How to Harden Off Seedlings: Complete Guide & Tips
Have you planted seedlings outdoors that looked healthy when growing inside (or in a greenhouse), only to have them burn up, wither, and die when you plant them? You may not have hardened them off properly before planting. Hardening off seedlings is the crucial last step of growing seeds indoors. In this post, you’ll learn how to harden off seedlings before you plant, along with tips for hardening off in hot climates, protecting seedlings from pests, and more.
Article Outline:
- What is Hardening Off?
- Using Grow Lights for Indoor Seed Starting
- How to Harden Off Seedlings
- How Long Does the Hardening Off Process Take?
- What Should I Do If the Weather Is Unpredictable?
- Signs That You Need to Harden Off Seedlings More Slowly
- When to Leave the Seedlings Outside All Night
- Protecting Seedlings from Pests
- Do I Need to Harden Off Nursery-Grown Purchased Transplants?
- Hardening Off Seedlings in Hot Summer Climates (Monsoon Planting)
- Final Thoughts
- References & Additional Resources
What is Hardening Off?
Hardening off is the process of gradually exposing seedlings to outdoor conditions. As the seedlings experience more sun, wind, and fluctuating temperatures, their cell walls harden and the plant’s cuticle thickens. This thicker cuticle helps protect the plant from moisture loss and damaging UV rays. Once hardened off, seedlings are much more likely to thrive outside.
Did you know?
The cuticle is the plant’s outermost layer, similar to human skin. It primarily acts as a water permeability barrier, helping reduce evaporation and protect the plant from external stressors.
Using Grow Lights for Indoor Seed Starting
Starting seeds under grow lights is a good idea when outdoor conditions are not yet suitable for tender seedlings. Grow lights allow you to control the intensity and duration of light, ensuring seedlings develop strong stems instead of becoming leggy. Get more tips for starting seeds indoors in this guide.
- Height of the Lights: Keep lights 2–3 inches (5–7.5 cm) above the seedlings. This encourages sturdy growth rather than spindly stems. (This may vary depending on the type of lights you use.)
- Light Duration: Most seedlings need about 12–16 hours of light per day. Using a simple timer can help maintain a consistent schedule.
- Transition to Natural Light: About a week before you begin the hardening off plant process, try gradually shortening the time under grow lights. This subtle shift mimics the reduced light time they’ll get outdoors.
How to Harden Off Seedlings
Start 7–10 Days Before Planting
Watering and Fertilizing:
- Before the hardening-off process, gradually reduce how often you water the plants. You don’t want them to dry out, but allowing the top of the soil to dry slightly helps strengthen the roots. Avoid letting them wilt.
- It’s also not recommended to fertilize seedlings before or during hardening off—this prevents tender growth that’s more susceptible to outdoor stressors.
Weaning Off Heat Mats:
If you’ve been using heat mats, begin turning them off or lowering the temperature a few days before you start taking the seedlings outdoors. By the time you’re in the middle of the hardening-off period, the plants should be completely off bottom heat.
Day 1: Place your seedlings outside in a shaded, protected location for 1–2 hours. Seedlings and transplants should avoid direct sun and wind initially. Then, bring them back inside.
Day 2–3: Gradually increase the time outside to 2–3 hours. Keep them in filtered sunlight or morning sun, avoiding intense afternoon rays.
Day 4–6: Extend outdoor time further. Introduce them to a bit more direct sunlight, but watch for signs of stress, such as wilting or leaf burn.
Day 7–10: By the end of 7–10 days, your seedlings should be ready for full sun exposure throughout the day. If they look healthy and strong, they can be transplanted into your garden.
Tip:
Keep the soil evenly moist throughout the hardening off process. Seedlings dry out faster outdoors due to wind and sun, so consistent watering is crucial.
How Long Does the Hardening Off Process Take?
Typically, the process takes between 7–10 days, though it can vary by plant type and environmental conditions. Cold hardy plants (like kale or broccoli) may adapt more quickly than very tender seedlings (like tomatoes or peppers). If you have a cold frame available, you can expedite the transition by placing seedlings in it for a few hours each day before fully exposing them to the elements.
What Should I Do If the Weather Is Unpredictable?
Unpredictable weather is common, especially in spring. Here are some tips:
- Check the Forecast
- If nighttime temperatures are expected to dip below 45°F (7°C) or if it is very windy, bring the seedlings back inside or cover them.
- Use a Cold Frame or Protective Cover
- A cold frame or row cover can help buffer temperature swings.
- Hold Off on Transplanting
- If there’s a risk of late frost or a drastic temperature drop, wait until after the final frost date to set seedlings outdoors. It’s better to delay planting than to risk losing all your hard work to a cold snap.
Signs That You Need to Harden Off Seedlings More Slowly
- Wilting
- A plant that wilts (not from lack of water) could be under heat or wind stress. Reduce exposure time to intense sunlight or strong winds.
- Burnt or Bleached Leaves
- Leaf burn or bleaching is a common sign of too much direct sunlight too soon. Return the seedlings to a shadier spot for a day or two, then reintroduce sunlight gradually.
When to Leave the Seedlings Outside All Night
- Warm-Season Crops (e.g., Tomatoes, Peppers)
- You can leave them out once nighttime temperatures stay above 50°F (10°C). If a sudden cold front hits, bring them back indoors.
