How Many Hours of Light Does a Countertop Garden Really Need?

Every kit maker has quietly answered this question differently, which tells you something. AeroGarden's classic salad-greens program ran lights 17 hours a day; the Harvest 2.0 defaults to 15. Click & Grow picked 16 and won't let you change it. iDOO gives you modes that land around 12. LetPot's app defaults to 16 and lets you drag the slider anywhere. These machines all grow the same lettuce and basil, so at most one of those numbers can be "correct" — and in truth none of them is, because the right answer depends on the crop, not the machine.

The good news is that the answer is stable. Plant photobiology hasn't changed since these kits were invented, and it won't change next year either. Set the schedule right once per planting and you can ignore it until you reseed.

Why the defaults disagree#

Plants don't count hours; they accumulate light. What drives growth is the total daily dose — intensity multiplied by time. Outdoors, a summer day delivers a huge dose in whatever hours it gets. A countertop kit's 10–24W hood is, charitably, a heavily overcast sky. The only lever a weak light has is time, so kits run long days to claw back the dose: 15 hours of dim light lands roughly where 8 hours of decent greenhouse light would.

That's why you can't take a light schedule from a greenhouse chart or a high-power tent guide and apply it to an AeroGarden. Those numbers assume intensity your kit doesn't have. It's also why the manufacturers' defaults cluster in the 14–17 hour range — they're compensating — and why the answer for bolt-prone crops has to break from the default entirely, as we'll get to.

The crop-by-crop schedule#

Hours per day, assuming a typical countertop kit hood (10–24W LED) positioned a few inches above the canopy. Where your kit's default disagrees, change it — every app-controlled kit and every AeroGarden with a "customize" option lets you.

Crop	Hours/day	Why
Leaf lettuce, butterhead	12–14	Long days + hood heat = tipburn and bitterness. More light does not mean more salad.
Arugula, mustard greens	12–14	Fast and bolt-prone; keep days moderate and harvest young.
Spinach	10–12	A long-day bolter — it flowers because days are long. The hardest common crop to keep vegetative in a kit.
Kale, chard, bok choy	14–16	Sturdy, bolt-resistant greens that use everything you give them.
Basil	14–16	The crop kit defaults were built for. Pinch early, run long.
Mint, parsley, chives	12–14	Happy at moderate days; parsley gets coarse under very long ones.
Thyme, oregano, rosemary	14–16	Mediterranean sun-lovers; under a weak hood, hours substitute for intensity.
Cilantro, dill	10–12	Long-day bolters like spinach. Short days buy you weeks of leaf harvest.
Dwarf tomatoes, peppers	14–16, never past 17	Fruiting needs the biggest dose in the kit world — but tomatoes are injured by near-continuous light. Keep ≥7 hours dark.
Day-neutral strawberries	12–14	Flowering isn't day-length-driven; moderate days keep fruit quality up.
Microgreens (post-blackout)	10–12	They're harvested in days; long schedules just spend electricity.

Two patterns worth internalizing. First, the sturdy stuff — basil, kale, fruiting crops — wants 14–16 hours, which is why the factory defaults mostly work if that's what you grow. Second, the bolters — spinach, cilantro, dill, and to a lesser degree lettuce and arugula — want less light than any kit ships with. If you've ever had cilantro rocket to flower in three weeks, the 16-hour default did that, not your water.

More light is not more yield#

The growth response to light flattens brutally at the top end. Moving lettuce from 10 to 14 hours produces a difference you can see across the room. Moving it from 14 to 17 produces a difference you'd need a kitchen scale to find — and it comes bundled with costs. Long days mean more hours of hood heat on the canopy, and warm, fast-grown lettuce develops tipburn (browned leaf edges from calcium not keeping up) and the bitter compounds that precede bolting. The internet is full of owners asking why their kit lettuce tastes like dandelion; the answer is almost always "17-hour default, warm kitchen, harvested too late."

For fruiting crops the ceiling is higher — dwarf tomatoes and peppers genuinely use 16 hours — but there's a hard stop past it. Tomatoes under continuous or near-continuous light develop a well-documented leaf mottling and stall out. Whatever you grow, the schedule that maximizes yield per problem is 14–16 hours, not the maximum the app allows.

Leggy seedlings are a distance problem, not an hours problem#

The other half of the perennial question — "will more hours fix my leggy, stretched seedlings?" — has a mostly-no answer. Legginess means the plant isn't receiving enough light intensity and is stretching to find more. Since light falls off sharply with distance, the fix is almost always geometric: drop the hood to 2–4 inches above the seedlings and raise it as they grow. Adding two hours to a light that's eight inches away treats the symptom at a tenth strength.

The place hours can't help at all is a physical mismatch: edge pods leaning hard toward a center-mounted light, or a tall kit whose mast is maxed out while a second planting germinates in the shade of mature basil. That's the one situation where a piece of gear earns its counter space:

Clip it to the side of the base, aim the goosenecks at the shaded pods, and match its timer roughly to the kit's schedule. It won't replace a kit hood, but as targeted fill light it fixes lean and stretch for less than a round of replacement pods.

Plants need the dark#

It's tempting to see the dark period as wasted time, especially when a kit's glow is the brightest thing in the apartment at 2 a.m. It isn't. Plants spend the dark hours respiring and translocating the day's sugars; some of the physiology of flowering and fruiting is keyed to uninterrupted night. As a floor, give everything 7–8 hours of true dark, and don't fret about brief interruptions — opening the fridge next to the garden at midnight does nothing.

The practical version of this: schedule the light window around your life, not around the sun. A 6 a.m.–8 p.m. window keeps the kit dark while you sleep; if the garden lives in a studio apartment, shift it earlier so "sunset" lands before your bedtime. The plants are indifferent to which 14 hours they get, as long as they get the same ones every day.

For a dumb clip-on or shelf light without a built-in scheduler, a mechanical timer is the entire solution:

The electricity question, retired#

Running a 20W hood 16 hours a day is 0.32 kWh — roughly $1.50 a month at average U.S. rates, less than a single seed pod. Trimming your schedule from 16 hours to 12 to save power saves about forty cents a month. Set the hours for the plants, not the meter.

Set it once#

The whole guide compresses to this: greens and herbs at 12–14 hours, basil and fruiting crops at 14–16, bolters (spinach, cilantro, dill) down at 10–12, everything dark for at least 7 hours, and the hood as close to the canopy as it can get without cooking leaves. Change the default the day you plant, write nothing down, and let the machine do the only thing it's truly good at — the same day, every day, until harvest.