How to harden off seedlings

Picture this, you have lovingly grown and tended to your seedlings over the course of a month or so. Visions of bountiful harvests dance in your brain. You proudly take your wee babies outside and lovingly plant them in your well-tended garden. You go to bed proud of your accomplishment and the next day you come out and it looks like a massacre. Dead! All dead! WHHHHYYY????

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Chances are you didn’t know you needed to harden off seedlings, which sucks. Or maybe you did but decided to roll the dice to “save” some time. Without this step, it’s like turning a coddled child out into the world. Completely unprepared for the harsh reality of the real world. So it’s crucial that you don’t skip this step. It takes a bit of time, but not too much and it’s worth the effort in order to avoid the above-mentioned plant massacre.

How difficult is it to harden off seedlings?

Hardening off seedlings isn’t really that hard at all, it just takes patience and discipline. Neither of these are my strong suits, but hey it has to get done and so I do it. So let’s take a look at what it is and why it is such a crucial step. I currently make twelve trips from the basement to the back deck each day Is it a pain in the ass? Yeah for sure! But it’s such an important part of starting your own seedlings. As a bonus, I get some extra exercise which is a good thing.

What is hardening off?

“Hardening off” is pretty much the process of moving plants outdoors for a portion of the day to gradually introduce them to the direct sunlight, dry air, and cold nights. It is a reality check for your plants about what they will be up against when they leave their cushy home. It is important that this process be done gradually. If you rush it then your efforts will be for naught and your seedlings will most likely be dead. They need a gradual introduction into the elements so that they can become accustomed to strong sunlight, cool nights and less-frequent watering. Typically the hardening off process takes place over a 7-10 day period.

Why harden off seedlings?

To put it mildly, if you start seedlings inside and you don’t harden them off, then chances are they are going to die when you move them outside. Either the sun will fry them, the wind will snap them or the chill will do them in. The point of hardening them off is to gradually introduce them to all three so that its not a shock to their little systems. You need to toughen them up by thickening the cuticle on the leaves.

This helps to make it so that the leaves lose less water when exposed to the elements. Ultimately this prevents transplant shock, where you watch your seedlings wither and die or become stunted from the sudden changes in temperature. And seriously, nobody wants that…so this method is your insurance policy for all your hard work.

What else can you do?

Two things I do to help prep for the hardening off process are to use a fan and a tray. All you need is a cheap oscillating fan. I have it on oscillate, on very low, from the time I plant my seeds. This way the seedlings are used to a breeze from the second they emerge from the soil.

When my seedlings are about 3-4 weeks old, I take them out of my Aerogarden and put them into the soil. When I do this, I put all the pots into one of those big trays you get from the nursery. Any tray would work, I just like those ones. So grab a tray and load up your little babies. This will make it easier to cart them in and out during the hardening off process.

When to start to harden off seeding

I like to start the week BEFORE the last frost date. If you are not sure what yours is you can check here if you are in Canada or here if you are in the States. I take a week to harden off my seedlings, so starting the hardening off process the week before the last frost date ensures I give them the longest growing season possible. We have such a short season in Canada, so every day counts!

If you ever struggle with knowing when is the right time to start your seedlings or harvest your crop, then check out my simple Seed Starting and Harvest Guide.

How to harden off seedlings?

There are three main ways that you can employ in order to hardening off your seedlings. They are as follows:

1. Gradually exposing them to sunlight, wind and cooler temperatures
2. Placing them in a cold frame (stay tuned for a future post on how to make a cold frame)
3. Withholding water (but only for a bit)

Option 1 – Gradually expose them to the elements

Some people suggest doing the hardening off over a period of 4 days. Because I start a week before the last frost date I like to hedge my bets and stretch it to 7 days just to be safe. One little suggestion I have is that you set an alarm each day so that you don’t get the deer in the headlights look when you realize you left them out waaaaaay too long!

***cough, cough***

I am sad to admit that I made this mistake before, but it is definitely one you only make once. So let’s jump into the day by day plan to get your babies out into the real world.

Day by day guide to harden off seedlings with gradual exposure

Day One: Set your tray(s) out in the shade, in an area that protects them from wind and direct sun. Leave them for about 3 hours, then bring the tray(s) back inside.

