Growing Microgreens in Winter: How to Handle Cold Weather

The first time a real cold snap hit our mobile grow trailer, the thermostat said 72°F and our sunflower trays germinated poorly anyway. The space was actually sitting closer to 67°F, and that gap showed up in the trays. Cold weather does that to microgreens. Seeds take longer to germinate, seedlings take longer to grow, and the medium takes longer to dry out, so a watering schedule that worked in September can quietly over water a tray in December. Here is how we handle temperature, humidity, windows, heat mats, and grow tents once fall turns to winter, based on growing microgreens since 2018 in a custom built insulated trailer in North Texas and in a normal house.

Quick Answer: What Cold Weather Does to Microgreens

Microgreens grow best with air around 65 to 75°F and a medium near 75°F during germination. Below that range, everything slows down.

• Germination slows, and the seeds that struggle first are large seeds like sunflower and small, slow seeds like amaranth and celosia.
• Growth slows, so a grow that takes about two weeks in warm weather can stretch to three or four weeks in a cold room.
• Trays dry out more slowly, so the same watering schedule over-waters faster, and over-watering invites mold, damping off, and fungus gnats.
• Cold glass and cold walls can damage seedlings that sit too close to them.
• A heat mat with a thermostat, steady airflow, a dehumidifier, and a space heater or a small enclosed space like a grow tent can bring a cold room back into range.

Why Cold Weather Changes Everything

Slower Germination

Seeds need moisture, oxygen, and warmth to wake up, and warmth is the first thing to go in a cool room. Most microgreen seeds germinate best with the medium around 75°F. Well below that, germination gets slow and uneven, with some seeds up early and others lagging behind. Sunflower and slow small seeds like amaranth and celosia show it first.

Air temperature and tray temperature are not the same number. A wet medium sits cooler than the air around it, especially overnight. We saw this in our trailer. On our first 30°F nights the thermostat said 72°F, but the space averaged closer to 67°F, and our sunflower germination was poor until we added a floor space heater. Now we check the temperature at the tray instead of trusting the thermostat.

Old seed and low temperatures are two of the most common causes of slow, uneven germination, so winter is a good time to start with fresh seed. Our microgreen seed quality guide covers what to look for, and our microgreen seeds page covers where we buy, including True Leaf Market.

Slower Growth

Once seeds are up, cold keeps stretching the clock. A crop you can plan to the day in warm weather becomes a guess in a cool room, and the slower crops can slip well past your usual harvest date.

We have not run a full year long trial of this ourselves, so we lean on the best winter data set we have found. Main Street Farms in New York seeded microgreens every week for a year in a minimally heated greenhouse with no supplemental lighting, and reported it to Northeast SARE. In their final report, trays averaged about 14 days to harvest from March through August, about 25 days in December, and about 32 days in January. They also found that once greenhouse air stayed below 60°F without added heat, trays stopped being dependable. Harvests either failed or took three to four weeks. Light dropped over the same months, so cold was not the only factor, but it matches what we see: cool rooms add days.

Short days make it worse, which is why grow lights matter more in winter. Our microgreen lighting guide covers the 16 to 17 hour schedule and 8 to 12 inch light height we use. Whatever your usual timeline is, plan on the long end of it, or past it, for anything growing in a cool room.

Slower Drying, Faster Over-watering

This is the one that catches growers off guard. Water leaves a tray through evaporation and through the plants themselves, and both slow down in a cool room. If you water on the same schedule you used in September, the medium stays wet longer. A wet, cool, still tray is exactly what mold, damping off, and fungus gnats are looking for. When we troubleshoot a winter tray, over-watering, poor airflow, and cool temperatures are the first three things we check.

Here is what we do when the weather turns:

• Check the bottom reservoir before every watering. If water is still there, skip that watering and check again later.
• Water in smaller amounts and let the tray tell you how much it drinks. A 10x20 tray can take up to about 2 cups per watering at peak growth in warm conditions, and a cold tray will not get there.
• Keep leaves dry by bottom watering. Wet leaves and stems invite disease, and cold air does not dry them off.
• Keep air moving across the trays so the surface can dry between waterings.
• Treat fungus gnats as an early warning that the surface is staying too wet.

Our complete watering guide walks through bottom watering and volumes. If you are already seeing fuzz or collapsing seedlings, start with our guides to microgreens and mold, damping off, and fungus gnats.

