How to Care for Seedlings and Harden Them Off: A Complete Guide
At Pandemonium Gardens, we learned the hard way more than once when it came to hardening off, or our lack of it. It's tempting to plant your gorgeous new seedlings right away, and it's heartbreaking when one sulks for weeks, sometimes to death, because you skipped a step. We wrote this guide so you don't have to learn it the same way.
TLDR (The short version)
Hardening off is the process of slowly exposing indoor-grown seedlings to outdoor conditions over 7 to 14 days before you plant them in the ground.
Skip it and your plants can stall out or scorch in a single afternoon.
Start with one hour in dappled shade on a mild day. Add an hour each day.
Increase sun exposure gradually.
Bring plants in if overnight temperatures drop below 50°F for warm-season crops like tomatoes and peppers.
Water before and after each session.Transplant on a cloudy afternoon or in the evening, not under midday sun.
When and how to transplant after hardening off
Transplant in the late afternoon or on a cloudy day. Midday sun on a freshly transplanted seedling is a hard combination, even after proper hardening off.
Water the seedling thoroughly an hour before you plant it. A well-watered plant transplants with less shock than a thirsty one.
Dig the hole slightly deeper than the cell. For tomatoes, plant deep enough to bury the lower set of leaves, since tomatoes grow roots along any buried stem. For peppers and herbs, plant at the same depth as the cell.
Water in immediately after planting, even if the soil is moist. The water settles the soil around the roots and eliminates air pockets that can dry out the root ball.
Skip fertilizer at transplant. The plant needs to focus on root establishment, not new top growth. Wait two weeks before the first feeding.
Keep reading for the full breakdown.
What is hardening off, and why does it matter?
Hardening off is the process of acclimatizing indoor-grown seedlings to outdoor conditions before transplanting. Indoor seedlings grow in protected conditions: filtered light, steady temperature, no wind, no rain. Outside, they meet full sun and real wind. They face temperature swings they have never experienced.
The transition is a physiological shock. A seedling that goes straight from a windowsill or grow light into a sunny garden bed can suffer leaf scorch within hours. The cuticle, the waxy outer layer of the leaf that protects against water loss and UV, has not developed enough to handle direct sun. The stem has not built up the structural fiber needed to stand against wind.
Hardening off gives the plant time to rebuild itself for the real world. Done well, the seedling develops stockier stems and thicker leaves. Its root system also strengthens to chase moisture in real soil. Done poorly or skipped entirely, your seedling spends two to three weeks recovering from transplant shock instead of growing, and in many cases it never fully catches up.
For warm-season crops, skipping hardening off can cost you weeks of growth at the start of the season. Plants that were ahead in May end up behind in July.
How long does it take to harden off seedlings?
Most seedlings need 7 to 14 days to harden off properly. The exact length depends on the variety, the weather, and how protected the seedlings were before you started.
A general rule: warm-season crops (tomatoes, peppers, basil, eggplant) need the longer end of the range, usually 10 to 14 days. Cool-season crops like lettuce, kale, parsley, and sage often acclimate in 7 to 10 days because they tolerate more variable temperatures from the start.
If a cold front or heavy rain interrupts the schedule, pause and resume where you left off. Pushing through bad weather sets the plant back further than waiting.
| Day | Time Outside | Location | Notes |
|---|---|---|---|
| Day 1 | 1 hour | Full shade, sheltered from wind | Pick a mild day, overcast if possible |
| Day 2 | 2 hours | Full shade | Watch for wilting; bring in if leaves droop |
| Day 3 | 3 hours | Dappled shade | Begin introducing gentle airflow |
| Day 4 | 4 hours | Dappled shade | Move toward filtered sun in late afternoon |
| Day 5 | 5 hours | Morning sun, afternoon shade | First real sun exposure |
| Day 6 | 6 hours | Morning sun, afternoon shade | Watch for leaf curl |
| Day 7 | Full day | Mixed sun and shade | Bring in at night |
| Days 8 to 10 | Full day | Increasing sun exposure | Leave out overnight if above 50°F |
| Days 11 to 14 | Full day and night | Final location | Transplant on day 14 if conditions allow |
Two things this schedule will not tell you:
Watch the weather. A 70°F day with high winds is harder on a seedling than a 60°F still day. Wind dries plants out faster than sun. If forecasts show sustained winds above 15 mph, keep seedlings in a sheltered spot or skip the day.
Watch the plant. Curled leaves and browned edges mean you have moved too fast. So do limp stems. Step back a day and resume more slowly.
Where should I put seedlings during hardening off?
