How to Grow Microgreens Without a Grow Light: A Windowsill Method

The Question Everyone Asks First

Almost nobody's first microgreens question is about seed density or growing medium. It's "do I have to buy a light?" — usually asked while looking at a $60 shelf rig and a kitchen that has exactly one sunny window and no spare outlet near it.

The internet gives you two bad answers. One camp says light doesn't really matter for something you harvest in ten days. The other says you're wasting your money without a full-spectrum rig. Both are wrong in the same way: they're answering a yes/no question that actually has a "depends on which crop, which window, and which month" answer.

So here's the useful version. Microgreens are the single most windowsill-viable crop in indoor growing, because you harvest them at the seedling stage — before the plant has spent down its seed reserves and started depending entirely on photosynthesis. That's a real structural advantage over, say, starting tomatoes. But light still matters, and when it's short you get a specific and recognisable set of results: longer, paler, floppier stems, slower runs, and a tray that leans hard toward the glass.

This guide walks the method from dry seed to scissors, using figures from university extension services rather than retailer copy. If you're still deciding whether trays are even the right project for you, sprouts vs microgreens settles that first — sprouts need no light at all. And if you'd rather buy the whole setup in one box, our microgreens kit guide covers what's actually in them and what's filler.

Step 1: Pick Varieties That Tolerate Lower Light

Variety choice does more work here than anything else you'll do. Two different mechanisms make a microgreen forgiving on a dim sill, and it's worth understanding both, because they let you pick well rather than guess.

Large seeds carry their own funding. Peas and sunflower are big seeds with substantial stored food reserves. A large fraction of the growth you harvest was paid for by the seed, not by photosynthesis. That's why they're the classic recommendation for anyone without strong light — you're essentially harvesting the seed's savings account before the plant needs a real income.

Fast crops don't give weak light time to compound. Radish, broccoli, and arugula finish quickly. Legginess accumulates over days, so a crop that's done in eight days simply has fewer days in which to stretch than one that takes three weeks.

Extension guidance backs the low-light tolerance directly. University of Maryland Extension states that "lettuce, arugula, mustards, and Asian greens will grow acceptably well in less-than-ideal levels of light" — a genuinely useful sentence, because it's an extension service saying out loud that some crops tolerate a compromised setup. Utah State University Extension is similarly plain: artificial lights give you the most control, but a southern-exposed window can also work.

Be equally honest about what doesn't work. Basil and cilantro are slow, want a long covered period, and take substantially more days to finish. On a dim windowsill they don't just underperform — they tend to produce a thin, stretched, disappointing tray that isn't worth the seed. Save them for when you have a light.

One non-negotiable regardless of light: buy seed intended for growing, untreated and uncoated. Penn State Extension advises seed that is not treated with chemicals or coated in any way, from suppliers following high food safety standards. Garden-aisle packets are often fungicide-treated and were never meant to be eaten.

Browse untreated microgreen seed on Amazon →

Variety Chart: What Actually Tolerates a Windowsill

Days-to-harvest figures below come from Utah State University Extension and reflect good growing conditions. On a windowsill, add a few days — sometimes several in winter. Where we couldn't find an extension figure for a crop, we've given a qualified range and said so.

General ranges above sit inside the overall 7 to 21 days that University of Maryland Extension gives for microgreens as a category, and are given as ranges precisely because we could not verify a species-specific extension figure for them.

Step 2: Choose the Window — and Assess It Honestly

This is the step where self-deception costs you a tray. "It's a bright room" is not an assessment. Direct sun hours are.

University of Minnesota Extension puts the hierarchy simply: "an unobstructed south-facing window will provide the highest level of natural light for plants." Working down from there, for the northern hemisphere:

• South-facing, unobstructed: your best and often only genuinely good option. This is the window the whole method assumes.
• Southeast or southwest: workable. Expect a slower run and slightly leggier stems.
• East or west: marginal. You'll get a half-day of direct light. Stick to radish, broccoli, arugula, and peas, and accept longer runs.
• North-facing: not viable for growing food. UMN classes north windows as low-light positions. Don't fight it — either use a different window or buy a light.

Now do the honest audit, because the compass direction is only half the story. Sit down and actually watch the sill for a day, or check it three or four times:

1. Count real direct-sun hours. Not "it's light in there" — hours when sun physically falls on the sill. Four is thin. Six is workable. Eight-plus is good.
2. Look for obstructions. The building opposite, a tree, a balcony above, a deep window reveal. All of these cut your hours without changing the compass reading.
3. Check the glass and coverings. Insect screens, heavy net curtains, and grimy glass each take a bite. Clean the glass and open the blinds fully — this is free light.
4. Note the temperature. A sunny sill can bake, and a single-glazed sill in winter can run cold. Microgreens germinate and grow well from 70 to 75°F per UMD Extension, with USU Extension suggesting around 74°F for germination and about 72°F once growing.

