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(Last Updated On: April 8, 2022)
You get really excited when your plants start to grow in the garden if you’re anything like me, and hardening off plants is decidedly BORING.
But if you want those plants that you worked so hard to grow to not just survive, but thrive, you need to take the time to harden them off.
What is hardening off? It’s a process that helps prepare your plants for life outside of the protective environment of your home.
This blog post covers everything you need to know about hardening off plants.
What Does Hardening Off a Plant Mean?
Some gardeners are familiar with the process of hardening off plants, but many are not.
Hardening off plants simply refers to the acclimation and gradual exposure of young seedlings to outdoor conditions.
Seedlings can be really delicate after spending several weeks indoors under artificial lights. They need to be slowly introduced to the harsher conditions of the outdoors before you plant them in your vegetable garden.
If you transplant seedlings directly into the garden without hardening off, they will experience extreme transplant shock and can quickly wilt, stunt, or die.
The hardening-off process usually takes about a week and involves gradually increasing the number of hours your delicate seedlings spend outside each day.
By the end of the week, they should be able to tolerate being outside for the entire day.
With proper hardening off, seedlings will be strong enough to transplant into the garden and will have a much higher chance of survival.
How Long Do You Need To Harden Off Plants?
The process of hardening off usually takes at least a week, sometimes ten days.
What Happens if You Don't Harden Off Plants?
Here’s a sad example of what happens if you don’t harden off your plants.
I put these snapdragons outside on the first day of hardening them off, but I forgot about bringing them in that day.
As an experiment, I decided to just leave them outside from that point on to see what would happen to them.
As you can see, many of them wilted and died almost immediately. Others remain alive, but are sunscalded.
What is the Best Way to Harden Off Tender Plants?
There are many ways to do this, and everyone has their own particular way to do it. This is one way that works for me.
Increase Outdoor Exposure Each Day
The key to hardening off your seedlings is to gradually increase their time outside so they can toughen up.
When we think of hardening off, we usually think of how the plants need to toughen up in response to direct sunlight.
But there are also other elements that your seedlings need to be exposed to in order for them to thrive once they are planted outside.
Temperatures and wind also add to the sun exposure to make hardening off a very necessary process.
Place your seedlings outside in the shade for up to 2 hours the first day. Starting on a cloudy day is a great idea, but if you don’t have a cloudy day just choose a sheltered position in a shaded location.
You’ll need to protect the seedlings you started indoors from harsh winds and sun during the first few days, so be sure they are in a sheltered location.
After the 2 hours is up, bring the seedlings inside and place them right back where they were inside.
TIP: Be sure to keep track of weather during hardening periods. Temperatures may drop rapidly in unseasonally low levels, and wind damage can knock over seedlings and tear the foliage.
Day 2 of Hardening Off Plants
The next day, increase the time outside to 4 hours, with one of those hours in full sun.
Day 3 of Hardening Off Plants
On the third day, increase the time outside to 6 hours, with at least 2 hours in full sun.
This process does not have to be followed to the letter, but you should be gradually increasing the amount of time the seedlings spend both outside and in the sun.
Be mindful of the wind and air temperature as well.
Try not to increase exposure to higher winds, higher temps AND more direct sunlight all in one day until around day 5. By that time the seedlings should have toughened up enough to handle that exposure level.
Increase Outdoor Exposure Each Day Days 4-7
Gradually increase the time spent outside by the seedlings.
Seedlings should be outside for 8 hours on this day, and half of that time should be overnight.
Instead of putting your seedlings out first thing in the morning, you might decide to wait until noon so you can put them out during a hotter part of the day. They can then transition from warm daytime conditions into cooler evening conditions during a single 8-hour period.
Put your seedlings out for between 8-12 hours on day five. Because that is close to a 12 hour period, you will probably put them out first thing in the morning and bring them in 12 hours later. You can leave the seedlings out unprotected for the most part, because they are pretty tough at this point.
Continue the process for hardening seedlings by moving them outside while temperatures are warm.
