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Hydroponics in winter has one big advantage over a garden outside: the plants live indoors, where nothing freezes. But an apartment in January is not the same room it was in July. The glass by the window turns cold at night, the days are short, the heating dries the air, and the water coming out of the tap is colder than roots would like. None of these will ruin a healthy setup on its own, yet together they explain most of the slow, sulky growth people notice once the heating comes on.
This guide takes each winter problem in turn, with the temperatures, light hours and humidity this site uses for every crop. It also covers the crops that actually enjoy a cool apartment, because some of them do. If you are new to growing without soil, the complete guide to apartment hydroponics covers the basics first.
Can You Do Hydroponics in Winter?
Yes, and an apartment is one of the easiest places to do it. Indoor hydroponics doesn’t care what the weather is doing outside, only what is happening within a foot or two of the plant. In winter, four things change there:
- Cold glass. A windowsill that was ideal in summer can chill jars overnight.
- Short days. Window light alone falls well short of what leafy crops need.
- Dry heated air. Radiators and vents pull humidity down and blow hot, dry air across leaves.
- Cold water. Winter tap water is cold, so a water change straight from the tap chills the roots.
Fix those four and most herbs and greens keep growing through the winter, while a few crops grow better than they did in summer.
Key takeaways
- Keep the root zone at 65 to 75°F (18 to 24°C) for most crops, even when the room is cooler.
- Move jars back from cold glass at night, and keep them out of drafts and the direct blast of heating vents.
- Run grow lights 14 hours on and 10 off on a timer, since winter daylight falls short; spinach runs 12.
- Humidity of 50 to 70 percent is ideal, but 40 is survivable in a dry winter apartment.
- Let water for a water change reach room temperature before it touches the roots.
- Cool-season crops such as spinach, arugula and bok choy are happy in a cool room.
How Cold Is Too Cold for Hydroponics?
There is no single number, because crops differ. Two temperatures matter: the water the roots sit in, called the root zone, and the air around the leaves. These are the ranges this site uses:
| Crops | Air | Root zone |
|---|---|---|
| Basil, mint, oregano, thyme, parsley, dill, chives | 65 to 75°F | 65 to 75°F |
| Lettuce | 65 to 75°F by day, nights to about 60°F | 65 to 75°F |
| Spinach, arugula, watercress, bok choy, cilantro | 60 to 70°F | 65 to 75°F (spinach cooler) |
| Tomatoes and peppers | 70 to 80°F | 65 to 75°F |
In Celsius, 60°F is about 15°C, 65 to 75°F is 18 to 24°C, and 70 to 80°F is 21 to 27°C. Spinach is the one crop that wants its water cooler, at 59 to 68°F (15 to 20°C), because warm water feeds the root disease that troubles it.
Below those ranges, growth slows rather than stops. Purdue showed how much it slows: butterhead lettuce grown in cool air, 60°F by day and 55°F at night, grew more than two thirds less than lettuce kept at 70°F by day and 65°F at night. So a cold room rarely kills a plant, but it can stall it for weeks. Basil is a warm-season herb, which is why it belongs on the warmest shelf you have.
Keep Jars Off Cold Glass
In the northern hemisphere, a south-facing window gets the most winter sun, which makes its sill tempting. But glass loses heat fast once the sun goes down, and a jar pressed against it sits in much colder air than the middle of the room. Because the jar is mostly water, it also holds that chill well into the morning.
- Pull jars back at night. Even a few inches from the glass helps, and a shelf beside the window is better still.
- Close the curtain on the window side of the plants, so they stay in the room rather than trapped between the curtain and the cold pane.
- Put something under the jars on a stone or tiled sill, such as a cork mat or a folded towel, so the sill doesn’t draw heat out of the water.
- Measure where the plants live. A small thermometer and hygrometer on the shelf shows both temperature and humidity at leaf level, which is often not what the hallway thermostat says.
Kratky jars need the same care. A Kratky jar has no pump, so steady conditions matter more: keep it shaded from strong sun by day and away from cold glass at night. The Kratky jar setup guide covers the rest, and common Kratky mistakes lists what else slows a jar down.
Drafts and Heating Vents
Two kinds of moving air cause trouble in winter. Cold drafts from a leaky window or an outside door chill leaves and water alike. Meanwhile, the hot, dry blast from a radiator or a forced-air vent dries leaves, warms one side of a reservoir, and swings the temperature up and down as the heating cycles on and off. So keep plants out of the direct path of both.
A gentle fan is still worth running, though, because still air invites mildew, and closed winter windows mean very still air. The clip-on fan guide covers small, quiet ones, and powdery mildew on hydroponic basil shows what stale air can lead to.
Making Up for Short Winter Days
Winter is when a grow light earns its keep. Even a bright window gives far less light in December than in June, because the days are shorter and the sun sits lower. Leafy crops respond by stretching toward the glass, with thin, pale stems and smaller leaves, and seedlings stretch the most; why seedlings stay small under grow lights explains the light side of that.
