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“Hardening off seedlings” refers to the process of weaning your seedlings off of their early (usually indoor) environment to face the real world outside. Though this article follows our earlier articles on growing vegetables, Beginner’s Cheat-Sheet to Starting Vegetable Seeds in 6 Easy Steps and Beginner’s Cheat-Sheet to Transplanting Seedlings – 5 Easy Steps, you can use our steps with any type of seedlings (i.e. not just vegetables).
The idea of hardening off seedlings can be nerve-wrecking, particularly for beginners. After spending weeks germinating and raising seedlings to become stronger, it’s understandable to fear accidentally killing them and having to start over.
The first time I had to harden off seedlings, I was overly cautious in ensuring they did not stay outside for too long and probably prolonged the process longer than necessary. In the end, the seedlings survived, which illustrates that hardening off seedlings doesn’t have to be rocket science.
In other words, don’t overthink the process and suffer more anxiety than you need to. Although seedlings are fragile compared to mature plants, by the time you need to harden them off, they should be well past the stage of their really early days and now strong enough to survive outside elements.
Of course, if you live in a very temperate climate or plan to grow your vegetables indoors indefinitely, then you needn’t worry about hardening off your seedlings. This article only applies if you plan to transfer your seedlings outside indefinitely, whether in containers on in the ground.
For the latter, this article provides an easy, step-by-step guide to hardening off seedlings.
Why You Need to Harden Off Seedlings
Most seedlings start out in a sheltered environment, whether indoors or in a greenhouse. Very young seedlings have delicate, thin stems and shallow root systems. Thus, harsh outside elements could easily uproot or kill them.
By growing indoors since their germination, seedlings are used to a sheltered, climate- and element-controlled life. Suddenly transferring them outdoors without warning can shock their systems, causing severe injury or death.
Thus, hardening off seedlings allows them to gradually acclimate to outside elements to avoid a sudden shock and change of environment. They can get used to outside temperatures, direct sunlight, wind, and other elements at a suitable pace until they are ready to be outside full-time.
Growing outside actually strengthens your plants as well, where they gradually grow stronger and sturdier to be able to withstand outside elements.
When to Start Hardening Off Seedlings
As a general rule, you should plan to harden plants off four to five weeks after the seeds germinate. As indicated in our Beginner’s Cheat-Sheet to Starting Vegetable Seeds in 6 Easy Steps, you should have planted vegetable seeds according to their proper planting season per the plant hardiness zone.
Thus, the time you should start hardening off your seedlings should coincide with their proper outdoor planting date. Plan to harden off your seedlings over the course of one week. Thus, you should ensure no harsh weather patterns may occur during the week, like heavy winds or rain.
If you foresee or experience harsh weather patterns, it’s easier to keep your seedlings indoor and to delay hardening them off until the weather clears up. Delaying the process of hardening off seedlings by a week or two shouldn’t matter.
Supplies Needed for Hardening Off Seedlings
If you followed out last article, Beginner’s Cheat-Sheet to Transplanting Seedlings – 5 Easy Steps, your seedlings should be an adequate-sized container for the hardening off step.
You shouldn’t have to transplant your seedlings into a bigger container or the ground until after they’ve hardened off, nor should you. Transplanting them at this stage will simply add another change that your seedlings need to accommodate that isn’t necessary.
Thus, you shouldn’t need any new supplies for hardening off seedlings unless you live in a really extreme climate, e.g. harsh winds or precipitation. If you are concerned about harsh weather conditions, you can get plastic cloches to help shelter them from extreme temperatures or wind.
You should consider a large tray or box to transport your seedlings easily. Since you may be shuttling several pots in and out daily over the week, having them in a tray will facilitate the process. I’ve used both shallow boxes and large baking sheets/trays.
If you use a box, just ensure the walls aren’t so tall to prevent your seedlings from accessing sunlight and fresh air.
4 Easy Steps for Hardening Off Seedlings
Step 1: Pick a Temperate Day to Start
Look at the weather forecast for the upcoming week to ensure no harsh weather patterns may present, such as strong winds, precipitation, or a cold snap. Since hardening off seedlings takes around a week, allot for a week of you having to take your seedlings in and out, while keeping an eye on them outside occasionally.
Step 2: Leave Seedlings Outside for 1-2 Hours (Days 1 and 2)
Pick a spot outside that doesn’t have direct sunlight, and leave your seedlings there for a couple of hours on days 1 and 2. You should ensure your seedlings have been adequately watered prior.
If you are hardening off seedlings in the late-summer/early fall and live in a really hot climate like us, then you also have to keep an eye on their soil moisture. Depending on the size of your pots, extreme heat outside may dry the soil out during this time.
If the seedlings look like they’re in distress at any point (e.g. wilting stems or leaves), then take them inside and reduce the time outside to one hour. If they look fine after one hour, however, then go ahead and let them stay out for two hours.
If you are in a hot climate and in late-summer/early fall, then putting your seedlings out in the morning or late-afternoon would be ideal, to avoid the hotter mid-afternoon heat. In the wintertime or early spring, the hotter mid-day temperatures are fine.
Step 3: Increase Time Outside
With each subsequent day, increase the seedlings’ time outside by another one to two hours. As before, keep an eye on them to see if they look fine and can last another two hours versus one with each increase.
If you have particularly harsh heat (i.e. above 89 degrees) and sunlight, then keep your seedlings out of direct sunlight most of the time. You can try putting the seedlings in direct sunlight for 30 minutes and then gradually increase the time in direct sunlight daily, while keeping a close eye on signs of distress.
Also, remember to occasionally check soil moisture to ensure the soil hasn’t dried out.
If the temperature cooler , then you can put your seedlings in direct sunlight for the full time outside.
Step 4: Leave Your Seedlings Outside Overnight
Around day 6 or 7, leave your seedlings out overnight. If your temperatures are pretty consistent without a lot of fluctuation between daytime and nighttime temperatures, then you can do this earlier in this week.
Once your seedlings can successfully spend a couple of nights outside without issue, then congrats! They should be fine outside for good, barring extreme weather patterns like sudden windstorms or heavy rain. However, these extreme circumstances only mean potentially having to take measures to protect your plants at the time.
What’s Next After Hardening Off Seedlings?
Once your plants are close to outgrowing their current containers, then you can transplant them into their final growing medium, whether a large container like these cloth pots, a raised garden bed, or in the ground.
When you eventually transplant them, if your soil medium does not already contain fertilizer, then you can sprinkle a tablespoon of fertilizer like this vegetable fertilizer in the soil under your seedling’s roots for it to access.
Regarding the type of soil to use, you can use regular potting soil mix or create your own blend. With a raised bed garden bed, for instance, one popular composition is the following by Mel Bartholemew, who popularized square foot gardening: 1/3 compost, 1/3 peat moss, and 1/3 course vermiculite.
Like with any prior transplanting, it’s best to transplant when your seedling’s soil is dry, and then fully water the soil in the new, bigger medium. Watering the transplanted seedling with its new soil helps settle the soil around the seedling’s roots and transplanted soil.
Depending on your climate, just ensure your plants remain adequately watered, whether by hand or an irrigation system. You can fertilize them every other month by putting a ring of fertilizer around the seedling, about an inch away from the stalk, and then watering over it. Alternatively, you can simply use a kelp fertilizer like this one as a foliar spray (i.e. spritzing a drip diluted with water on the leaves) monthly, which works great.
If you have any questions, tips, or comments about hardening off seedlings or thereafter, please leave them below. We would love to hear from you!