Grow lights went from specialist kit to mainstream houseplant accessory in about five years. The technology improved, the prices dropped, and a generation of north-flat dwellers discovered they could keep plants they would otherwise kill. The problem is that the category is now full of products making vague claims. Here is what the science says and what actually matters when buying and positioning a grow light for houseplants.
The only light type worth buying
Full-spectrum LED is the correct choice for houseplants. It covers the full photosynthetically active radiation (PAR) range from 400 to 700nm efficiently, produces minimal heat, and consumes far less electricity than older options.
Avoid incandescent bulbs: they produce mostly heat with negligible useful light and are energy-wasteful. Traditional fluorescent tubes work but are inefficient and awkward to position. Red and blue "blurple" LED panels are effective for plant growth but produce an unpleasant purple light that is disruptive in a living space; full-spectrum white LEDs do the same job with better aesthetics. HPS and metal halide are horticultural-scale lighting, overkill for houseplants and generates significant heat.
How much wattage do you need
For low-light plants such as snake plant, ZZ, pothos, and peace lily on a desk or shelf, 10 to 20W is enough. For medium-light plants such as monstera, philodendron, and most tropical foliage on a standard shelf or as a window supplement, 20 to 40W is the right range. For high-light plants including herbs, succulents, cacti, and flowering plants needing full-sun simulation, 40 to 80W.
These are actual draw wattages, not "equivalent" wattages. Check the product specification for "actual draw" or "real wattage." A 100W "equivalent" bulb that actually draws 15W produces 15W of light. The equivalence claim is marketing.
Distance and placement
The distance between the light source and the plant canopy is as important as the wattage. Too far and the plant receives dim diffused light, equivalent to moving away from a window. Too close and there is a risk of light burn on leaves nearest the source.
General guidelines for LED: low-light plants should be 30 to 60cm from the light; medium-light plants 20 to 40cm; high-light plants 15 to 30cm. Adjustable-height pendant or clip lights make this easier to calibrate. A simple check: if leaves nearest the light are yellowing or bleaching, the light is too close. If all growth is stretching toward the light, it is too far.
How long to run the light
Most houseplants need 12 to 14 hours of light per day when a grow light is the primary or sole light source. High-light plants and seedlings benefit from 14 to 16 hours. If supplementing natural window light rather than replacing it, 8 to 12 hours is typically enough.
Always use a timer. Plants need a consistent dark period for healthy hormone cycles. Running a grow light 24 hours causes stress and disrupts dormancy cycles. Run the light during daytime hours rather than overnight so plants experience a natural day and night transition. Plants do sense the difference.
What a grow light cannot fix
A grow light addresses one variable: light. It does not fix overwatering, root rot, nutrient deficiency, or temperature stress. Adding a grow light to a plant with root rot will not save it. A grow light in a room below 15C will not meaningfully help tropical plants, which need warmth as well as light. Grow lights do not increase humidity, which ferns, calatheas, and maidenhair ferns also need in addition to light. The grow light supplements light. Other care factors still apply.
FAQ
Can I use a regular LED bulb as a grow light?
A standard warm-white LED bulb produces some usable light for plants but is weighted toward yellow and red wavelengths and provides limited blue, the spectrum plants use for leafy growth. It will keep a very low-light-tolerant plant such as a ZZ or snake plant alive at close range on a timer but is not effective for most plants. A dedicated full-spectrum LED is not much more expensive and is significantly more effective.
Do grow lights run up electricity bills significantly?
A 30W grow light running 14 hours per day uses 0.42 kWh per day, or roughly 12 to 13 kWh per month. At UK electricity rates around 24 to 28p per kWh as of 2026, that is approximately 3 to 4 GBP per month per light. Running several lights in a dedicated plant room adds up, but for one or two supplemental lights the cost is modest.
How do I know if my plant is getting enough light from the grow light?
The clearest indicators are growth rate and leaf size. A plant getting sufficient light under a grow light grows at roughly the same rate it would near a good window. If new leaves are small and growth is very slow, the light is too dim, too far away, or not running long enough. If leaves nearest the light are bleached or have crispy patches, the light is too close.
Can I use a grow light on plants that are already near a window?
Yes. Supplementing a dim north window with a grow light during winter is a very effective strategy. Run the grow light during the hours when natural light is lowest, such as early morning and evening or overcast days, or simply keep it on a consistent 12 to 14 hour timer alongside the window. The plants receive both sources simultaneously without harm.
My plants are on a bookshelf with no window access. Will a grow light work?
Yes, with caveats. The grow light needs to be close enough to be effective, around 20 to 40cm from the canopy for most plants, and you need appropriate plants: low to medium-light varieties rather than cacti or herbs. A clip-on or shelf-mounted LED strip with a timer is the typical solution for a fully interior bookshelf plant setup.