What Temperature Should You Keep Your Grow Space?

These are the numbers we aim for:

• Air temperature: 65 to 75°F. Our winter target in the trailer was 67 to 75°F.
• Medium temperature during germination: about 75°F.
• Below 60°F: you will feel the slowdown, and germination and growth both get unpredictable.
• Around 50°F: most crops survive, but they crawl.
• Above 75°F: not a winter problem, but it is where disease pressure and uneven germination start to climb.

Seed suppliers and universities land in the same neighborhood. Johnny's Selected Seeds recommends air at 65 to 75°F and a medium near 75°F until germination, and the Penn State Extension microgreens guide gives 60 to 70°F for the air. If you see slightly different numbers in different places, that is why. The ranges overlap far more than they disagree.

Measure the Tray, Not Just the Room

Put a thermometer at tray height, not on the far wall. Check it at night too, since that is when a house cools off and when a space heater earns its keep. If you use a heat mat, measure the medium itself with a probe, because that is the number the seeds care about.

What Humidity Should You Aim For?

In the trailer we aim for about 45 to 50% humidity. At home, anywhere up to about 55% is workable as long as the air is moving. High humidity favors mold and damping off, and winter makes it worse because closed windows and cool air hold moisture in the room.

Here is how we handle moisture and airflow:

• Dehumidify the room, not just the tray. In the trailer, a smaller 30 pint dehumidifier with a continuous drain into the sink beat a larger 70 pint unit, because it never filled up and shut off mid cycle. At home we keep a dehumidifier in the grow room.
• Keep air moving. In the trailer we have one fan per shelf on high, pointed toward the back wall, plus a central fan pointed the other way to create a room wide vortex so air never stagnates. At home we use a fan in the center of the room.
• Use more than one hygrometer. We once had two units disagree by 20% on humidity, and bad data leads to bad decisions.
• Remember that heating dries the air. A heater lowers humidity, which helps with mold, but it also dries tray surfaces faster, so keep checking the reservoir.

The dehumidifier and space heater we recommend are on our Amazon environment control list.

Keep Microgreens Away From Cold Windows

Natural light is tempting in winter, but a window works against you in two ways. Winter daylight is weak, and the glass itself gets cold. On a freezing night the glass can sit far colder than the room, so leaves that touch it can pick up frost damage. Seedlings also lean toward light, which means a tray set right at the glass can have leaves pressed against it by morning.

The same goes for cold exterior walls. Cold transfers through glass and walls into the air next to them, and that changes how your crops grow even when nothing is touching. Our rule is to keep racks about 1 to 2 inches away from walls and windows. Here is a simple test: put the back of your hand near the wall or glass. If you can feel the cold, so can your microgreens.

• Pull trays back from the glass on cold nights, or move them off the sill entirely.
• Leave a gap between the leaves and the window, and check for leaning.
• Hang a thick curtain between the trays and the glass. In our home grow room we have thick curtains over the window.
• Add grow lights so you are not depending on a cold window for light. Our grow lights and setup page pulls our lighting guides together.

How to Use a Seedling Heat Mat With a Thermostat

A heat mat warms the root zone instead of the whole room. Seedling mats typically warm the medium roughly 10 to 20°F above the air, which makes them the easiest way to get a cool tray up to germination temperature. Here is how to use one properly:

1. Choose a mat made for seed starting and pair it with a thermostat controller that has a soil probe. Look for waterproof, UL listed equipment, and plug it into a GFCI protected outlet, since water and electricity share the shelf.
2. Set the mat on an insulated surface. Bare concrete, cement, and steel pull heat out of the mat, so put a wood or foam board under it if your shelf is metal.
3. Place the seeded tray on the mat. Your normal blackout stack, with the flipped tray or dome and any weight, goes on top of the tray as usual.
4. Push the probe into the medium of a seeded tray, not between the mat and the tray. You want to control the medium temperature, not the mat surface.
5. Set the thermostat to about 75°F. Give it up to a day to level off, then check the medium with a separate thermometer to confirm the controller is reading true.
6. Check moisture daily. A warm mat dries the bottom of the tray faster, so keep checking the reservoir and mist the surface if it looks dry.
7. Take the tray off the mat once the seedlings have germinated and the tray moves into the light. Once seeds are up, they no longer need high medium temperatures.