Here's the number that frames everything else. UMD Extension's microgreen guidance assumes roughly 16 hours of light each day; USU Extension uses 18 hours on, 6 off; UMN Extension puts seedlings generally at 16 to 18 hours per day. No window on Earth delivers 16 hours of daylight for most of the year, and none delivers it at grow-light intensity through glass. You are, by definition, running a compromised setup. The method below is about compromising well.

Step 3: Set Up the Tray, Medium, and Depth

Nothing here is windowsill-specific, but two details matter more without a light: drainage and fresh medium, because a dim, cool sill is a friendlier environment for mould than a warm shelf under lights.

The trays. You want two: a shallow tray with drainage holes for growing, sitting inside a solid tray with no holes for watering. That pairing is what makes bottom watering possible in step 6, and bottom watering is what keeps the surface — and your greens — dry.

The medium. USU Extension recommends a seed-starting mix, coconut coir, or a hydroponic grow mat. UMD Extension specifies a depth of 1 to 1½ inches of soilless growing media. That's plenty; you're growing a plant for ten days, not a season. Pre-moisten before sowing to what USU nicely describes as the consistency and feel of a wrung-out sponge — damp throughout, not dripping.

Use new mix. University of Minnesota Extension is explicit on this point for damping-off prevention: "Use new potting mix to fill trays. Don't reuse potting mix and don't use garden soil or compost." Reused mix is the single most common self-inflicted mould problem.

Sowing. Microgreens are sown far denser than anything else you grow — you want a solid carpet, not spaced plants. UMD Extension gives two approaches: space seeds "3/16-inch to ¼-inch apart, or about 5 seeds per linear inch," or simply broadcast evenly at a rate of about one tablespoon of seed across the tray. Small seed (radish, broccoli, arugula) goes on dry and gets pressed gently into contact with the surface. Large seed gets soaked first: USU Extension gives about 6 hours for peas and about 12 hours for sunflower, with most other varieties needing no presoak at all.

Don't bury the seed. Surface contact plus a covered blackout period is all it needs.

Browse microgreen trays with drainage on Amazon →

Step 4: The Blackout Period — the Step Everyone Skips

If you take one thing from this guide, take this one. Ironically, the step that makes windowsill microgreens work involves no light at all — and it's the step most first-timers skip, because putting seeds in the dark feels backwards when you're already worried about not having enough light.

After sowing, cover the tray so no light reaches it, and press weight onto the seed for part of that time. UMD Extension explains the mechanism directly: "excluding light for a few days after germination (giving the seedlings a 'blackout period') will cause the stems to lengthen, making harvesting easier." USU Extension's protocol has the medium in direct contact with the seed for roughly half the dark period, with most varieties covered for about 1 to 2 days — longer for slow crops like cilantro.

So what does the blackout actually buy you?

• Even germination. The seed stays uniformly humid under cover, so the whole tray comes up together instead of in patches.
• Stem length you can cut. Deliberate, controlled stretching in the dark gives you enough stem to get scissors under. A tray with no stem is genuinely miserable to harvest cleanly above the medium.
• Hull shedding. Weight on sunflower and peas forces the shoots to push against resistance, which helps them shuck their seed coats. Un-weighted sunflower carries hulls up on its heads and you'll be picking them off one by one.
• Root drive. Pressure and darkness push the seedling to root down before it reaches for light.

Practically: a second tray inverted on top with a couple of tins or a book on it works fine. Sunflower and peas want real weight, a few pounds. Small seed just wants darkness and light contact pressure. Lift the cover once a day to check moisture, mist if the surface has dried, and vent briefly — trapped humidity for days is how mould starts.

Step 5: Move to the Window and Rotate the Tray Daily

Uncover when most of the tray has germinated and the shoots are lifting the cover — usually day 2 or 3, and typically pale yellow at this point. That's normal. Colour arrives within hours of light exposure.

Set the tray as close to the glass as it will sit without touching it. Intensity falls off fast with distance from a window; a tray eighteen inches back on a counter is in a meaningfully dimmer position than the same tray on the sill, even though both look "bright" to your eye. Your eye is a terrible light meter — it adapts, cameras and plants don't.

Then the habit that separates a decent windowsill tray from an embarrassing one: rotate the tray 180° every single day.

