When to Start Kratky Outside in Spring

Basil growing outdoors in a container, ready once it is warm enough to start Kratky outside

Go by night temperatures, not the date. Knowing when to start Kratky outside comes down to the crop: lettuce and leafy greens can go out once nights stay above about 40°F (4°C), while basil, tomatoes, peppers and cucumbers should wait until after your last frost, when nights stay above about 50°F (10°C).

A container of water outside cools down overnight just like the air around it. Starting too early leaves roots sitting in cold solution, which stalls growth, and a hard frost can freeze the container.

Cool-Season Crops First

Lettuce, spinach, other leafy greens, parsley and chives like cool weather. They can go out in early spring, weeks before warm-season plants. Spring is often their best season outdoors, because summer heat makes lettuce bolt.

Nights mostly above about 40°F (4°C).
A sheltered spot against a wall, out of cold wind.
Bring them in, or cover them, if a frost is forecast.

Warm-Season Crops After the Last Frost

Basil, tomatoes, peppers and cucumbers are damaged by cold, even above freezing. Basil can blacken on a single cold night.

Wait until after your area’s average last frost date.
Wait for nights that stay above about 50°F (10°C).
Give large containers time to warm up; a big tote of solution stays cold longer than the air.

Hardening Off

Plants started indoors, on a windowsill or under lights, need a week or so to adjust to sun and wind. Without it, leaves can scorch or wilt.

Day 1 to 2: an hour or two outside in shade, then back in.
Day 3 to 4: a few hours, with a little morning sun.
Day 5 to 7: most of the day outside, including some afternoon sun.
After that: outside full time, as long as nights are warm enough for that crop.

Move containers gently so the solution doesn’t slosh up into the air roots. Carry them by the base, not the lid.

Moving Indoor Jars Outside

If you grow in mason jars indoors over winter, many of those plants can move outside in spring. Check the cover on each jar: outdoor sun is much stronger than window light, and a thin cover that kept algae out indoors may let light through outside. Read the algae article for cover options.

Watch the Forecast

Spring weather swings. A warm week can be followed by a cold night. Keep an eye on the forecast for the first few weeks and have a plan: bring small containers inside, and cover larger ones with a frost cloth or an old sheet.

Common Mistakes

Putting basil out on the first warm weekend.
Skipping hardening off and scorching indoor-grown leaves.
Forgetting that rain will start falling into containers; see the rain article.

Common Questions

Can lettuce go outside before the last frost?

Yes, once nights stay above about 40°F. Bring it in or cover it if a frost is forecast.

When can basil go outside?

After your last frost, once nights stay above about 50°F (10°C). Basil is easily damaged by cold.

Do I need to harden off Kratky plants?

Yes, if they were grown indoors. Give them about a week of gradually longer time outside.

Related guide: growing Kratky on a balcony or patio.

When Can You Start Kratky Outside in Spring?