- Cool-Season Crops (e.g., Lettuce, Broccoli)
- If nighttime temperatures stay above 45°F (7°C) and the seedlings have been outdoors all day without stress, they’re typically safe to remain outside overnight.
Protecting Seedlings from Pests
Young seedlings can be vulnerable to birds, slugs, and other pests.
- Physical Barriers
- Cover seedlings with tulle or netting to keep birds away.
- Use DIY cloches made from plastic bottles to protect individual seedlings from insects.
- Natural Predators
- Encourage beneficial insects like ladybugs to help control pests. Eliminating pesticides can help keep these beneficials around. Learn more about how to attract beneficial insects in this guide.
- Healthy Transplants
- Strong, well-hardened seedlings are more resistant to pests. Ensuring they’re healthy before planting reduces the likelihood of major pest damage later.
Do I Need to Harden Off Nursery-Grown Purchased Transplants?
Often, nursery plants are partially hardened off. However, giving them an abbreviated hardening-off period is still a good idea.
- Day 1: Place them in a part-shade area.
- Day 2: Move them to full sun.
- Day 3: Leave them outside overnight (if temperatures are suitable) and plant them the next day.
Hardening Off Seedlings in Hot Summer Climates (Monsoon Planting)
When summer temperatures soar and it’s time for monsoon planting, hardening off seedlings can be especially challenging. Learn more about monsoon planting in this guide. Here are a few tips to help your young plants transition successfully:
- Provide Extra Shade
- During this period, protect seedlings from the harsh afternoon sun with shade cloth, a temporary canopy, or by placing them under taller plants or structures. This prevents leaf scorch and reduces water loss. Get more tips for providing shade in this guide.
- Use Larger Transplants
- Larger seedlings have more established root systems. A bigger root ball holds moisture longer and reduces the risk of roots drying out quickly in hot, windy conditions.
- Water Generously but Strategically
- Increase watering frequency so transplants stay consistently moist (but not waterlogged). Monitor them closely—hot weather can dry the soil faster.
- Water at the base of the plant early in the morning and again in the evening if needed.
- Avoid Heatwaves
- When hardening off plant transplants, try to wait for a break in extreme heat. A severe heatwave can lead to wilting, leaf scorch, or even plant death. Get more tips for heatwave gardening in this guide.
- Transplant in the Evening
- Planting in cooler evening temperatures allows seedlings to settle in overnight. This helps them recover from transplant shock before facing the next day’s sun.
- Monitor and Adjust
- Keep a close eye on leaves for signs of stress—wilting, scorching, or curling may indicate too much sun or insufficient water.
- Gradually move the seedlings into areas with more sun as they acclimate, but continue providing afternoon shade until you’re sure they can handle full exposure.
By following these guidelines, you’ll ensure your transplants are well-prepared for hot summer climates and variable monsoon conditions. If you’re ready to transition from seed starting to a fruitful harvest, check out our comprehensive guide on how to grow peppers for even more tips.
Final Thoughts
Learning how to harden off seedlings correctly will give your garden a strong start. Healthy transplants become robust plants that are less susceptible to pests and diseases—and will reward you with bountiful harvests for months to come.
Once your seedlings are hardened off and ready to plant, check out “How to Plant Transplants” for additional tips on planting in your garden.
Feel free to ask if you have any questions about hardening off plant seedlings, timing, or best practices.
Please share this post if you found it helpful!
References & Additional Resources
University of Illinois Extension. (2020, April 6). Starting a Garden: Hardening Indoor Seedlings (Good Growing Blog). https://extension.illinois.edu/blogs/good-growing/2020-04-06-starting-garden-hardening-indoor-seedlings
University of Maryland Extension. (n.d.). Hardening Off Vegetable Seedlings in the Home Garden. https://extension.umd.edu/resource/hardening-vegetable-seedlings-home-garden
Just what I needed. Thank you!
Hello, I planted chili tepin seedlings in an aerogarden. They are ready to be planted. How do I go about planting them in a pot and hardening them? First time doing anything like this. I live in southern Arizona.
This post may be helpful as you pot them up: https://growinginthegarden.com/when-and-how-to-pot-up-seedlings/ and then follow this blog post’s steps about hardening them off.
Thanks for providing such informative content—I’ve watched so many of your vids trying to get ready for this season.
I grew transplants from seed this season for the first time in all my few years of gardening. Hardened them off for about a week before transplanting into my garden beds. It’s been about 10 days now and I feel like a lot of my peppers, tomatoes, basil, and eggplants are just… stuck. They’re not dying or wilting, but they’re not really growing in height or bearing new leaves. Fed them fish emulsion a few days ago, deeper infrequent watering, but nothing. I figure okay, a week of root growth underneath the surface… are they just in shock? Is there a way to overcome this or do I consider this a loss??
My guess is they will take off soon, especially as temperatures are consistently warm. (all of the crops you mentioned are happiest with nighttime temps above 50) I’m rooting for them to grow for you, best of luck!!!
@Angela Judd, Thanks! I guess I just have to keep waiting then—while day temps were in the 70s I think night temps have still been in the upper 40s/low 50s. I’m in Tucson, so a little cooler than Mesa.
I’m new to the hardening off process, and I thought I was doing good until today. All my plants that looked healthy and happy are droopy and wilted. Will they bounce back or did I just kill off all my plants?
Sometimes they wilt and then bounce back – other times that means we have to back off a bit and take it more slowly. I hope yours recovered!