Grow Lights 101: How to Feed Your Houseplants When the Days Get Short

Every November, your plants' meals get smaller, and dinner ends earlier.

Maybe you've already noticed: a yellow leaf at the bottom of the pothos, a fiddle that dropped two leaves the week the heat came on, a monstera that's been sitting on the same three leaves since September. If that's had you wondering what you're doing wrong, chances are you're not doing anything wrong. Most of the time your plant is simply eating less, because there's less to eat.

This post is about what a grow light can do about that, if you want one. We'll walk through how we measure whether a grow light is actually feeding a plant and how we light our shop year-round, then get into what's worth buying. None of it is required, and plenty of plants coast through a Hamilton winter on a smaller meal. But if you've ever stood in a store aisle staring at purple bulbs and wondering what any of it means, read on.

Light Is the Meal (Fertilizer Is Still the Multivitamin)

If you've read our fertilizer post, you know where we stand: your plant eats light. Fertilizer is the multivitamin that supports the process, not the food itself.

So think about light the way you'd think about a meal. Two things decide how much a plant gets to eat in a day:

How full the plate is. That's intensity: how bright the light is right at the leaf.
How long it sits at the table. That's duration: how many hours that light lasts.

Multiply the two and you get the day's meal. A modest plate served all day can add up to the same meal as a big plate served for an hour. That trade between brightness and hours is what grow lights are built around.

A Hamilton winter shrinks both at once. The sun sits lower in the sky, so the plate gets lighter, and the days get short: around the winter solstice we get about nine hours of daylight, compared with about fifteen and a half in late June. Add our grey Great Lakes skies, and a window that fed your plants generously in July serves a much smaller meal by December.

The Usual Suspects Behind November Leaf Drop

We covered the winter slowdown in Houseplant Hibernation: Winter Mode, so here's the short version: when the meal shrinks, plants often let go of older leaves they can no longer afford to keep running. Lower leaves usually go first, because they eat the least (you'll see exactly why in our readings below).

Light isn't the only suspect this time of year, though, so read the signal before you reach for a fix:

Most commonly, it's the smaller meal. Slow, gradual yellowing of older leaves, with no other drama.
Sometimes, it's the heat coming on. Our eight-foot fiddle leaf fig drops a handful of leaves every winter when the heaters start, then settles. Dry air from vents and cold glass at night both play a part.
Sometimes, it's watering. A plant eating less also drinks less, so the soil stays wet longer. The finger test keeps you in step: first knuckle dry, water; still damp, wait.
Less often, it's pests. Spider mites love dry, heated air. Our spider mites post covers what to look for.

A grow light only answers the first one. If the heat or the watering is the real issue, more light won't fix it.

Reading the Plate: How We Measure Light

I learned to measure light from two teachers. Darryl Cheng's The New Plant Parent (chapter 5 especially) is what first got me holding a light meter up to my plants, and his House Plant Journal is still where I send anyone who wants approachable numbers. Leslie Halleck filled in the technical side: her book Gardening Under Lights and her Bloom and Grow Radio episode, Grow Lights 101, are where I really understood what a grow light is doing, and why the numbers on the box don't tell you much.

The two free tools from our natural light post, the shadow test and the sky check, still work as a first read. But grow lights are where an actual number earns its keep, because your eyes judge artificial light just as poorly as daylight.

How we take a reading

We use a free phone app called Light Meter, which reads in foot-candles, and take every reading the same way:

Find your phone's light sensor. On most Android phones it sits near the top edge of the screen, by the front camera. (iPhone apps generally estimate from the camera instead.)
Take the phone to the leaf. We rest it right against the leaf, with the sensor facing the light.
Let the number settle for a few seconds, then read it.

Measuring at the leaf matters, because two leaves on the same plant can be eating very different meals. Phone apps aren't lab equipment either. We treat the numbers as comparisons, like "this leaf gets about three times that one," and for most decisions that's plenty.

What we found in the shop

On a sunny morning in early November, between about 8:30 and 9:00, we measured a few plants around the shop. Each number was taken at the leaf.

