How to harden off seedlings (don't skip this!)

If you've been growing your seedlings indoors for the past few months like I have, then I'm sure you're also anxiously counting the days until you can plant them outside. I've been hardening off my seedlings for about two weeks now, which is an important step before they can go into the garden. Transplanting them without first hardening off can result in damaged and struggling plants.

Plants on porch getting used to outside weather, aka hardening off.

To harden off seedlings is to gradually expose them to the environment they will be living in. Even though you may have your seedlings growing under bright grow lights inside your house, they are nowhere near ready for a full sun garden! They are also probably weaker than they should be.

Why harden off seedlings?

Lets face it, the climate inside your house isn't too exciting for a plant. No sudden winds, no rain, no temperature fluctuations and no burning sun. Outside though, well that's another story entirely...and that is exactly what we need to get these plants ready for.

Outside the plants will have to deal sun, rain, cold and high winds and the only way to get them used to that, it to take them outside. You have to do it slowly though, or they run the risk of getting sunburned or possibly wind torn. If you don't take the time to get them used to it first, they could die from the sudden exposure to the elements. Wind can easily break the stems of plants that aren't used to it. 

Plants can suffer from sunburn if suddenly exposed to full sun after sprouting inside. While a plant can recover from a mild case of sunburn, they will be spending the next few weeks healing instead of growing and you'll find these plants slightly behind others that have not experienced sunburn.

Here is how to acclimate your seedlings to the outside climate without stressing them.

When to start hardening seedlings?

This should take about 2 weeks to do, so you want to start about 2 weeks before you plan on transplanting them into your garden. You don't want to transplant cold sensitive plants to the garden until the chance of frost has passed. Here is How to determine your last frost date

Once you have that date simply count back 2 weeks to determine when you should start hardening your plants off.

You want to give yourself the full 2 weeks in case there's a day or so that you can't take the plants outside because of weather or other issues. If you do manage to get them out every single day, you can probably get it done in 10 days though. 

Seedlings and transplants sitting on a sunny porch.

How to harden off seedlings before transplanting

You'll want to wait till daytime temps are at least 50°F on a consistent basis. You'll start by taking your seedlings outside on a nice day. Put them in a bright area for an hour or two. Bright but not direct sun. Make sure it's not a windy day, as the stems haven't built up their strength yet. Don't choose a rainy day least not yet. 

I like to set them on my porch where the sun hits them as it's going down, but not from directly above them. The picture above is from 3:45 pm, note the angle of the shadows the sun is creating. Leave them out for an hour or two, then bring them back in. Do this for 2-3 days adding and extra hour every day.

Overcast days are great for hardening off seedlings, as long as it's warm out. In fact, you can allow your plants more time outside without worry of burn on an overcast day. While this doesn't acclimate them to the sun, they are still getting exposure to wind and temperature fluctuations. 

After a few days start putting your seedlings in direct sun for an hour, then moving them back to a less direct sun area for another few hours. Slight winds and a light drizzle are ok, but bring the plants in if it gets really windy or starts pouring rain.

Check plants often for signs of sunburn: white or brown spots on leaves.

Each day you will extend the amount of time the seedlings are outside in full sun until they are out for around 8 hours. As long as your nighttime temperatures are warm enough, your final step before transplanting is to let your seedlings stay out overnight. 

Once your plants are staying outside for 24 hours a day, they are ready to be transplanted into the garden.

A few tips to make this easier:

Some people like to put a fan on low on their seedlings for a few hours a day while they're still inside. This helps to get them used to normal winds outside. If you do this remember to turn the trays or move the fan daily so the 'wind' hits them from all sides.

Don't forget to check the soil moisture before taking the seedlings out and during their time outside. The soil will dry out more quickly outside in those little pots, especially in the sun.

Bring the seedlings inside immediately if the wind gets severe as this can break the stems. 

Even after your seedlings are completely hardened off and transplanted into the garden you'll still need to protect them from frost. Keep an eye on the weather forecast and if it is predicted to go below 34°F you'll want to place some type of cover over them to keep them safe. 

In case of frost, I just use a piece of plastic sheeting and some hula hoops. It's simple but does the job and it's cheap for when you suddenly have to cover the whole garden real quick! Instructions here: Quick and easy low tunnel to protect plants.

Seedlings in a cold frame greenhouse

Hardening off seedlings using a cold frame

A cold frame can cut down on some of the work of hardening off the seedlings. I move my seedlings to a portable cold frame on about day 3 of hardening them off. After that simply open the top for more time each day. You no longer need to bring them in each night or during rain since the cold frame will keep them protected. 

The frosted panels keep the seedlings from being exposed to direct sun when closed, though it does still get warm inside and will need vented. Keep aware of frost though as that can still damage delicate seedlings. 

You still need to keep an eye on the plants, but you no longer need to carry them in and out each day.

Do all plants need hardened off?

No. If you bought your plants from the garden center they should not need to be hardened off. Most greenhouses have industrial fans, windows they open up for cross ventilation and are under direct sun, so your plants should already be somewhat acclimated ot the outside weather. Most home stores display their plants outside where they are exposed to regular weather. 

It's only when you're bringing plants out to a climate they are not accustomed to that they need hardened off. If you take houseplants  outside for summer you will want to bring them out gradually also, or risk sun and wind damage.

Want to learn more about growing your own food? Click here for my other posts on growing your own food, herbs and flowers.


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