The ideal first location is north-facing, against a wall or hedge, where seedlings get bright indirect light but no direct sun. A covered porch works well. So does a spot under a tree canopy or alongside a fence that blocks afternoon sun. If you do not have a sheltered spot, you can build one. A simple lean-to using shade cloth or an old bed sheet propped against patio furniture will protect seedlings during the first few days. A folding screen works just as well. The point is to filter the sun, not block it entirely.
Keep seedlings off cold ground for the first week. A bench or small table holds heat better than bare grass and reduces overnight chill at the root zone. Pavers work too.
Do I need to harden off seedlings I bought, or only the ones I started myself?
You need to harden off any seedling that has been grown under protected conditions, whether you started it yourself or bought it from a nursery.A common mistake is assuming nursery seedlings are ready to plant because they look healthy. Most nursery seedlings have been kept in a hoop house or greenhouse with filtered light and controlled temperatures. They look strong, but they have not built up tolerance for direct sun or wind.
At Pandemonium Gardens, our seedlings are hardened off before they go to market or delivery, so they are ready to plant within a day or two of pickup. If you buy from another grower, ask whether the plants have been hardened off. If the answer is no or unclear, give them 5 to 7 days of acclimation in your own garden before transplanting.
How do I water seedlings during hardening off?
Water seedlings the morning before you take them outside, and check them again when you bring them in.
Outdoor conditions, especially wind and sun, dry seedlings out faster than indoor conditions. A seedling that needed watering every other day inside may need water every morning once it is outside for full days.
Check the soil with your finger, not just your eye. The top of the soil can look dry while the root zone is still moist, and the surface can look fine while the roots are bone dry. Push your finger an inch into the soil. If it feels dry, water until the bottom of the cell drains.
Avoid watering in the evening once seedlings are outside overnight. Wet foliage going into a cool night invites fungal problems. Water in the morning whenever possible.
What temperatures are safe for hardening off?
For cool-season crops like lettuce, spinach, kale, broccoli, parsley, cilantro, sage, and thyme, you can start hardening off when daytime temperatures are consistently in the 50s and overnight lows stay above 40°F.
For warm-season crops like tomatoes, peppers, basil, eggplant, cucumbers, and squash, wait until daytime temperatures are in the 60s and overnight lows are reliably above 50°F. Tomatoes and peppers can be permanently stunted by exposure to temperatures below 50°F, even if the cold does not kill them outright. Basil is even more sensitive and will show black spots on leaves after a single night below 50°F.
In Zone 5b/6a (Northwest Indiana, where we grow), this typically means starting cool-season hardening off in mid-April and warm-season hardening off in mid to late May. The last expected frost in our area is around May 15, but we wait until Memorial Day weekend for tomato and pepper transplants.
Hardening off tomato seedlings: what to watch for
Tomatoes are responsive to hardening off and reward the time you put in. A well-hardened tomato seedling will have a thicker stem and deeper green leaves. The undersides of older leaves often take on a slight purple cast, which is a normal response to cooler nights and sunlight.
Things to watch for:
Leaf curl. Some upward leaf curl during the first few days of sun exposure is normal. If leaves are still curled after a week, you are moving too fast.
Stem bend. Tomato stems should stiffen as they harden off. If a stem is still flopping after a week, the plant needs more wind exposure, not less. A small fan blowing on indoor seedlings before hardening off helps build stem strength early.
Yellowing lower leaves. This usually means the plant is hungry. A light feeding with half-strength fish emulsion can help, but do not over-fertilize during hardening off. The plant is rebuilding cell structure, not putting on new growth.
For ribbed heirloom varieties take an extra two or three days. These varieties are slower to establish but produce some of the best-flavored tomatoes you will grow.
Shop our tomato seedlings →
Hardening off pepper seedlings: what to watch for
Peppers are slower than tomatoes and more sensitive to cold. They want warmth above all else.
A pepper seedling that gets a single night below 50°F can stall for two weeks. You will not see immediate damage, but the plant simply stops growing. Leaves stay the same size. New leaves do not appear. The plant looks fine but is doing nothing.
For peppers, we recommend:
Wait until daytime highs are consistently above 65°F before starting hardening off. Overnight lows should be above 55°F for the first week of outdoor time.
Use a heat mat in the early days if your nights are still cool. Bring the seedlings inside or into a garage and place them on a heat mat overnight. This is especially important for superhot varieties like habaneros and scorpions, which need consistent warmth to thrive.
Hold off on transplanting until soil temperature at 4 inches deep is above 65°F. A soil thermometer is one of the best small investments you can make for pepper success.