Plant	Where it sits	Distance to light	Reading
Manjula pothos	Under a bar on our big plant shelf	About 14 in. from the bar	120 to 140 fc
Monstera 'Thai Constellation', top leaf	Same shelf unit, reaching up toward the bar	About 8 in.	130 fc
Monstera 'Thai Constellation', lower leaf	Same plant, under hanging baskets	About 16 in.	40 fc
Jade, side facing the window	Bench about 19 ft. from our east window	About 55 in. from the nearest bar	130 fc
Jade, side facing the shelf	Same plant	Same	48 fc
Fiddle leaf fig (our eight-footer)	About 2 ft. from the east window, in direct sun	About 2 ft. from the glass	About 4,700 fc

What the readings are telling us

The Thai Constellation's top leaf gets more than three times the light of a lower leaf eight inches further down. Same plant, same light, a minute apart. Distance and shade are working together there (the lower leaf also sits under hanging baskets), and it's a big part of why lower leaves tend to yellow first in winter. The jade shows the same split sideways, with the window side getting nearly three times what the back gets. A quarter turn every so often spreads the meal around for free.

Left: Monstera Thai Constellation, Right: Manjula Pothos

Distance doesn't explain everything, though. The Manjula sits farther from its bar than the Thai Constellation's top leaf, yet reads about the same, thanks to the white wall behind it bouncing light back onto its leaves. Distance, shade, and reflection all change the meal, so a reading from your own spot will always beat a chart, ours included. Variegated plants like the Manjula have less green tissue to work with and tend to feel a smaller meal first; our variegation post explains why.

The jade photo also shows how easily eyes get fooled. Sunbeams stretch across the floor, so it looks like a sunny spot, yet the jade itself read 130 foot-candles. That's good window light, and a small fraction of the 4,700 the fiddle got with sun landing directly on its leaves. Your eyes adjust so well that both spots look bright, while the meter tells two very different stories.

That fiddle reading puts grow lights in perspective. The bars give our shelf about 130 foot-candles, and the sun gave the fiddle more than thirty times that. Grow lights aren't brighter than the sun, and they don't need to be. Their advantage is time, which is where the math comes in.

The nerdy layer: from foot-candles to a day's meal

Foot-candles measure light the way human eyes see it, which leans toward yellow-green. Plants count light differently, so horticulturists use two units built for them:

PPFD is how much usable light lands on a spot each second (in µmol/m²/s). That's the plate.
DLI, or daily light integral, is the total that lands over a whole day (in mol/m²/day). That's the meal.

Virginia Tech's extension guide to DLI is blunt that foot-candles and lux aren't recommended for measuring light for plants. You don't need a special meter to use the idea, though. A foot-candle reading can be roughly converted once you know your light source, and Darryl Cheng's House Plant Journal has charts that do the conversion for you, so you can skip the math entirely.

Run our readings through those charts and two pictures emerge. Our shelf gets a small plate (low intensity), but it's served around the clock (24 hours of light); on a 12-hour timer, the same bars would deliver about half the daily total. The fiddle in direct sun gets an enormous plate (high intensity) for only part of the morning, and even one hour of it adds up to more light than our bars give the shelf in a full day.

The bars still do their job; they simply work at a much smaller scale than the sun. Scale is also why light "bands" disagree from source to source. Cheng's book calls 50 to 150 foot-candles very low light, while UGA Extension calls 75 to 200 medium light (for eight hours a day). Our shelf is "very low" in one and "medium" in the other. The daily total is the more useful idea.

How We Light the Shop

Our setup is simple. Linkable full-spectrum LED bars in a warm white run along our big plant shelf, connected end to end, and a few grow bulbs sit in ordinary light fixtures. We chose warm white so the shop still feels cozy, like a room you'd want to sit in. The bars run 24/7. I think they've been switched off twice since we opened.

Most of our plants fit under them. Tougher plants, like our sansevierias, move around and do fine on the smaller amounts of light that spill and reflect off the shelf. Bigger plants sit in our east-facing front windows on Sherman, getting morning sun plus a little extra from the bars. And apart from the big fiddle, everything rotates: plants move around the shop, some spending time in brighter window spots, some on the shelf. So the bars set the floor: a dependable baseline that keeps everything growing well whatever the sky is doing, with window light added on top as plants rotate through.