What a Heat Mat Will and Will Not Do

A mat solves a cold tray, not a cold room. In our experience, warmth matters most for sunflower and for slow small seeds. When our trailer sat at about 67°F, a space heater fixed our sunflower germination. For amaranth, celosia, and red veined sorrel, a heat mat at 75°F is how we get them going. A mat is a good tool for those crops. It is not a substitute for a room that is warm enough.

Keep your expectations realistic, too. Main Street Farms used heat pads in their winter trial and saw a boost in the first few days after seeding, but found the overall benefit hard to measure. Their controller was unreliable, and pads sometimes overheated trays so young plants dried out. That is the reason for the probe in the medium and the daily moisture check.

How We Control Our Space: Trailer vs. Home

In Our Mobile Grow Trailer

Our mobile grow trailer is an 8.5ft x 20ft insulated cargo trailer in Texas zone 8A, and controlling the environment is what lets us grow microgreens all year. Our winter target is 67 to 75°F. Here is what we use:

• An AC and heat unit on a thermostat. Heating is not what it does best, so we treat it as a last resort. On our first 30°F nights it could not keep up, and the space averaged about 67°F even though the thermostat said 72°F.
• A small electric space heater with its own thermostat. We call ours Ol' Trusty. It is a 1200 watt model that heats the whole trailer quickly and then shuts off, because the insulation holds the heat. It got us through two winters.
• One fan per shelf plus a central fan, so air never stagnates.
• A dehumidifier draining straight into the sink, aiming for 45 to 50% humidity.
• Multiple temperature and humidity readers in different spots.

Then came a week in early February 2021 when Texas temperatures floated between 0°F and 30°F. We ran two 1300 watt heaters around the clock to keep the trailer from freezing, and some of our water lines still froze because a cabinet door was closed and warm air could not reach them. Leaving cabinet doors open fixed it for the moment, and venting the doors is the better long term fix. The uninsulated metal wheel wells also collect condensation in winter when the heater is on. After that week we planned better insulation, since that is where the trailer was most prone to failing. We call that failing forward.

At Home

At home it is a different story. We have a normal house thermostat, so the room is whatever temperature the house is. In the room we have a fan in the center, a dehumidifier, and thick curtains over the window.

If your setup looks like ours at home, here is what that means in practice. You control moisture and airflow, but not temperature, so your levers are germination warmth with a heat mat, crop choice, and careful watering. A space heater on its own thermostat is the next step up if your room runs cold.

We broke down both setups, and what went wrong before we fixed them, in this video on how temperature ruined our microgreens and how we fixed our grow space climate control. We share setups like this on our YouTube channel, which has 152,000+ subscribers, and you can find more on our Video Library.

What Does It Cost to Keep a Grow Space Warm?

Control is great, but it comes with an electric bill, and winter is when you feel it. Here is what we have measured:

• On an average day, heating or cooling our insulated 8.5ft x 20ft trailer ran about $1.00 to $4.00. Our floor space heater in winter was about $1.80 per day, tracked with a Kill A Watt meter.
• During that 0°F to 30°F week, two 1300 watt heaters pulled 62.4 kWh every 24 hours. At the $0.12 per kWh we paid then, that was about $7.50 per day. At roughly $0.14 per kWh, which is closer to the Texas average today, it would be about $9 per day.
• In our guest bedroom, two racks with five shelves each use about 1 kWh per shelf per day with the lights on for 17 hours and off for 7. Each shelf has three 20 watt LEDs and a 120mm fan. That is about 10 kWh per day for the whole setup, or about $1.20 per day at $0.12 per kWh.

To estimate your own cost, multiply watts by hours of use, divide by 1000, and multiply by your rate per kWh. A 1500 watt heater running nonstop for 24 hours uses 36 kWh, so a heater that cycles on and off with a thermostat costs far less than one running all day. Your electric rate will differ from ours, so use your own number.

The biggest savings come from keeping the heat you pay for. Insulate, seal drafts, keep trays off cold walls, and heat a smaller space. That last one is the reason we like grow tents, which we cover below.