Day Two: Set your tray(s) out with access to a little sun, but mostly shade. Today leave them out for about 6 hours and then bring them back inside.

Day Three: Set your tray(s) outside, yet again, but give them a bit more access to direct sunlight. Leave them out for 9 hours and then bring them in for the night.

Day Four: Today is the day they get to experience a little bit of sun, but not too much! They can also be a little less sheltered from the wind today. They can stay out for 9 hours again today but they still need to come in at night.

Day Five: Same as Day 4, but the 9 hours can include about 1-2 hours of direct sunlight (don’t lose track of time, set a timer!).  Bring them back inside for the night.

Day Six: This is a big day. It’s going to be the first time that you leave the babies out all night. During the day give them a bit more sun and as long as the temperature isn’t going to drop crazy low, leave them outside.

Day Seven:  Today let them have sun most of the day and leave them outside for the night.

Day Eight: This is it, today is the day you plant your plants outside.  It is best if today is a cloudy day, but you get whatever Mother Nature gives you. Just make sure to water well after planting. A great way to get the most from you space and seedlings is to use square foot gardening.

Option 2 – Using a Cold Frame 

Another option is to place your plants in a cold frame. Currently, I don’t have one but I am on the hunt for some bricks to attempt to build a DIY cold frame. My dad already gave me some old windows that are stashed in the garage. I will be using them along with the bricks to make my cold frame. This method is handy in that you don’t have to shlep the plants in and out every day. So let’s take a look at what’s involved.

I would start this process the same way I did with gradual exposure. Starting a week before the last frost date so that you give your plants the maximum time outside for the growing season. So make sure to determine your last frost date and then work back a week.

Day by day guide to harden off seedlings in a cold frame

Day One: Move your seedlings out to the cold frame. Make sure that the temperature in the cold frame stays above 10C (50F) and below 27C (80F).

Day Two: Today you will need to open the cold frame for about 3-4 hours. It is also very important to make sure to check the soil daily while they are in this warm, sheltered environment. You don’t want them to dry out!

Day Three: Open the cold frame for 4-5 hours, make sure to close it at night.

Day Four: Open the cold frame for 5-6 hours, make sure to close it at night.

Day Five: Opens the cold frame for 6-7 hours (you see the pattern here right?), make sure to close it at night.

Day Six: Open the cold frame for 7-8 hours, make sure to close it at night.

Day Seven: Open the cold frame from 8-9 hours, make sure to close it at night.

Day Eight: Move those babies into their permanent location. As with the gradual method it is best if this is done on a cloudy day. Make sure to water them well.

Option 3 – Withholding Water

This is my least favourite method to harden off seedlings. But in the interest of full disclosure this is an option and some people swear by it. So let’s take a look at what is involved. It may seem counterintuitive, but apparently allowing seedlings to wilt has the same effect as gradually exposing them to the elements. Weird right?

1. This method starts a week earlier than the other two methods. You will want to start about 2 weeks before your transplant date. On this fateful day stop watering your seedlings. Yup…no water.
2. Watch them closely looking for signs that they are starting to wilt. You need to pay really close attention with this method as you do not want them o be dry and wilted for too long.
3. Drying out can happen REALLY quickly if they are in small seedling pots (be warned).
4. As soon as they start to wilt, you can water them again.
5. Once they perk up then you need to withhold water again until they wilt again.
6. After 2 weeks of this process, seedlings should be ready to transplant.
7. As with the other methods, transplanting to their final locations should be done on a cloudy day if possible.

I’m not going to lie, this method seems a bit like torturing the poor babies, but to each their own. I normally try to keep a few plants black regardless of the hardening off method, just in case something goes wrong. I never like to put all my eggs in one basket….except I don’t have eggs because I’m not allowed to have chicken ***sniff***

So regardless of the method that you use to harden off seedlings, just make sure you do it. After all the work you put in getting them started you want to make sure you give them the best chance possible. Another important aspect to making your suburban homestead a success is to give them what they need once you plant them outside. Consider using square foot gardening as it is your best friend when you don’t have a lot of space to work with. Also, make sure they have good soil including lots of good compost to help them grow strong.

How do you hardened off seedlings? Do you just start them outside?

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