Kratky Windowsill in Winter: Cold Glass and Short Days

Herbs and greens in small containers on a kitchen windowsill, where Kratky jars sit by cold glass in winter

A Kratky windowsill keeps producing in winter if you deal with two things: cold glass at night and short days. Move the jars an inch or two off the glass on cold nights, grow cool-season greens and herbs, and expect slower growth, or add a light to make up the hours.

Winter is when an indoor windowsill is most useful, because the garden outside has stopped. It is also when the windowsill is least like the rest of the house: the glass gets cold, the days are short and the sun sits low.

Cold Glass

A window can be much colder than the room on a winter night, and a jar touching the glass feels it. Cold solution slows root growth, and leaves pressed against frosty glass can be damaged.

Leave a small gap between the jars and the glass, and between the leaves and the glass.
On the coldest nights, slide a piece of cardboard between the jars and the window, or move the jars to a table nearby until morning.
If you close curtains or blinds at night, keep the jars on the room side of them.
Stand jars on a folded towel or a cork mat rather than a cold stone or tile sill.

Heat From Below

The opposite problem is a radiator or heating vent under the window. Warm, dry air blowing over the jars dries the leaves and warms the solution. If the sill sits above a heater, move the jars to a different window or a shelf next to it.

Short Days

In winter, even a south-facing window gets fewer hours of light, and the light is weaker. Plants keep growing, just more slowly. Lettuce that took five weeks in spring may take eight. That’s normal, not a sign that something is wrong.

To make the most of the light, keep the glass clean, turn jars every few days and choose the brightest window you have. A south-facing window is best in the Northern Hemisphere.

What to Grow in Winter

Loose-leaf lettuce and other leafy greens, which like cool conditions.
Chives, parsley and mint, which handle cool rooms and lower light.
Green onions regrown from their root ends: quick, easy and useful in the kitchen.

Basil struggles on a cold winter sill. It likes warmth and long days, so save it for spring or grow it under a light.

A Winter Routine

Start new jars every few weeks, so a slow crop doesn’t leave you with nothing to pick.
Check the water level weekly. Plants drink less in winter, so a jar lasts longer.
Harvest outer leaves rather than whole plants, so each plant keeps producing.
Watch for algae where the low sun reaches under a loose net cup.

When to Add a Light

If the plants stretch toward the glass, or the window faces north, a small grow light extends the day. A light on a timer for 12 to 14 hours keeps lettuce growing at close to its summer pace. The grow lights guide covers how close to set it.

Common Mistakes

Leaving jars touching the glass on a frosty night.
Putting jars on a sill directly above a radiator.
Expecting summer growth rates from a winter window.

Common Questions

How cold is too cold for Kratky jars on a windowsill?

If the jar feels cold to the touch in the morning, the roots are cold too. Lettuce tolerates cool solution, but growth slows a lot. Move jars off the glass at night.

Should I top up jars more often in winter?

Usually less often. Plants drink less in cool, dim conditions, so the level falls more slowly.

Can I grow basil on a windowsill in winter?

It is hard. Basil wants warmth and long days. Under a grow light in a warm room it can work.

How Do You Grow Kratky on a Windowsill in Winter?