Choosing and Using a Space Heater

If your room runs cold, a space heater is the simplest fix. This is what we have learned from using one for winters in the trailer:

• Pick a heater with a built in thermostat. Ol' Trusty starts heating the moment the room drops below its setting and shuts off once it catches up, which keeps the temperature steady and the power bill down.
• Find out what it does after a power outage. Ours does not turn itself back on. You have to press the on button, so a power cut can leave the grow space cold until you notice.
• Look for tip over shutoff, overheat protection, and the seal of an independent testing laboratory.
• Plug it straight into a wall outlet, not an extension cord or power strip. Most household heaters draw up to 1500 watts, so give it a circuit that is not also feeding other big loads.
• Keep anything that can burn well clear of it. Fire safety guidance, including from the National Fire Protection Association, is to keep at least three feet between a portable heater and anything flammable, which includes curtains, cords, and cardboard.

Fire safety groups also advise turning portable heaters off when you leave the room. A grow space that needs steady heat is a harder fit for that advice, so follow the manufacturer's instructions, and if you need constant heat, consider insulating better or asking an electrician about a more permanent option.

Grow Tents: A Smaller Space Is Easier to Control

The trailer taught us that the smaller and more enclosed the space, the easier it is to hold a temperature. A grow tent applies the same idea to a corner of a room. Instead of heating and dehumidifying a whole room, you are managing a few cubic feet. We have grown trays in grow tents too, and these are the benefits we notice:

• A small air volume is easier to warm with a small heater on a thermostat, and it costs less to keep warm.
• Reflective walls bounce light back onto the trays instead of letting it escape into the room.
• The tent buffers trays from the swings of the rest of the house, including cold nights and drafts.
• It keeps lights, humidity, and cords in one contained area.

The trade-offs are worth knowing. Lights add heat in a small space, which can help in winter and hurt in summer. In the trailer we use only LEDs because high output fluorescent lights would push the space past 100°F within hours, and the same logic applies inside a tent. Do not count on LED lights alone to warm a tent. Humidity can also build in a sealed tent, so keep a fan moving and the tent ventilated. Heaters dry the air inside a tent, so watch a hygrometer.

Our Amazon grow tents list is a good place to start. If you would rather build a shelf, our grow rack build guide walks through it.

Choose Crops That Handle Cold and Plan for Slower Grows

When the room is cool, pick crops that cope with it. Radish is our fastest and most forgiving crop at 6 to 10 days, and broccoli at 8 to 11 days is another reliable one. The Main Street Farms trial also found that mizuna, red Russian kale, red cabbage, and kohlrabi gave the most consistent yields in cooler temperatures, while amaranth was not dependable once temperatures dropped below about 65 to 70°F.

• Fast brassicas make a good winter base. Our guides to radish, broccoli, and red cabbage are good places to begin.
• Large seeds like pea shoots and sunflower are worth growing, but sunflower is the seed that showed us the cost of a cold room, so give it warmth to germinate.
• Save slow crops like cilantro, basil, amaranth, and celosia for a warmed space or a heat mat. Most of them already take 14 to 21 days in good conditions.

Plan your seeding around the slower pace, too. Time your next tray to when the last one will actually be harvested, not to what the calendar said last summer, and space your plantings further apart than you would in warm weather so you do not end up with a gap or a glut.

This is where tracking helps. If you keep your grows in Microgreen Tracker, a responsive web app that installs to your home screen, stage timing is set per crop and per variety and can be overridden for a single tray. A crop that normally harvests in 8 days can carry its slower winter schedule without throwing off your task list. You can also override waterings per day for a tray, and skipping a watering slot records a dismissal instead of a fake watering, which fits the check the reservoir first habit. Log winter notes on the tray so you remember which room and which week a grow ran slow. We have a 7-Day Free Trial Plan.

What About Greenhouses in Winter?

A greenhouse sounds like the obvious answer to winter, and it solves one problem: free light. It does not solve the other two. A greenhouse loses heat fast on cold nights, and closed winter greenhouses tend to run humid. We have even talked about adding large windows to our trailer to save on lighting, but we would still need to solve heating and cooling.

The farm research we found backs that up, though these are trials in different climates with different crops, so treat them as a guide:

• The Main Street Farms trial in New York showed winter microgreens could break even or turn a small profit with the right varieties, but winter growing needed heating pads, a limited mix of cold hardy crops, and sometimes supplemental lighting.
• The University of Minnesota's Deep Winter Greenhouse project, which uses passive solar heat, logged average temperatures around 53 to 56°F from November through February, with humidity averaging in the high 80s. That is well outside the range we want for microgreens, and it is why moisture problems show up in closed winter greenhouses.
• Fat Peach Farm in New Hampshire took a different route. It converted an 8 by 7 foot cold storage room into a grow space with lights, fans, and a ceramic heater on a temperature controller set around 75°F, described in this UNH Cooperative Extension guide. It is the same idea as our trailer: a small, insulated, controlled space.

