Hardening off seedlings is a critical step, and it’s something that many new gardeners miss. The process takes a little time, but you definitely should not skip it.
Don’t worry, I’m going to make it very easy for you, and show you exactly how to do it the right way.
In this post, I’ll explain what it means and why it’s so important. Then I’ll tell you when to start moving your seedlings outside, and show you the exact process to properly harden them, step-by-step.
What Does “Hardening Off Seedlings” Mean?
Hardening off seedlings is the process of slowly acclimating them from being indoors to living outside, which prepares them for the transition to your garden.
When you don’t do it the right way, full sun can burn their tender leaves, strong winds can break their weak stems, fluctuating temperatures can cause them to wilt, and rain or hail can quickly crush them.
Why Do You Need To Do This?
If you don’t harden off your seedlings, they probably won’t survive the transition to your garden. That’s because they aren’t acclimated to the harsh outdoor environment.
Think about it – seedlings live a very protected life inside the house. They’re used to perfectly warm temperatures, mild and consistent lighting, gentle waterings, and evenly moist soil.
If you take them from their cozy indoor environment and plant them directly into your garden, they would shrivel up and die. Eek!
After spending all that time caring for your seedlings, you certainly wouldn’t want that to happen. That’s why it’s so important to slowly introduce them to life outdoors.
When To Harden Off Seedlings
You can start hardening them off 7-10 days before you plan on transplanting your seedlings into the garden, and once the daytime temperatures are consistently above 60°F.
I recommend starting on a weekend, or during a time that you’ll be home for the first few days. Also, do it on a week when there’s no rain or heavy wind in the forecast.
Preparing Them For Life Outside
A week or so before you plan on moving your seedlings outside, there are a few steps you should take to get them ready for the transition:
- Remove the plastic cover from the seed tray at least 1 week before moving them outside.
- To speed things up, I recommend using an oscillating fan to help to strengthen their stems. Simply run it on the lowest setting, and allow it to gently blow over your trays during the day.
- If you recently potted up your seedlings, give them at least a week to recover before beginning the hardening process.
- Remember to check the forecast a few weeks before, so you can plan to start putting them outside during a period of mild weather days.
How To Harden Seedlings
Step 1: Find a protected spot – Begin by moving your seedlings to a shady location where they will be shielded from any wind or rain. A porch, covered back patio, or a front step with an overhang are perfect for this.
Step 2: Bring them back indoors – Plan to leave them outside for only a few hours the first couple of days. Then bring them back inside, and put your seedlings under the grow lights again.
Step 3: Gradually introduce them to sunlight – It’s best to start with partial or dappled morning or evening sun. Avoid the strong afternoon rays for the first few days, or the leaves could burn.
Step 4: Give them more sun every day – Each day you can leave them out a little longer, and expose them to 1-2 more hours of sunlight. After about a week, your seedlings will be outdoors all day long and acclimated to the full sun.
Step 5: Leave them outside overnight – Once your seedlings are used to being out all day, and the nighttime temperatures are consistently above 60°F, you can leave them outside overnight. After they spend a night or more out there, you can plant them in your garden.
Schedule For Hardening Off Seedlings
Here’s a sample schedule that you can use as a guide. But keep in mind that some seedlings may take longer to acclimate to the great outdoors than others will.
If at any time the leaves start to fade, or turn brown, white, or gray, then move them back into the shade right away. That means they’re getting too much sun, and you need to slow down the process. So give them a few more days in the shade.
| Day 1: 2-3 hours in full shade | Place your seedlings in a shady spot that’s protected from wind and rain. |
| Day 2: 3-4 hours dappled to partial shade | Move them to a spot that gets dappled or partial sunlight in the morning or evening. |
| Day 3: 4-6 hours morning/evening sun | Give them a little more sun, but still avoid the harsh afternoon rays. |
| Day 4: 6-9 hours morning/evening sun | If you can, give them a little shade in the afternoon. |
| Day 5: 9-10 hours full sun | Give them full exposure for most of the day. |
| Day 6: 12+ hours full sun and outside overnight | Leave them in full sun all day, and let them spend the night outside. |
| Day 7: 24 hours outside in their final full sun location | Once they’re outside for 24 hours a day, they’re ready for planting. |
Tip For The Best Success
- Protect them from pests – Be sure to protect your seedlings from any animals and bugs that may disturb or eat them. If you have pesky ground-dwelling pests, then put your trays on a table or other raised surface, so they can’t reach them. Otherwise you can put chicken wire or netting around them.