This matters far more on a sill than under a light, and it's worth being clear about why. A grow light sits above the canopy — UMD Extension gives 6 to 12 inches above the foliage as typical — so light arrives from directly overhead and every seedling gets a roughly equal share, straight up. A window delivers light from one side, at an angle. The row nearest the glass gets substantially more than the row eight inches back, and every seedling in the tray bends toward the source.

Leave a tray unrotated for a week and you get a visibly wedge-shaped crop: tall, leaning, pale stems at the back, shorter greener ones at the front, the whole mat tipped toward the window like a crowd at a concert. Rotate daily and you average out both problems. It takes two seconds and it's the highest-return habit in this entire method.

A few supporting details while the tray is in the window:

• Give it air. UMD Extension recommends good air circulation to discourage plant diseases — "a gentle breeze from a nearby fan." A cracked window or a small clip fan on low is enough. Still, humid air on a crowded tray is how mould and damping-off get started.
• Watch the heat. A south window in summer can push a shallow tray well past the 70–75°F sweet spot, and hot media dries out fast. Pull it back a few inches on the hottest days.
• Watch the cold. Single-glazed sills in winter can sit far colder than the room. Cold plus wet plus dim is the worst combination for damping-off.

Step 6: Water From the Bottom

Once the tray is uncovered, stop watering from above. UMD Extension's sequence is exactly this: "mist the seeds for the first few days to keep the media surface moist, then water from the bottom (by placing a container or tray into a pan of water)."

The method: lift the drainage tray, set it into the solid tray holding roughly half an inch of water, leave it for a few minutes until the medium has drawn moisture up, then lift it out and pour off whatever's left. Never leave the tray standing in water. You want the guidance UMD gives throughout — "keep the growing media moist but not saturated."

Bottom watering matters more without a light, for a reason that isn't obvious: a dim windowsill dries more slowly than a warm shelf under lights. Less radiant heat, less evaporation. So a wet surface stays wet longer, and wet surfaces are where fungal problems live.

Damping-off: what it is and how to not get it

Damping-off is the disease that kills seedling trays, and it's worth knowing by name because it's fast and total. University of Minnesota Extension describes it as a disease of seedlings that can kill "a large section or an entire tray," caused by the fungi Rhizoctonia and Fusarium species plus the water mould Pythium.

The symptoms UMN lists are unmistakable once you know them: seedlings fail to emerge at all; cotyledons and stems go "water soaked, soft, mushy"; stems become "water soaked and thin, almost thread like" at the base and the seedling topples; and under high humidity you get "fluffy white cobweb-like growth" on the affected parts.

Prevention is entirely cultural, and every measure is something you control:

• New mix every time. Don't reuse potting mix, and don't use garden soil or compost (UMN).
• Clean containers and tools. UMN recommends sterilising used pots and trays by soaking in a 10% household bleach solution for 30 minutes, and cleaning any tools used in planting and maintenance.
• Good drainage. A mix that drains, in a tray with holes. This is why the two-tray setup exists.
• Don't overwater. Moist, never saturated. Bottom watering, poured off.
• Air circulation. A gentle fan or open window (UMD).
• Reasonable warmth. UMN suggests clean warm water at 68–77°F and soil around 70–75°F. A freezing sill works against you.

Step 7: Read the Signs of Insufficient Light

This is the diagnostic section, and it's the honest heart of the article. Your tray will tell you clearly whether the window is enough. You just need to know what it's saying — and to check while there's still time to act rather than at harvest.

University of Minnesota Extension describes the pattern with useful precision. Plant stems become "leggy," meaning "stems become long and thin." Plants don't produce enough chlorophyll and can "turn pale green to yellow to white." And "a lack of sufficient light causes the plant to grow long spaces on stems between the leaf nodes."

Translated into what you'll actually see in a microgreen tray:

Long, thin, floppy stems

The classic sign. The stems are noticeably taller than the variety should be, thin enough that the tray can't hold itself up, and the whole mat collapses sideways when you look at it. Some stem length is what you wanted from the blackout period — this is different: it keeps going after the tray is in the light, and the stems get thinner rather than thicker as they lengthen.

Pale colour

Healthy microgreens colour up within a day or two of coming out of blackout. If your tray is still yellowish-green after three days in the window, or the green is washed-out rather than deep, it isn't making enough chlorophyll.

Small cotyledons

The seed leaves stay small and stay closed rather than opening out into fat, well-formed leaves. The plant is spending its budget on stem — reaching for light — instead of leaf.