That's also why we run them so long. Our bars set a small plate (about 130 foot-candles), so we make up the difference with time. Served around the clock, that small plate (low intensity for 24 hours) adds up to roughly the same daily meal as a plate twice as full served for twelve hours. Commercial growers use the same idea on purpose: greenhouse research describes long hours of lower-intensity light as a way to hit a daily light target with fewer fixtures.

So should your light run all night?

I want to be honest about what the research does and doesn't say here. Most studies on continuous lighting have been done on food crops, not houseplants, and tolerance turns out to be species-specific. Researchers at Agriculture and Agri-Food Canada's centre in Harrow, Ontario note that lettuce and cucumber handle 24-hour light well, while tomatoes and peppers can develop leaf injury (yellowing between the veins) without a dark period. Healthy foliage plants on that shelf since we opened in June 2023 are real evidence. They're also one shop's experience, and nobody has run a controlled study on it.

So for most homes, a timer with a nightly dark stretch is the more predictable choice. Extension guidance commonly lands somewhere around 12 to 16 hours of light a day, and UGA Extension gives 16 hours on and 8 off as an example. Start there and adjust. Nothing about a timer setting is permanent.

The plants that need the dark

Some plants bloom only after long, uninterrupted nights, and holiday cacti are the classic example. Light at night can cancel their buds; our holiday cactus post explains how. Ours live away from the shelf on purpose. The one on top of our fridge, which gets light on one side from the back door, blooms every year. We moved the other to the top of our Doctor's Appointments shelf this year, and it hasn't hit a bloom season there yet, so we're watching it.

One more shop plant worth knowing: as we shared in our fiddle leaf fig guide, our four-foot fiddle sits in a corner near a grow light and a transom window, but gets direct light from neither. It's been healthy for about seven years and has grown very slowly. Near a light isn't the same as under it, and light sets the pace.

What the Big Growers Already Figured Out

For a lot of people, cannabis is the first place they ever heard about grow lights. It's a legal, regulated industry here in Ontario, and because lighting is one of its biggest costs, it has produced some of the most useful lighting research around. A University of Guelph study put lighting at roughly 60% of the energy used in indoor cannabis production. When light costs that much, growers want to know exactly what it does.

A few of their findings apply just as well to a pothos on a bookshelf:

The same Guelph team noted that lighting vendors have long sold fixtures using single-leaf studies, which only loosely predict how whole plants grow. Measuring your own plants is the better guide.
They also found that small changes in distance from the light make big differences at the leaf, which is exactly what our Thai Constellation showed.
A Utah State University study concluded that a fixture's efficiency and cost mattered more than its exact colour mix at commercial light levels. (A lighting manufacturer helped fund it, which the paper discloses.)
A Guelph study with a Canadian licensed producer found no commercially relevant benefit to adding UV.
Cannabis is a short-day plant, like poinsettias and holiday cacti. NC State researchers found that adding light in the middle of the night, or stretching the day, keeps it from flowering: the holiday cactus lesson at commercial scale.

One big difference: cannabis is a sun crop that keeps photosynthesizing at light levels many times higher than anything a houseplant wants. Most houseplants come from the shade of a forest canopy, as we covered in our sunburn post. So borrow the growers' habits of measuring, thinking in daily totals, and tuning out the hype, and leave their light levels in the grow room.

Shopping for Grow Lights Without Getting Dazzled

Grow lights come in every shape and colour, and finding a good one locally can take some looking.

About the purple ones

A lot of people picture grow lights as red and purple, and those lights do work. The thinking behind them is that chlorophyll absorbs red and blue light most strongly. But plants use green light too: research shows green light travels deeper into leaves and down into lower foliage, driving photosynthesis where red and blue don't reach.

For a home, white light also has two practical advantages. It looks like normal light in your living room. And it shows you your plant's true colours, so you can spot yellowing, pests, or scorch early. Under purple light, everything looks a little alien. For most homes, our pick is full-spectrum white, and we lean warm white in particular. It reads like an ordinary lamp, so it doesn't feel aggressive in your living space. It's what we use in the shop.

