How to Harden Off Your Seedlings

I remember when I first started gardening from seed, one of my biggest struggles was keeping my seedlings alive when I planted them outside.

They looked so healthy inside. They were growing, their leaves looked great, some of them were even starting to produce flowers! But then I would put them outside and within a day they would be completely wilted, their leaves would be blotchy and within three days: half of my seedlings would be dead and the other half would look like they just survived a war.

What was I doing wrong? Was it the soil? Was I not watering them enough? Was I watering them too much? This never happened when I planted out seedlings I bought from the nursery.

It turns out the reason for my failure was that I wasn’t giving my plants enough time to acclimate to the outside world. Plants, just like animals, need time to adjust to a new environment.

This acclimatization process is called “hardening off” and it is time consuming, but really easy to do and so totally worth it.

My seedlings on my front patio on an overcast day.

5-10 days before you plan to permanently settle your seedlings into the soil, start bringing them outside for field trips. The first day, the field trip should be 1-2 hours in a shady location with no wind. After 1-2 hours, bring your seedlings back inside. If your plants do well, increase the amount of time on day 2. Normally on day 4-5 I’ll move my seedlings into an area that receives partial sun and allow them 1-2 hours of direct sunlight followed by some more outside shade. Slowly but surely give your plants more time in the sun and elements each day until they can withstand the conditions of their permanent home. The final day before planting, let them sit outside in their little grow-out pots overnight.

Sounds simple right? There are some signs that you should look out for to know that your seedlings are not ready to be permanently placed outside.

If your plants are incredibly wilted, floppy, and soft after their time outside, then they are not ready to be permanently planted outside.

If your plants have white marks on their leaves, this is sunburn or “sunscald,” and your plants are not ready to be planted permanently outside.

Sunburn (aka sunscald) on my tomatillo seedlings.

If your plants fall over in the slightest breeze, your plants are not ready to be planted permanently outside.

If you run into any of these problems here’s what you can do:

Plant is overly wilted: Bring the plant back inside, give it a good water and keep it inside until it has perked back up, this may be multiple days. If you have already planted it outside, add a shade cloth (I use a tablecloth) to protect it from the direct sun of the afternoon.
Plant is sunburned: Bring the plant back inside, give it a good water, and keep the plant in the shade for the next few days when you bring it outside. Start introducing the sun again at a slower pace. If you have already planted it outside, add a shade cloth (I use a tablecloth) to protect it from the direct sun of the afternoon.
Plant can’t handle wind: When you bring your plant back inside add a small fan to your seed-starting station and allow the plants to feel a slight breeze. Alternatively, move your seed-starting station to an open window and allow the natural wind to do the work for you. Do not put your seedlings outside on super windy days.

I use table cloths as shade cloths with newly planted seedlings on unseasonably hot days.

One last note: the beginning of the hardening off process is going to be very stressful for your seedlings. You are going to notice that they stop growing as fast and that their soil is very dry after each adventure outside. This is normal! Give them a good drink via bottom-watering and once they’re acclimated to the outside world and in their permanent home, you’ll see their growth rate increase again.

Growing from seed is one of the most rewarding parts of gardening and also one of the best budget gardening methods if you’re successful. With a little bit of patience you can avoid mistakes that will cost you your harvest and create a beautiful and abundant growing space.

Good luck!