Hardening off is the process of gradually exposing indoor-grown seedlings to outdoor conditions (sun, wind, temperature changes) over 7-14 days. Start by placing seedlings outdoors in a sheltered, shaded spot for 1-2 hours on the first day. Increase exposure by 1-2 hours daily, gradually moving them into more sun. Bring them indoors at night if temperatures drop below 50°F (10°C). After 7-14 days, seedlings are ready to transplant. This process prevents transplant shock—sudden exposure to sun and wind can damage or kill tender seedlings. The goal is to toughen them up so they thrive in the garden.
What you will need
Know When to Start Hardening Off
Seedling readiness: Start hardening off when seedlings have 2-3 sets of true leaves and are 3-4 weeks old (for most vegetables). They should be sturdy and well-established.
Weather conditions: Choose a period with mild weather (50-70°F / 10-21°C). Avoid extreme heat, cold, or windy days for the first few sessions.
Last frost date: Start hardening off 1-2 weeks before your expected last frost date. This gives you time to transition before planting outdoors.
Check the forecast: Look for a 7-14 day window of mild weather. If a cold snap or heat wave is predicted, delay starting.
Don't start too early: Seedlings that are too small (only 1-2 true leaves) are more vulnerable to stress. Wait until they're sturdy.
Start Slowly: Day 1-3
Day 1: Place seedlings outdoors in a sheltered, shaded spot for 1-2 hours. This could be under a tree, on a covered porch, or in a shady corner. Protect from direct sun and wind.
Day 2: Increase to 2-3 hours. Still keep in a sheltered, shaded spot.
Day 3: Increase to 3-4 hours. Continue in a shaded, protected location.
What to watch for: Check seedlings after each session. They should not be wilting or showing signs of stress. If they look stressed, reduce time the next day.
Bring indoors at night: Always bring seedlings indoors at night during the first few days. Outdoor temperatures can drop significantly.
Gradually Increase Sun Exposure: Day 4-7
Day 4: Start introducing morning sun for 1-2 hours. Keep them in shade for the rest of the outdoor time. Total outdoor time: 4-5 hours.
Day 5: Increase morning sun to 2-3 hours. Total outdoor time: 5-6 hours.
Day 6: Increase morning sun to 3-4 hours. Total outdoor time: 6-7 hours.
Day 7: Morning sun for 4-5 hours. Total outdoor time: 7-8 hours.
Stay protected from wind: Even as you increase sun exposure, keep seedlings protected from strong winds. Wind can dry out and damage tender leaves.
Continue bringing indoors at night: At this stage, you may be able to leave them out all night if temperatures are above 50°F, but check the forecast carefully.
Full Outdoor Exposure: Day 8-14
Day 8: Morning sun for 5-6 hours. Total outdoor time: 8-9 hours.
Day 9: Morning sun for 6-7 hours. Total outdoor time: 9-10 hours.
Day 10: Morning sun for 7-8 hours. Total outdoor time: 10-11 hours.
Day 11-14: By now, seedlings should be able to handle full sun all day (if temperatures are moderate). Leave them outdoors all day, bringing them in at night if needed.
Goal: By day 14, seedlings should be in outdoor conditions full time and ready to transplant.
If you're short on time: A minimum of 7 days is recommended. If you can't do 14 days, prioritize sun exposure over time outdoors.
Protect Seedlings from Extreme Conditions
Wind: Wind can dry out and damage tender seedlings. Use windbreaks (fences, walls, or other plants) or place seedlings in a sheltered location.
Rain: Heavy rain can damage seedlings or cause disease. Bring seedlings under cover during heavy rain.
Hot sun: If temperatures rise above 85°F (29°C), provide shade with a shade cloth or move seedlings to a spot with afternoon shade.
Cold nights: If temperatures drop below 50°F (10°C), bring seedlings indoors. For cold-hardy crops (cabbage, kale, broccoli), they can tolerate lower temperatures once hardened.
Frost: Never leave seedlings out during a frost. Check the forecast and bring them indoors if frost is predicted.
Use Cold Frames or Cloches
What is a cold frame: A cold frame is a simple structure with a clear lid (glass or plastic) and four sides. It traps heat and protects seedlings from wind and cold.
How to use: Place seedlings in the cold frame during the day, starting with the lid partially open. Gradually open the lid more each day. This gives them more outdoor exposure with protection.
Cloches: Individual covers for seedlings (glass jars, plastic bottles, or commercial cloches). They protect single seedlings from wind and cold.
Row covers: Lightweight fabric row covers provide shade, wind protection, and frost protection while letting light through.