What to look for

White, full-spectrum light. Read the label carefully. "Full spectrum" red-and-blue bulbs often sit right beside the white ones on the same shelf.
A format that fits your space. A grow bulb that screws into a lamp you already own is the easiest start. Linkable LED bars, which is what we use, are great under shelves: they connect end to end and spread light evenly across a row of plants.
A timer. It's inexpensive and gives you consistency without having to remember. Some lights come with one built in.
Skip the extras. UV, special "bloom" recipes, and wattage claims like "replaces 500W!" aren't worth paying for. What matters is how much light reaches your leaves at your distance.

Where to find them

Grow lights aren't widely stocked around here. Most hardware stores carry only a few options, if any. Three places worth looking:

Online, where the selection is widest. Look for listings that state the colour (warm white, daylight) and, ideally, how much light reaches a plant at a given distance.
A garden centre with a big houseplant selection. We love a day trip out to The Watering Can in Vineland, with its thousands of tropicals, a café, and an energy a lot like ours. If a light is the main reason for the trip, call ahead, and note that they're closed Sundays.
Canadian Tire, which as of this writing carries white full-spectrum grow bulbs that fit standard lamp sockets, plus T5-style LED strips.

Then test it

Once your light is set up, take a reading at the leaf. That number tells you more than anything printed on the box.

You don't need a grow light to be a good plant parent, either. Moving a plant closer to a window costs nothing, and it's often enough.

Setting Up a Grow Light at Home

How far should a grow light be from your plants? Closer than most people expect. In our shop, leaves about 8 to 14 inches from a bar read 120 to 140 foot-candles, and the reading dropped fast past that, down to 40 at 16 inches on a shaded leaf. A lot of people hang their lights far too high.

Ease in, though. A plant that's been living in a dim corner can scorch if it's suddenly parked right under a light, the same dose-and-acclimation idea from our sunburn post. Start a little farther away and move closer over a week or two, and give pots a quarter turn now and then so every side gets fed.

Expect the watering to change, too. A plant that's eating more drinks more, so the soil will dry faster under a light; keep using the finger test to decide when. And keep holiday cacti and other long-night bloomers out of the glow.

Risk overall is low. If something isn't working, move the light, move the plant, or change the timer. Everything here is reversible.

Feeding Alongside the Light

Our rule from the fertilizer post still holds: fertilize in response to growth, not the calendar. That's why we usually ease off in winter. Most plants slow down, and a plant that isn't growing can't use the extra.

A grow light can change that. If your plant is pushing out new leaves under a light in January, it's growing, and the rule says you can support it. Two ways to go:

Go easy. Keep feeding light and occasional, and only while you see active growth.
Microdose. If your light is serving a steady, modest meal every day, a steady, modest dose of fertilizer suits it: a very, very small amount added to every watering. At the shop we do this seasonally, mostly in spring, with a little Kelpy in the watering can. At home, it's how I feed my own plants.

Either way, gentle wins indoors. A pot can't flush the way garden soil does, so the smallest dose that works is usually the right one.

Seasonal Notes for Hamilton

Start whenever you notice the meal shrinking as the days shorten through October and November; there's no wrong week to begin. Ease off in spring, once your windows start serving bigger meals again, by shortening the timer or moving lights to your dimmest spots.

Before buying anything, try the free fixes: move plants closer to the glass, wipe dusty leaves, and clean your windows, since winter grime cuts into an already small meal. On frigid nights, ease plants an inch or two back so their leaves aren't pressed against the cold glass.

Next time a plant looks like it's sulking this winter, try holding your phone up to its leaves before you change anything else. Often the answer is a smaller supper, and then you get to decide whether to top up its plate or let it rest until spring.

If you'd like a second set of eyes on your own space, that's what our Growing Consultations are for: I'm happy to help you read your light, window by window, and work out whether a grow light makes sense. And if a plant is dropping leaves in a way that doesn't fit any of this, bring it by for a Doctor's Appointment. No booking needed.

Notes for Stef (rem

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