At this point, you are almost ready to transplant your seedlings. All you have left to do is work your way up to a full 24 hour period of hardening off for your seedlings.
On Day 6, you can put them outside first thing in the morning, and leave them out until 10 or 11 pm.
On days 7-10, go ahead and leave them out all night long. Be sure to inspect the foliage after they’ve been out for 24 hours to be sure that you don’t have pest damage or damage from exposure.
Leave them outside for 2 or 3 24-hour periods if possible, but don’t sweat it if you can only leave them out one time overnight.
Introduce Plants to Cool Nights Gradually
Don’t forget to introduce your seedlings to cool nights as well. Having your seedlings see the temperature difference between day and night is key in their development and hardening.
When Should I Start Hardening Off Plants?
Count 10 days back from your desired transplant date and that will be the date you should begin exposing your young seedlings to outside elements. You want to plant them after their first frost date for most plants.
What Happens if I Have to Stop Hardening Off Plants Mid-Way Through?
Sometimes you aren’t able to harden off completely at one time. It could be that the temperatures turn very cold or you have to travel and aren’t able to complete the process.
When that happens, don’t ever cut the process short. You will never regret taking the proper amount of time to harden off, but you will regret planting full trays of weak plants that end up losing weeks of growth because they are stunted.
Keep Seedlings Indoors Until You Can Continue the Process
If you have to cut hardening off short, just bring them back inside under a grow light and treat them the same way you had been before you started. When the coast is clear or the calendar allows, start over.
Can you harden off seedlings too early?
Hardening off your seedlings too early in good weather can cause them to become stressed and more susceptible to pests and diseases. If they are hardened off but aren’t transplanted, they can begin to get root-bound or stunted in their containers. If you harden off your seedlings in weather that’s too cool, you run the risk of stunting them and killing them.
How do you harden plants quickly?
There is no way to rush the process, and why would you want to? You’ve spent anywhere from 4-8 weeks or longer growing your seedlings with love and care. Why skimp on this part of the process just to stunt all your seedlings?
Do plants from a nursery need to be hardened off?
No. They are already hardened off. The nursery would not take the chance of leaving that up to the consumer, risking returns.
Can I harden off plants in an unheated greenhouse?
For the night exposure, it would be fine, but I wouldn’t recommend it for daytime exposure. The temperatures inside the greenhouse during the day would reach 90 degrees plus, burn your seedlings, and likely kill them.
How long does it take to harden off in a cold frame?
You can use a cold frame to store your plants overnight during the week it takes to get them ready to transplant. This is a great option if you don’t want to drag them all back inside every night during the hardening-off process. Just be sure that the temperature inside the cold frame does not go below freezing. And don’t forget to take your plants out of the cold frame before the temperature heats up past 85-90 degrees or you’ll stunt/kill them.
Using a cold frame doesn’t change the amount of time it takes to harden off your plants, but it is something that can streamline the process.
Tips for Hardening Off Plants
Here are some tips for hardening off plants:
Keep Track of the Weather During the Hardening Off Period
Use a weather app to track the amount of sun, wind, and temperatures. Try to start the period on a cloudy or overcast day. This way, if you forget the seedlings outside, you won’t do a ton of damage to the seedlings.
Use Reminders to Put Out and Bring In Seedlings on Time
Technology is your friend when it comes to hardening off seedlings! Use your phone’s timer function or reminder features to alert you when it’s time to put out your seedlings in the morning and bring in your seedlings in the evening.
Start Over if You Can't Finish the Process
If you can’t finish the process, put your seedlings back inside and restart the process when you are able to finish it completely. You’ll be rewarded with stronger plants when you do this!
Hardening off plants is necessary for the gardening process, allowing your plants to slowly adjust to outdoor conditions before being transplanted into the ground.
By hardening off your plants properly, you can help them thrive in their new environment and avoid common problems like stunting or disease.
I hope that this article has provided you with all the information you need to get started hardening off your plants. Happy gardening!
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