The common thread is that winter success comes down to heat, light, airflow, and the right crops, whether you grow in a greenhouse, a cold storage room, a trailer, or a spare bedroom.

Our Winter Checklist for Microgreens

• Aim for 65 to 75°F air and about 75°F in the medium during germination.
• Measure at tray height, day and night, with more than one thermometer or hygrometer.
• Use a heat mat with a thermostat probe in the medium for slow or large seeds, and take the tray off once it germinates.
• Keep racks 1 to 2 inches away from cold walls and windows, and use thick curtains over windows.
• Check the bottom reservoir before every watering and skip it if water is still there.
• Keep air moving and use a dehumidifier to hold humidity in the mid range.
• If you use a space heater, choose one with a thermostat and tip over shutoff, plug it into the wall, and keep it clear of anything that can burn.
• Add grow lights, since winter daylight is not enough on its own.
• Choose cold tolerant crops and stretch the gap between plantings.
• Use fresh seed.

Frequently Asked Questions About Growing Microgreens in Winter

What temperature is too cold for microgreens?

Microgreens can survive down to about 50°F, but growth slows to a crawl, and below about 60°F germination and growth both become unpredictable. For trays you want to harvest on a schedule, we aim for the mid 60s to mid 70s.

Do microgreens need a heat mat in winter?

Not always. A heat mat helps most with germination, especially for sunflower and slow small seeds like amaranth and celosia. A mat does not warm the room, so think of it as a germination tool. Use a thermostat with a probe in the medium and take the tray off the mat once it germinates.

How long do microgreens take to grow in winter?

It depends on your room. In a minimally heated greenhouse, one New York farm saw averages of about 25 days in December and 32 days in January, compared with about 14 days in spring and summer. In a room held in the mid 60s to mid 70s, you should stay much closer to your normal timeline.

Can microgreens freeze on a windowsill?

Yes, they can be damaged by cold glass. Leaves pressed against a freezing window can pick up frost damage, and seedlings lean toward the light. Keep trays 1 to 2 inches back from the glass, pull them away on cold nights, or hang a thick curtain between the trays and the window.

What humidity should I keep my microgreen grow space at?

In our trailer we aim for about 45 to 50%, and up to about 55% works at home if the air is moving. High humidity favors mold and damping off, so pair a dehumidifier with steady airflow, and use more than one hygrometer to check your readings.

Why do my microgreens get moldy more often in winter?

Cold slows evaporation, so trays stay wet longer, and closed windows and still air let humidity build. Check the reservoir before watering, keep air moving, and see our microgreen troubleshooting hub and our free troubleshooting PDF for help telling mold from root hairs.

How much does it cost to heat a microgreen grow space?

It depends on the size of the space, how well it is insulated, and your electric rate. For our insulated 8.5ft x 20ft trailer, heating or cooling usually ran about $1.00 to $4.00 per day, and a single floor space heater in winter was about $1.80 per day. A small space like a grow tent costs far less to keep warm.

Can I grow microgreens in a garage or unheated room?

It is hard to do reliably. Without added heat, trays in the New York trial took three to four weeks or failed once the air stayed under 60°F. If you want to try, choose cold tolerant crops, add a heat mat for germination, and consider a small enclosed space with a thermostat controlled heater, such as a grow tent.

Final Thoughts

Winter does not have to end your microgreens. It changes the rules: seeds wake more slowly, seedlings take more time, and trays stay wet longer, so you adjust your temperature, humidity, watering, and crop choices to match. Start by measuring what your room actually does, then add one tool at a time, whether that is a heat mat, a space heater, a dehumidifier, or a grow tent. If you track your grows, Try Microgreen Tracker Free for 7 Days and see how your winter trays compare with the rest of the year.

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The results and opinions shared in this post are based on our own firsthand experience in our specific grow spaces. Electricity costs reflect the rates and equipment we had at the time, and your rates, climate, water, seed, and setup are different from ours, so your results can vary. Farm and university findings are reported as published and come from different climates and crops. We always encourage you to run your own experiments and see what works best for you.

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Happy growing!