Sweet peppers and hot peppers both follow the same rules, whether you're growing mild varieties like Padron and Aleppo or superhots like habaneros and scorpions. The difference is patience. A pepper transplanted into 55°F soil in May will produce less than one transplanted into 70°F soil in early June.
Hardening off herb seedlings: what to watch for
Herbs vary widely. Mediterranean herbs like rosemary, sage, thyme, and oregano tolerate cooler temperatures and full sun faster than most vegetables. Tender herbs like basil, shiso, and cilantro need the same care as warm-season vegetables.
For Mediterranean herbs:
Move to full sun within 5 to 7 days. These plants want the brightest spot you can give them. Overwatering during hardening off is a bigger risk than sunburn.
Watch for root rot if conditions stay damp. Mediterranean herbs prefer to dry out between waterings, even as seedlings. If the weather turns rainy during hardening off, move them under cover.
For tender herbs:
Treat basil like a pepper. Black spots on leaves after a cool night mean the plant has been damaged. The leaves will not recover, though the plant may push out new growth.
Shiso is hardier than basil but still sensitive to cold. Wait for overnight lows above 50°F before leaving it out overnight.
Cilantro will bolt (go to flower and seed) if exposed to heat too quickly. It prefers cool weather and partial shade. If your spring is warm, you may want to skip hardening off in midday sun entirely and keep cilantro in dappled light.
Common hardening off mistakes
We have made most of these ourselves. Here is what to avoid:
Moving too fast. The most common mistake. A sunny weekend tempts you to leave seedlings out all day on day two. The damage may not show up until day four, when leaves brown and stems flop. Stick to the schedule.
Forgetting about wind. Wind dries plants faster than sun and can break unhardened stems outright. Always check the wind forecast, not just the temperature.
Watering at the wrong time. Morning watering is best. Evening watering can leave foliage wet overnight, which invites disease, especially in cool spring weather.
Skipping hardening off because the weather looks perfect. Calm, mild weather is when hardening off is easiest and most effective, not when it becomes optional. Use the good weather to your advantage.
Leaving seedlings in cell trays too long after hardening off. Once a plant is hardened off, transplant within a week. Plants left in cell trays start to become root-bound and lose vigor.
When and how to transplant after hardening off
Transplant in the late afternoon or on a cloudy day. Midday sun on a freshly transplanted seedling is a hard combination, even after proper hardening off.
Water the seedling thoroughly an hour before you plant it. A well-watered plant transplants with less shock than a thirsty one.
Dig the hole slightly deeper than the cell. For tomatoes, plant deep enough to bury the lower set of leaves, since tomatoes grow roots along any buried stem. For peppers and herbs, plant at the same depth as the cell.
Water in immediately after planting, even if the soil is moist. The water settles the soil around the roots and eliminates air pockets that can dry out the root ball.
Skip fertilizer at transplant. The plant needs to focus on root establishment, not new top growth. Wait two weeks before the first feeding.
Need more advice? Sign up for our Seedling Care Services (available in Northwest Indiana only) or drop us a line
FAQs
How long do I need to harden off seedlings? Most seedlings need 7 to 14 days to harden off. Warm-season crops like tomatoes and peppers need closer to 14 days. Cool-season crops like lettuce and kale need closer to 7.
Can I skip hardening off if I bought my seedlings from a nursery? No. Most nursery seedlings have been kept in protected conditions and still need hardening off, even if they look strong. Ask the grower whether plants have been hardened off. If not, give them 5 to 7 days of acclimation before transplanting.
What temperature is too cold for hardening off tomatoes and peppers? Below 50°F. Even a single night below 50°F can stall a tomato or pepper for weeks, even if the plant does not die. Wait until overnight lows are reliably above 50°F.
Why are my seedling leaves turning white after I put them outside? That is sunburn. The leaf cuticle was not developed enough to handle direct sun. The damaged leaves will not recover, but the plant can. Move it back into shade, water lightly, and resume hardening off more gradually.
Do I need to harden off seedlings every spring, or just the first year? Every spring. Hardening off is about acclimating that specific plant to outdoor conditions. New seedlings each season need new hardening off.
Can I harden off seedlings if it is raining? Light rain is fine for cool-season crops once they are partway through hardening off. Heavy rain is hard on any seedling and can damage tender leaves. Move plants under cover during storms.
What is the best time of day to start hardening off seedlings? Late morning, after the dew has dried. Avoid putting seedlings out during the hottest part of the afternoon for the first few days.
Do herb seedlings need to be hardened off? Yes. Tender herbs like basil and shiso need the same care as warm-season vegetables. Mediterranean herbs like rosemary and sage harden off faster but still benefit from the process.
