You think they’ll be ready to enter the world outside the confines of that warm, cozy bubble called your kitchen (or bathroom or greenhouse or wherever they’ve been shacking up) in a few weeks.
But like a good parent, you don’t want to set them out before preparing them for what it’ll really be like out there.
You need to harden them off first.
It sounds kind of gangsta, to harden off seedlings, but that’s exactly the attitude they’ll need when they make the transition from being indoor to outdoor plants.
What does it mean to harden off seedlings?
Hardening off your seedlings is the vital step of acclimating them to the outdoors to assure their survival. You don’t want to transplant seedlings directly in the garden without “weaning” them first, and this goes for vegetables, herbs, and flowers.
Up until this point, your seedlings have been protected from wind, rain, cold, heat, and intense sunlight. They need to toughen up before you fling them into the deep end of the pool, so to speak.
Why? For this simple reason: All plants have a protective waxy coating on the leaves (called a cuticle) that repels water, reduces the rate of dehydration, and filters harmful ultraviolet light.
Seedlings that have been grown in controlled conditions indoors (in a sunny window, under grow lights, or in a greenhouse) haven’t fully developed the cuticle and need time to build up their “armor” against environmental disturbances.
By gradually exposing seedlings to the outside elements, you stimulate their natural defenses and give them time to adapt to their new environment.
What happens if you don’t harden off plants?
Plants that were started indoors are susceptible to transplant shock and heat stress if they’re not hardened off properly.
If seedlings are left out in direct sun all day, their leaves could get scorched, curl under, or even fall off because they aren’t used to the intensity of unfiltered light.
Strong winds could weaken their stems (especially if you’re already dealing with leggy seedlings) or snap them in half.
Wide swings in temperature caused by hot days and cool nights could further stress delicate seedlings, and even if they do manage to survive, they could become more vulnerable to pests and diseases. Sudden cold temperatures, in particular, can even stunt seedling growth or kill them outright.
Do you need to harden off plants purchased from a nursery?
Nursery-grown seedlings usually start in greenhouses before moving outside. The commercial process of propagating, transplanting, packaging, and sometimes shipping them means they’re already adjusted to the outdoors by the time you buy them.
While nursery plants that are displayed in full sun don’t need to be hardened off, smaller seedlings that live under a shade structure are susceptible to shock if not hardened off properly. You’ll have the best chance at success if you harden off vegetables, herbs, and flowers before planting them in your garden.
In general, store-bought seedlings need less time to acclimate to the conditions in your yard. Set them out in dappled shade all day the first day, then full sun all day the second day, and finally overnight on the third day, making sure they have plenty of water every day.
How long do you have to harden off plants?
In general, the process of hardening off will take about one week, and sometimes up to two weeks if the weather has an unexpected and dramatic drop in temperatures.
If your region has frost, you should time your seed starting in spring so the seedlings can start to go outside after the last frost date. (Use my planting calendar to get a custom sowing and transplanting schedule for your zip code.)
Wait until nighttime temperatures are consistently in the low 50°Fs before you start hardening off leafy greens and other cool-weather plants, or in the high 50°Fs for warm-weather plants like tomatoes, peppers, and squash.
In some cases you can start to harden off seedlings earlier, but they’ll need some kind of frost protection at night, like frost covers, cold frames, cloches, or unheated greenhouses.
How to harden off seedlings in 7 days
Day 0: Start hardening off seedlings when they’re a couple inches tall with their first set of true leaves.
That’s right: the process of hardening off seedlings should start indoors first.
Every day, as soon as your seeds germinate, brush your hand back and forth across the seedlings to simulate a breeze. This simple motion strengthens their stems and prepares them for the stronger breezes they’ll face in the garden.
Day 1: Set your seedlings outside in dappled or partial sun for 2 to 3 hours.
Make sure the area is free of wind and above 60°F. This could be under an eave, a covered porch, or a shady tree.
Give your seedlings only 2 to 3 hours of sun in the afternoon (the warmest part of the day) and be sure to check their moisture levels, as seedlings have a tendency to dry out quickly once they’re outside.
Bring them back inside for the night. The next few days is what I like to call the “spring shuffle” as your seedlings move in and out of the house.
Day 2: Set your seedlings outside in partial to full sun for 3 hours.
The next afternoon, put the seedlings outside for 3 hours in partial to full sun. Somewhere with a light breeze is also good for them at this point.
Be sure to watch the weather for any dramatic changes, and don’t forget to bring them back inside.
Day 3: Set your seedlings outside in full sun for 4 hours.
Put them someplace warm with a soft breeze, if possible, and make sure they don’t sit dry and wilted for a prolonged period of time.
Bring the seedlings inside for the night.
Day 4: Set your seedlings outside in full sun for 5 to 6 hours.
Bring them outdoors earlier in the day so they experience some cooler temperatures, and give them 5 to 6 full hours of direct sun.
Keep the potting soil moist, and bring the seedlings inside at the end of the day.
Day 5: Set your seedlings outside in full sun all day.
Leave your seedlings outside all day in the sun and the breeze. They’ll most likely need to be watered at least once if it’s a very warm day, so be sure to keep an eye on them.
Also — speaking from personal experience — you’ll want to watch for garden visitors that might think your seedlings are a free buffet. Squirrels, birds, and rabbits have a tendency to take a nibble if your seedlings are looking especially tasty.
Before it gets dark, bring the seedlings back inside.
Day 6: Set your seedlings outside all day and all night.
Today will be their first sleepover in the garden! Leave your seedlings outside in full sun and through the night as long as temperatures stay well above freezing.
It’s important to keep seedlings off the ground, as mice, voles, and other pests have an appetite for tender new growth and will be quick to decimate your seedlings overnight.
Day 7: Graduation day!
Your seedlings are hardened off and can be transplanted to their permanent location in the garden, whether it’s in soil or a larger container outside.
Your best chance at transplant success is on a cloudy day (to give seedlings time to adjust to yet another new environment), though at this stage, they should be able to take direct sun, spring rains, and gentle winds.
If frost is expected over the next week or two, be sure to protect your plants with frost cover. Unfortunately with hail and strong, cold winds, your plants can suffer damage, no matter how carefully you’ve hardened them off.
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How to harden off seedlings in less than 7 days with these garden shortcuts
Sometimes, you can shorten the amount of time it takes to harden off seedlings. I don’t recommend skipping this important step, but if you’re pressed for time or need to move those seedlings out of your kitchen stat, you can try one of these shortcuts.
Shortcut #1: Transplant your seedlings outside during a period of cloudy weather.
If the weather is on your side, with at least a week or two of mild, overcast days, you can plant your seedlings in the soil immediately.
This method is risky, especially for climates that have dramatic shifts between day and night (like we do here in Central Oregon) or higher elevations with blazing sun.
But if you live in a mild climate and are confident of your local weather patterns, you can save at least a week by transplanting your seedlings as soon as they have their first true leaves.
Shortcut #2: Use a portable “mini greenhouse” to protect your seedlings outside.
I’m a big fan of Wall-o-Waters, which look like water-filled teepees that you place around your plants. I’ve seen red versions and green versions, but they’re pretty much the same product and you’ll need for each plant. (You can see them in action here when I grew tomatoes in containers.)
The Wall-o-Water collects heat from the sun during the day (while also protecting small seedlings from wind) and radiates the heat back out at night, keeping them snug and warm.
By the time the seedling outgrows the Wall-o-Water, it’s hardened off naturally — all without you doing the spring shuffle.
This post updated from an article that originally appeared on March 24, 2014.