Benefits: Cold frames and cloches make hardening off easier—they provide a buffer that reduces the risk of transplant shock.
Water Properly During Hardening Off
Check moisture levels: Outdoor conditions (wind, sun) dry out pots much faster than indoors. Check soil moisture at least twice daily.
Water before going out: Water seedlings before placing them outdoors. This helps them withstand the stress of sun and wind.
Water when they come in: Check seedlings again when you bring them inside. If the soil is dry, water gently.
Avoid wilting: Wilting is a sign of stress. If seedlings are wilting, reduce their outdoor time or provide more shade.
Use a gentle spray: Use a spray bottle or watering can with a fine rose to avoid damaging tender seedlings.
Don't overwater: While seedlings need consistent moisture, avoid waterlogged soil—it causes root rot.
Know When to Transplant
Signs seedlings are ready: They've spent 7-14 days outdoors, they can handle full sun, and they show no signs of stress (wilting, sunburn, or leaf damage).
Weather conditions: Transplant on a cloudy, calm day if possible. Avoid hot, sunny, or windy days—these cause transplant shock.
Time of day: Transplant in the late afternoon or evening. This gives seedlings overnight to recover before facing the next day's sun.
Water before transplanting: Water seedlings well before transplanting to reduce stress.
After transplanting: Water thoroughly after planting. Keep the soil moist for the first 1-2 weeks as roots establish.
Provide some shade: For the first 2-3 days after transplanting, provide shade (a row cover or shade cloth) to help seedlings adjust.
Success: If seedlings are growing within a week, they've successfully transitioned to the garden.
Common problems and fixes
| Problem | Likely cause | What to do |
|---|---|---|
| Seedlings are wilting after being outside | Not enough water or too much sun/wind too soon. | Water immediately. Reduce outdoor time. Move to a shadier, more sheltered spot. |
| Leaves are turning white or brown | Sunburn from too much direct sun too quickly. | Reduce sun exposure immediately. Provide shade. Start with more shade and gradually increase. |
| Seedlings look okay, then flop over | Damping off (fungal disease) or severe transplant shock. | If damping off, discard. For shock, water and move to a sheltered spot with shade. |
| Seedlings are growing slowly | Cool temperatures or not enough light. | Keep them in the sun during the day. Bring in at night if temperatures are cool. Be patient. |
Gardener's checklist
Test what you learned
How long should the hardening off process typically take?
Answer: 7-14 days
How much outdoor time should seedlings get on day 1?
Answer: 1-2 hours in shade
At what temperature should you bring seedlings indoors at night?
Answer: Below 50°F
What is the best time of day to transplant hardened seedlings?
Answer: Late afternoon or evening
Common questions
What happens if I don't harden off seedlings?
Without hardening off, seedlings can experience severe transplant shock. Sunburn, wind damage, and temperature stress can weaken or kill them. It's a critical step.
Can I harden off seedlings without moving them daily?
While the ideal method is daily movement, you can use a cold frame. Place seedlings in the cold frame and gradually open the lid more each day. This provides a protected transition.
Do all plants need hardening off?
Yes, all indoor-grown seedlings need hardening off—from tomatoes and peppers to flowers and herbs. The duration may vary, but the process is essential.
How can I tell if my seedlings are fully hardened?
Signs include: they can handle full sun all day, they're not wilting, leaves are a normal green color, and new growth appears. If you've followed the 7-14 day process, they're ready.
Can I harden off seedlings in a greenhouse?
Yes! A greenhouse is an excellent intermediate step. It provides protection while gradually exposing seedlings to more outdoor conditions. Move them to a cold frame or outdoors for the final stages.
Hardening off is the essential bridge between the cozy indoors and the harsh outdoors. It's a simple but critical step that prevents transplant shock and sets your seedlings up for success. Start slowly—1-2 hours in a sheltered, shaded spot on day 1—and gradually increase exposure over 7-14 days. Pay attention to moisture, wind, and temperature, and be ready to protect seedlings from extreme weather. By the end of the process, your seedlings will be tough, resilient, and ready to thrive in the garden. Don't skip this step—it's the difference between plants that merely survive and those that thrive.
Frequently asked questions
How often should a vegetable garden be watered?
Frequency depends on weather, soil and plant size. Check the soil rather than watering on a rigid schedule.
Is compost enough fertilizer?
Compost improves soil and provides nutrients slowly. Heavy-feeding crops may still need additional feeding.
The Sproutstead Editorial Team
We create clear, practical gardening and homesteading guides designed for real homes and beginner growers.
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