The fix is a light on a timer, run at the site’s standard schedule of 14 hours on and 10 hours off for herbs and greens. Spinach is the exception: it is a long-day plant that bolts when days run long, so it runs 12 hours. Hang the light 6 to 8 inches above leafy herbs and greens, or 10 to 12 inches above seedlings before their true leaves. The grow light distance guide has the other crops.
Using the Window and the Light Together
You don’t have to choose between the window and a light. Keep the plants near the window for whatever daylight there is, and let the light make up the rest, so the plants get the same 14 hours every day; combining window light with a grow light explains how. A compact timer does the switching, which matters more in winter, when it is already dark by the time you get home.
If your windows face north, grow lights for a north-facing apartment covers the darkest case. The grow light calculator sizes a light for your shelf, while the best grow lights for low-light apartments compares budget lights that do the job. There is also a small winter bonus: LEDs still give off some heat, as grow light heat in apartments explains, so the canopy under a light runs a little warmer while it is on.
Dry Heated Air
Heating dries indoor air, and in a cold climate the humidity in an apartment can fall a long way. The ideal for most crops is 50 to 70 percent. Even so, plants manage at 40, which is common in a dry winter apartment, so there is no need to panic at a low reading. Microgreens even prefer 40 to 60, since damp air invites mold on a tray.
Dry air shows up in two ways. First, leaves lose water faster, so plants drink more and an active reservoir drops faster between checks. In a pumped system, top it up with plain water as topping off hydroponic water describes. Kratky jars work differently, because they are filled once and left to run down, so a jar that runs low long before harvest is simply too small for its crop; the container size chart has the right jar for each plant.
Second, dry, warm air is exactly what spider mites like. So check the undersides of leaves now and then over the winter, using spider mites on hydroponic herbs as a guide to what they look like. If your meter sits well below 40 for weeks, a small room humidifier is the simplest fix, placed a few feet from the plants rather than pointed at the leaves.
Warm the Water, Not Just the Room
Here is the most useful winter finding for hydroponic growers. Purdue grew butterhead lettuce under cool air, 60°F by day and 55°F at night, and tested four nutrient solution temperatures. Growth rose with every step up, and the best result came from the warmest solution tested, 71.5°F. In other words, warm roots can partly make up for a cool room.
So the winter goal is to keep the root zone in its usual range, 65 to 75°F (18 to 24°C), even when the air around the leaves is cooler. Location does most of the work: a shelf on an inside wall, away from the window and the door, stays warmer and steadier than anywhere near the glass. The guide to water temperature in small systems covers the summer side of the same range.
Water Changes With Cold Tap Water
Winter tap water often comes out cold, and pouring it straight into a reservoir drops the root zone in one go. The sudden swing is the problem, for the same reason ice cubes are a bad idea in summer. Instead, fill a jug the night before and let it stand at room temperature. Then mix in the nutrient, measure the EC and adjust the pH last, as mixing two-part nutrients describes. How often to change hydroponic water has the schedule, and using tap water for hydroponics covers what your water already carries. The pH and nutrient calculator sizes the mix for your container.
Heating a Bucket or Tote
For an aerated bucket or tote in a cold room, a submersible aquarium heater with a built-in thermostat is the usual tool. Set it inside the 65 to 75°F range and check it against a thermometer for the first few days. Leave a drip loop in the cord, a dip below the socket, so any water running down the cord drips off before it reaches the plug. Also unplug the heater before it ever comes out of the water, since many heaters are not made to run dry. Jars are too small for a heater, so for them the answer is location rather than equipment.
Starting Seeds in a Cold Room
Seeds sprout slowly in a cool apartment. A seedling heat mat under the starter tray speeds them up, while a thermostat stops the mat from overheating the tray; the seed-starting kit guide covers a mat and thermostat set. Take the tray off the mat once most seeds are up, then move the seedlings under the light.
What to Grow in a Cool Apartment
Some crops like winter better than summer. Cool-season crops grow best in air of 60 to 70°F (15 to 21°C), which is close to what many apartments sit at once the heating is set low.
- Spinach is the clearest winner. It bolts in warm rooms and long days, so a cool apartment and its 12-hour light suit it well; growing spinach in an apartment has the details.
- Arugula, bok choy and watercress all belong to the cool-season group, so they need no extra warmth; see arugula, bok choy and watercress.
- Cilantro bolts faster when its reservoir runs warm, so a cool room helps it hold its leaves longer, as growing cilantro indoors explains.
- Swiss chard handles anything from 55 to 75°F, which makes it one of the easiest winter crops; see hydroponic Swiss chard.
- Lettuce is fine with nights down to about 60°F, as long as days stay at 65 to 75°F; the countertop lettuce guide covers the rest.