Everything leaning one way

Universal on a windowsill to some degree, since light comes from one side. But if the tray is at a serious angle after a day, or if it's leaning despite daily rotation, you're seeing a strong phototropic response — a plant working hard to find light it isn't getting.

Slow growth

The most easily-missed sign, because there's no dramatic moment. Your radish is at day 14 and looks like it should at day 8. Nothing looks wrong, exactly. But a run consistently running 30–50% longer than the extension figures is a light problem, and it also means more days of exposure to mould and damping-off before harvest.

What to do about it

Work through these in order — the first four are free:

1. Move the tray right up to the glass. Inches genuinely matter.
2. Clean the window and open the blinds completely. Free light you're currently throwing away.
3. Rotate daily without fail. Fixes the leaning half of the problem outright.
4. Add a reflector. White card or foil on the room side of the tray bounces light back onto the shaded flank. Genuinely helps on a one-sided light source.
5. Switch to a faster or larger-seeded variety. Radish, broccoli, peas, sunflower. Drop the slow crops until you have a light.
6. Harvest earlier. Microgreens are edible from cotyledon stage. A slightly smaller tray beats a collapsed one.
7. Sow less densely. Crowding makes seedlings compete and stretch, and reduces airflow. Ease off the seed a little.
8. Accept the result, or buy a light. Covered in step 9.

Seasonal Honesty: Winter Changes the Answer

Everything above assumes a reasonable amount of daylight. From roughly March to October at mid-latitudes, a good south window is a legitimate microgreen setup. Winter is a different conversation, and pretending otherwise does nobody any favours.

Three things stack against you simultaneously in winter, and it's the combination rather than any one of them that bites:

• Day length collapses. Extension guidance assumes microgreens get around 16 hours of light daily (UMD), or 18 (USU), with UMN putting seedlings at 16 to 18. A December day at 45°N gives you roughly nine hours of daylight — before you subtract for the hours the sun isn't actually on your window.
• Intensity collapses too. A low winter sun travels through more atmosphere and strikes your glass at a shallow angle, so even the hours you get are weaker hours. Overcast winter skies cut it further.
• Sills get cold. Microgreens want 70–75°F, and a winter windowsill — especially single-glazed — can run far below the room temperature. Cold slows growth, which extends the run, which extends the window in which a damp tray can develop damping-off.

So what does that mean practically? At southern US latitudes, winter windowsill growing is generally fine — slower, but fine. Across the northern US, Canada, the UK and northern Europe, roughly November through February, a windowsill alone is a genuinely marginal proposition. You can still get pea shoots and sunflower, because their seed reserves carry them regardless. Radish and broccoli will produce something. Everything else struggles.

The honest framing is this: at higher latitudes in winter, a grow light isn't an upgrade — it's the only practical route. If year-round microgreens are the goal rather than a summer hobby, that's the season that makes the purchase decision for you. Our apartment grow light guide covers what you actually need, which is far less than most people assume.

Step 8: Harvest With Scissors Above the Medium

Timing is a window, not a deadline. UMD Extension's cue is to cut "when the true leaves have expanded" — in practice, most people harvest when the cotyledons (seed leaves) are fully open and the first true leaves are just showing. Penn State Extension describes the edible portion as single shoots with the cotyledons and/or first true leaves.

On a windowsill, watch the tray rather than the calendar. Extension day-counts assume proper light; yours will likely run a few days longer. And if a tray is stretching badly, harvest early — legginess only gets worse, and a small crisp tray beats a tall collapsed one.

The cut. UMD Extension is specific: "cut the stems off just above the top of the media (the 'soil line') when the true leaves have expanded. Use clean, sharp scissors or a knife." Gather a handful of stems gently with one hand, hold them slightly taut, and cut in a straight sweep just above the surface.

Cutting above the medium isn't only about tidiness. It keeps growing medium — and whatever lives in it — out of your food, and it's the structural reason microgreens don't carry the same raw-consumption risk profile as sprouts. You're harvesting the plant, not the root mass and the germination environment together.

One tray, one harvest. Most microgreens don't regrow usefully after cutting; peas will sometimes give a modest second flush, but the honest expectation is a single cut. Compost the mat, wash the tray, start again.

Washing and storage

Rinse and dry with real intent. UMD Extension's guidance is to "rinse harvested microgreens with water, pat dry, and use right away or refrigerate for up to 10 days." That 10-day figure assumes properly dried greens — microgreens stored damp turn slimy in a day or two, exactly like sprouts.

Pat between clean towels or spin very gently in a salad spinner (they're delicate — go light). Store in the fridge in a loosely covered container over a dry paper towel. Fresh microgreens smell green and clean; anything sour or musty gets binned.

