Hardening off seedlings is a critical step of successfully growing seeds indoors, and it’s one that many new gardeners miss. In this post, I’ll explain what it means and why it’s important. I’ll also tell you when to put seedlings outside, and show you exactly how to harden off seedlings, step-by-step.
Growing seeds inside is fun, and it allows us to get our hands dirty several weeks earlier than we can start digging in the garden.
After spending all that time caring for seedlings indoors, it’s exciting when the time finally comes to plant them!
But you can’t simply take them from their cozy indoor environment and plant them directly into the garden. You must first harden off indoor seedlings before planting outdoors, and this is a crucial step.
Don’t worry, I’ve got you covered. I will walk you through step-by-step to show you exactly how to harden seedlings below.
First, let’s answer some common questions, like what is hardening off, why is it important, and when can seedlings go outside.
What Is Hardening Of Seedlings?
Hardening off is the process of slowly preparing seedlings for the transition to the garden. When you harden off seedlings, you are toughening them up by acclimating them to life outdoors over a period of several days.
Transitioning seedlings from growing inside to outside
Why Do Seedlings Need To Be Hardened Off?
Why can’t we just plant our seedlings directly into the garden? Well, your seedlings aren’t used 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 lighting, gentle waterings, and consistently moist soil.
If you put them directly into the harsh sun, wind, rain, and fluctuating temperatures outdoors, they would probably just shrivel and die. Eek!
It’s important to take the time to slowly acclimate your seedlings to the outdoors, so that they will be strong enough to survive being planted into the garden.
How Important Is Hardening Off Seedlings?
Hardening off seedlings is the most important step that you must take before planting indoor seedlings into the garden. Many new gardeners miss this step, and it’s one of most common causes of seedling death.
When you don’t properly harden seedlings, full sun can burn their tender leaves, strong winds can break their weak stems, and rain or hail can crush them.
Do I Need To Harden Off Seedlings?
YES, absolutely! Hardening off your seedlings is a crucial step, and one you definitely do NOT want to skip.
Sometimes we get busy and feel so rushed in the spring that it’s tempting to shorten or even skip this step.
But don’t! Always be sure to properly harden your seedlings, otherwise all that time you’ve spent babying them could be for nothing.
Hardening off plants and seedlings on my deck
When Should I Start Hardening Seedlings?
You can begin to harden off seedlings once the daytime temperatures are consistently above 50F degrees.
Start the process 7-10 days before you plan to plant your seedlings into your garden. Find out when to transplant seedlings into the garden here.
How To Harden Seedlings Step-By-Step
Before moving seedlings outside, remove the plastic cover from the seed tray. Be sure the seedlings have had several days to get used to living without the dome lids before moving them out.
To speed things up, you can use an oscillating fan to help to strengthen seedlings. Simply plug the fan into the same outlet timer as your grow lights, and allow it to gently blow over the seedlings during the day.
Also, if you recently potted up your seedlings, give them at least a week to recover before beginning the hardening off process.
Seedlings moved outside to a shady spot
Steps For Hardening Off Seedlings
Check the forecast before hardening off seedlings, and plan to start on a mild weather day. Also, it’s easier to start this on a weekend when you’re home during the day.
If at any time your seedlings 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.
- Step 1: Put seedlings outside in a shady spot – Begin by moving your seedlings to a shady, protected location. Be sure to also protect your seedlings from animals that may disturb or eat them. A porch or a front step with an overhang is perfect for this.
- Step 2: Protect seedlings from wind, rain and sun – Protect your seedlings from sun, wind and rain at all times for the first few days. So don’t put them out on a windy day or if there’s a storm in the forecast.
- Step 3: Bring them back indoors – Plan to leave your seedlings outside for only a few hours the first couple of days, then bring them back inside. Be sure to put them back under the lights while they’re inside.
- Step 4: Gradually introduce your seedling to sunlight – After a few days, begin slowly exposing your seedlings to sunlight (keep the shade-loving plants in the shade though). It’s best to start by exposing them to morning or early evening sun. Avoid the strong afternoon sun, or the leaves could get sunburn.
Gradually introducing seedlings to sunlight
- Step 5: 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 them more than once a day. Allowing them to dry out slightly is fine. But never let them dry completely, especially to the point where they start wilting.
- Step 6: Repeat steps to harden off seedlings for 5-7 days – Each day you can leave them out a little longer, exposing them to more sunlight every day. So eventually, your seedlings will be outside all day long, and acclimated to the full sun.
- Step 7: Expose seedlings to the (gentle) elements – Light wind and rain are great for your seedlings at this point. So if it’s breezy, or there’s a light sprinkle, leave them outside so they can get used to the elements. Just be sure to remove the bottom trays while it’s raining so your seedlings don’t drown.
Seedlings drowning in a tray after heavy rain
- Step 8: Leave them outside overnight – Once your seedlings are out all day long, and the nighttime temperatures are consistently above 50F, you can leave them out overnight. But you’ll still want to protect them from strong winds, heavy rain, and hail. So be sure to keep an eye on the forecast.
- Step 9: Always protect seedlings from frost – Once they’re hardened off, cold hardy seedlings (like root crops, salad greens, and brassicas) can handle a light frost. However, if a hard frost is in the forecast, then it’s best to move them back inside rather than take a chance on losing them.
How Long Do You Harden Off Seedlings?
Allow 7-10 days to fully harden seedling, and don’t rush it. Once your seedlings are outside 24 hours a day for a few days in a row, they are hardened off ready to be planted in the garden!
My seedlings hardened and ready for transplant
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 harden off seedlings will ensure that they will survive being transplanted to 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 (Everything You Need To Know)
- How To Repot Seedlings Into Larger Containers
Share your tips or questions about hardening off seedlings in the comments section below.
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!