Warm-season herbs need more care. Basil, mint, oregano and thyme want 65 to 75°F, so give them the warmest shelf. Tomatoes and peppers want 70 to 80°F, which is hard to hold in a cool apartment, so they are better started in spring unless you have a warm room. For a ready-made winter salad shelf, growing arugula, kale and spinach together shows how three cool-season crops share one setup.
Feeding Basil in Winter
Johnny’s Selected Seeds suggests a slightly stronger feed for basil in winter. This site runs mature basil at an EC of 1.0 to 1.6 and lets it rise toward 1.8 in winter. Raise it gradually and watch the leaf tips, as EC for hydroponic herbs explains. The hydroponic pH and EC chart has every other crop, and the complete basil guide covers everything else basil needs.
Storms, Outages and Holiday Travel
Winter storms bring power cuts, and an outage in a cold apartment stops pumps and heaters at the same time. Kratky jars carry on without power, while pumped systems need a plan; what to do in a power outage covers it, battery air pumps included.
Holiday travel is the other winter risk, and a feeding schedule for travelers shows how to leave plants for a week or two. Before you go, check that the timer still runs 14 hours and that no jar sits against cold glass.
Where these facts come from
🔗 Where this comes from The cool-air lettuce trials are from Purdue University’s controlled environment program: growing hydroponic lettuce with a heated nutrient solution under cooler air. The 50 to 70 percent humidity range is from the Cornell CEA hydroponic lettuce handbook, and the winter basil feed from Johnny’s hydroponic basil guide. Every other temperature and light figure is this site’s standard, with its sources on the crop sources page.
Tip. The seed-to-harvest calculator shows when a crop should be ready from its sowing date. In a cool room, though, treat that date as the best case, since cooler air slows growth even when everything else is right.
One Last Tip
If you change only three things this winter, make them these. Move every jar a hand’s width back from the glass on the first cold night. Set the timer to 14 hours and leave it there until spring. And keep a jug of water standing on the counter, so it’s never cold when a water change comes round. Then put the spinach on the coolest shelf and the basil on the warmest, and let the season do the rest.
Frequently Asked Questions
How cold is too cold for hydroponics?
For most crops, growth slows once the root zone drops below 65°F (18°C) or the air around warm-season herbs falls below 65°F. In a Purdue trial, lettuce in 60°F days and 55°F nights grew more than two thirds less than lettuce at 70/65°F. Cool-season crops such as spinach are happy in air of 60 to 70°F.
Can you do hydroponics in winter?
Yes. Indoor hydroponics works all year, because the plants only feel the conditions around them. In winter, keep jars off cold glass at night, run a grow light 14 hours a day, keep the root zone at 65 to 75°F and let water for a water change reach room temperature first.
Do hydroponic plants need a heater in winter?
Usually not. Moving plants away from the window to a shelf on an inside wall solves most cold problems. For an aerated bucket or tote in a cold room, a submersible aquarium heater with a thermostat, set within 65 to 75°F, keeps the root zone in range. Jars are too small for a heater.
How many hours of light do hydroponic plants need in winter?
The same as in summer: 14 hours on and 10 hours off for herbs and greens, with spinach at 12 hours. Winter daylight falls short, so a grow light on a timer makes up the difference, and you can keep plants by the window for whatever daylight there is.
Is a windowsill OK for hydroponics in winter?
By day, yes, especially a south-facing one. At night, though, the glass gets cold and chills any jar against it. Pull jars back a few inches or onto a nearby shelf at night, close the curtain on the window side, and put a cork mat or towel under jars on a stone sill.
What humidity do hydroponic plants need in winter?
Aim for 50 to 70 percent, but 40 is survivable and common in a dry winter apartment. Microgreens prefer 40 to 60, since damp air invites mold. Keep plants out of the direct airflow of heating vents, and only add a humidifier if the reading stays well below 40.
What grows best in hydroponics in winter?
Cool-season crops: spinach, arugula, bok choy, watercress, cilantro and Swiss chard all like air of about 60 to 70°F, and lettuce copes with nights down to about 60°F. Basil and other warm-season herbs grow too, but they need the warmest spot and 65 to 75°F.
Should I use cold tap water for a hydroponic water change?
Not straight from the tap in winter. The sudden drop chills the roots. Fill a jug the night before, let it reach room temperature, then mix the nutrient, measure the EC and adjust the pH last before the change.
Do I need to change nutrient strength in winter?
For most crops, no: keep the usual EC for each crop from the pH and EC chart. Basil is the exception. Johnny’s suggests a slightly stronger feed in winter, so this site lets mature basil rise from its usual 1.0 to 1.6 toward 1.8, raised gradually while you watch the leaf tips.
Why is my hydroponic basil growing slowly in winter?
Usually cold and short days. Basil is a warm-season herb that wants air and water at 65 to 75°F and 14 hours of light. Move it to the warmest shelf, away from the window at night, run a grow light on a timer, and consider letting its EC rise toward 1.8.