- Check soil moisture a few times a day – The soil will dry out much faster once the seedlings are outside, so check on them often. You may need to water more than once a day. Allowing them to dry out slightly is fine, and it’s part of the hardening process. But never let them dry completely, especially to the point where they wilt.
- Always protect seedlings from frost – If there’s a chance of freezing temperatures in the forecast, then you should protect your seedlings, even if they are hardened off. It’s best to either move them back inside for the night, or cover them with a blanket or frost cloth, rather than take a chance on losing them.
- Gradually expose them to the (gentle) elements – Light wind and rain are great for your seedlings after the first few days of hardening them. So if it’s breezy, or there’s a sprinkle of rain, leave them outside so they can get used to the elements. Just be sure to remove the bottom trays so your seedlings don’t drown, and bring them inside during harsh weather or heavy rain.
How Long To Harden Off Seedlings
Allow 7-10 days to fully harden off all of your seedlings, and don’t try to rush it. Once they are outside 24 hours a day for a few days in a row, they are ready to be planted in the garden.
FAQs
YES, you absolutely need to harden your seedlings! This is a crucial step, and one you definitely should NOT skip, shorten, or try to make it faster. Otherwise all that time you’ve spent babying them over the past few months could be for nothing.
Your seedlings should be at least a couple inches tall with a few sets of their true leaves before you start hardening them off.
Hardening off seedlings can be a bit of work. Moving them in and out every day is a chore, especially if you have a lot of seedlings. But taking the time to do it properly will ensure that they will survive and thrive in your garden.
If you’re ready to learn everything you need to know about how to grow your favorite plants from seed, then enroll in my online Seed Starting Course. It’s a fun, in-depth, self-paced training that will teach you how to grow any plant you want from seed. Enroll and get started today!
Otherwise, if you just need a refresher, then my Starting Seeds Indoors eBook is perfect. It’s a quick-start guide that will get you up and running in no time.
More Seedling Care Posts
- How To Fix Common Seedling Problems
- How & When To Thin Out Seedlings
- How To Get Rid Of Mold Seedlings
Share your tips or questions about hardening off seedlings in the comments section below.
Gracie says
Can tomillias be grown in a green house?
Amy Andrychowicz says
Sure, as long as it’s warm enough.
Larry Grech says
I placed seedlings outside and was called to work. I was on call. I ended up being there all night my tomatos and cucs and peppers took too much sun and dont look so good they have been back in the house for a while will they come back is there anything I can do
thank you Larry
Amy Andrychowicz says
Oh no, sorry to hear your seedlings got too much sun outside and now they’re suffering. All you can do at this point it keep them in their spot inside and leave them there, hopefully they will recover in a few days. Wait to start hardening them off again until they have recovered. Good luck!
Stacy J Emery says
This is so helpful. This is my first spring growing seedlings inside. This information is going to help me a lot.
Amy Andrychowicz says
Great, so happy to hear this article helped you! 🙂 Good luck hardening your seedlings.
mary kay johnson says
I have a small green house I put all my plants into after they come out of the basement is a green house the same as hardening them off or do I need to still get them use to the full sun
Amy Andrychowicz says
Putting them into the greenhouse is a great first step for hardening them. But you’ll still need to slowly get them used to the full sun, wind and rain before you move them to their final outdoors location.
Moses says
Great tutorial! Thanks for the clear step by step instructions. Hardest thing for me, will be to be patient and not rush the suggested 7-10 day process!
