the December solstice, and a south window that is comfortably bright in June can sit near the 200 foot-candle floor at midwinter. A pothos or a snake plant coasts on window light. A monstera or a winter herb pot needs a grow light from mid-November.