A brief word on food safety

Microgreens sit in a meaningfully better position than raw sprouts, and it's worth being clear why rather than just asserting it. Sprouts are grown in warm, humid, enclosed conditions and eaten whole — seed, root, and all. Microgreens are cut above the medium, so the seed and root stay behind, and they're grown in an open tray with airflow rather than a sealed humid jar. That's a real structural difference, and it's why the strong FDA cautions around raw sprouts don't transfer wholesale. We cover the contrast in more detail in sprouts vs microgreens.

That said, this is still a fresh raw green grown in your kitchen, so the basics still apply: buy untreated seed from a supplier with real food safety standards (Penn State Extension), start with clean trays and tools and new medium, use clean scissors, wash your hands, rinse the greens before eating, and refrigerate promptly. None of that is onerous — it's the same care you'd give a bag of salad leaves.

Step 9: When to Give In and Buy a Light

We've spent this whole guide arguing that you can skip the light. Here's the other side, plainly, because the goal is a useful answer rather than a reassuring one.

Buy a light when:

• You want to grow through winter at a northern latitude. The single strongest case. Nine hours of weak, low-angle daylight against a 16-hour target is a gap technique can't close.
• Your best window is east, west, or north. If a good south-facing sill simply isn't available, a light isn't a luxury — it's the enabling piece of equipment.
• Trays keep coming out leggy despite doing everything right. If you've run a proper blackout, kept the tray against clean glass, and rotated daily, and it's still stretching and pale, you've exhausted the free fixes. The window is the constraint.
• You want to grow anywhere other than the windowsill. A light frees you from one sill and one orientation. Suddenly a shelf, a cupboard top, or a corner of the counter becomes growing space, and you can run several trays instead of one.
• You want the slow, light-hungry crops. Basil, cilantro, and strongly-coloured varieties like amaranth are simply better under a light.
• You want consistency. A light removes weather from the equation. Same result in February as in June, which matters if you're growing to actually eat rather than to enjoy the process.

The good news is that microgreens are the cheapest possible crop to light. They're short, so you don't need clearance; they occupy a small footprint; and UMD Extension's positioning guidance — 6 to 12 inches above the foliage, around 16 hours a day on a timer — is modest. A simple LED strip or small clip light over one tray is a genuinely small purchase, not a growing-tent project. Our grow light guide covers what's worth buying and what's marketing.

And it's worth saying: starting on the windowsill first is a perfectly sensible order of operations. Run two or three trays, see whether you actually keep it up, and let the results tell you whether a light is worth it. Plenty of people find the sill is enough for what they want. If you'd rather start with everything in one box, our microgreens kit guide compares what's included — and if you find yourself wanting to move on to full-size plants, seed starting kits is where that road goes next.

Frequently Asked Questions

Sources

Growing figures in this guide — days to harvest, blackout period, light hours, medium depth, temperature, watering method, damping-off prevention, harvest technique, and storage — were verified against the following university extension publications.

• University of Maryland Extension — Growing Microgreens and Baby Greens Indoors (7–21 days to harvest; blackout period and its purpose; ~16 hours light and 6–12 in above foliage; 1–1½ in media depth; seeding rate; mist then bottom water; 70–75°F; air circulation; cutting above the soil line; refrigerate up to 10 days; low-light tolerance of arugula, lettuce, mustards and Asian greens)
• Utah State University Extension — Grow Your Own Microgreens (days to harvest by variety: pea 8–10, sunflower 9–10, radish 8–12, broccoli 8–12, arugula 6–8, kale 8–12; presoak times; 1–2 day covered period for most varieties; 18 hours light / 6 dark; germination 74°F, lights-on 72°F; medium options and wrung-out-sponge moisture; southern-exposed window as a workable alternative to artificial light)
• University of Minnesota Extension — Lighting for Indoor Plants and Starting Seeds (unobstructed south-facing window as highest natural light; north windows as low-light; seedlings 16–18 hours per day; symptoms of insufficient light — leggy long thin stems, pale green to yellow to white colour, long spaces between leaf nodes)
• University of Minnesota Extension — How to Prevent Seedling Damping Off (pathogens Rhizoctonia, Fusarium, Pythium; symptoms; new potting mix only; 10% bleach soak for 30 minutes to sterilise used trays; clean tools; good drainage; warm water 68–77°F and soil 70–75°F)
• Penn State Extension — The ABCs of Microgreens (7–21 days from germination; harvest by cutting at the base right above the growing medium; untreated, uncoated seed from suppliers with high food safety standards)