Question: If I germinate a seed inside (ie. wet paper towel method / instant pot method / peroxide method) and get the seed to “pop”, then immediately put outside in the soil (vs in a seed tray), will that seed survive? in otherwords, at how early of a stage can a seed be put outside to mimic direct sowing and thus bypass the need to be hardened off? Or should any seed that germinates indoors, be required to grow and harden off before going outside?
Amy Andrychowicz says
You’re welcome, glad you found it helpful. Interesting question, this is something I have never tried before. I honestly don’t think that germinating the seeds indoors and then planting them directly into the garden will give you much of a benefit. I would recommend just direct sowing the seeds. But to answer your question, as long as the soil is warm enough for the respective seeds, and the “sprouts” are fully covered by soil, then I don’t think you would need to harden them off before planting them. It would be fun to experiment with it so see if it works, and if it makes any difference with the maturity dates.
shali says
i’m starting seedlings indoors WAY before spring, and plan to keep growing them in bigger pots (started in Jiffy pellets) under my indoor UV LED light setup until april when i can move them outside (suburban Maryland, zone 7). Do these “indoor-only for first two months” seedlings need hardening before i move them to the outside or can i wait till april to harden them..?
(not sure if that made sense)
Amy Andrychowicz says
Yes, any seedlings started indoors absolutely need to be hardened before you move them outside. But wait until spring when you’re ready to move the outside for good.
Laurie says
Your website is amazing! So much thorough information for a newbie veggie gardener like me 🙂 Thank you, I appreciate all the great advice!
Amy Andrychowicz says
Wow, thanks so much! Glad you find it helpful, and thanks for your nice comment about my website! 🙂
Lisa Mcwilliams says
Thank you for this tip! I didn’t know to do this, attempting to start from seed for the first time??
Amy Andrychowicz says
You’re welcome! Yes, this is the most common mistake that new gardeners make with their seedlings. Have fun planting everything! I hope they will grow well for you!
SGA says
I live alone and work full time. I am never home long enough to go through this process. Is there a way to harden them indoors?
Amy Andrychowicz says
Unfortunately, there’s no way to simulate the process of hardening off seedlings indoors. I recommend that you start the process on the weekend (or whatever days you have off during the week), so they have two days where you have control. Then, on the day you go back to work, put them outside in a spot where they are completely protected from the sun, wind and rain. Move them out right before you head off to work, then bring them back inside as soon as you get home. As long as your seedlings don’t get any direct sun, wind, rain or cold temps, they should be fine. They may look a little sad after the first few days of being outside all day, but should pop back quickly after you move them back inside. If you have a porch that you could use, or a well protected area outside, those would be great. I put my seedlings on my north-facing front step. There they are completely protected from the elements. Hope this helps, good luck!
James says
I usually put cloches over my young plants after I plant them outside in the spring. Is this equivalent to hardening them off by moving them outside gradually?
Amy Andrychowicz says
I would be worried that the seedlings would get fried in the sun being under cloches all day long (the greenhouse effect). So I would still harden them off first. But if this method works for you, then stick with it. 🙂
Mindy says
Great advice! And very nice pictures! We learned the hard way back in the day, and ruined a tray of seedlings by not hardening. A really tough lesson to learn the hard way! 🙁
Amy Andrychowicz says
Thanks! Yes, learning the hard way that seedlings need to be hardened before transplanting them into the garden is NOT fun. Sorry you had to go through that.
Carol Walters says
Thank you for such thorough coverage of getting my new plants hardened off.
I’ve hardened my seedlings off before, but I usually lost some because I just played it by ear.
This year, I’ll be dealing with more expensive plants and I don’t want to pay for them again.
I can hardly wait to get started!!
Thanks
Amy Andrychowicz says
Great, glad you found these instructions for hardening your plants helpful! I hope that all of your seedlings will survive this time, and thrive in your garden!
rmgales says
I'm going to start hardening mine off this week. Hoping to start planting them out through-out May.
Amy Andrychowicz says
Awesome, hope the hardening off process goes well for you!