In my Basics of Growing from Seed article, I mention that you MUST harden off your seedlings before planting outside. Since then, I’ve received a lot of questions about what this means exactly or how I do it. Therefore, I thought I would write down my process for hardening off seedlings and share it with you. Keep in mind that every gardener has their own process, so this is just how Sam and I harden off our seedlings.

Did you start your seedlings indoors or in a greenhouse?

If not, then the hardening off process does not apply to you! For example, if you direct sowed your seedlings in the garden or if you started them in cells outside, you do not need to harden off your seedlings because they have already adjusted to the outdoor elements.

Hardening off your seedlings will protect them from going into transplant shock. In general, the term transplant shock refers to any kind of stresses your plant will experience when moved, which can result in poor establishment, weak root development, or even death. This can happen when you are simply moving plants in your garden or changing containers, but right now we will be discussing the move from indoors/greenhouse to your garden.

For example, if your seedlings were only grown under grow lights and you suddenly put them in the sun without hardening them off, their leaves can burn and will be unable to photosynthesize—leading to death.

When should I start hardening off my seedlings?

The actual process of hardening off, should occur approximately 7-14 days before transplanting out in the garden. Usually, you want to give them ample time to adjust.

Additionally, you typically want to start strengthening your seedlings weeks before they go out into the garden (while they are still young plant starts). Honestly, a successful hardening off process starts with the strongest seedlings possible, so follow my tips HERE and your seedlings will be off to a great start!

Things to check before hardening off seedlings

Are you out of frost danger? You can look up your last frost date here but please keep in mind that weather is different every year, so you can’t depend solely on that date. Keep up on local weather and future forecasts for the current year.

Look at your weather forecast. Will there be any high winds? Rain? or cold swings? If any of these things are in the upcoming forecast, do not start hardening off your seedlings. You want to wait until the weather looks mild and gentle so that your seedlings have an opportunity to adjust.

Lighting. Your seedlings will still need to be brought inside (or in a greenhouse) when they are not outside hardening off, and whatever light setup you were using needs to stay the same. You don’t want to take away their usual light source all of a sudden. For my seedlings I usually keep my lights on for about 12 hours a day, but gradually decrease the length of light to better match daylight hours the closer we get to transplant day.

Heat. If you use a heat mat, you should be gradually weaning your seedlings off of them in the weeks prior to starting the harden off process. Their root systems do not need to be kept as warm because eventually they will be going into soil that’s most likely much lower. 

Watering. It’s time to stop babying your seedlings! Remember how you carefully monitored the moisture while they were germinating and growing their first set of leaves? As you start the hardening off process, don’t water your seedlings as much. It is highly beneficial for them to struggle just a little bit. I’m not saying to let them dry out completely, but let them start to get drier than usual so their roots wants to reach out and search for water. This makes them stronger!

10-Day Plan for Hardening Off Seedlings

*Note: morning and late afternoon sun tends to be less harsh than midday sun. Also, this is just a guideline, so don’t feel bad if you don’t follow it exactly. Sometimes we have to tweak it depending on work schedules, etc.

Day One-Put your plants outdoors in a shaded area for a couple hours.

Day Two– Put your plants outdoors in a shaded area for four hours.

Day Three-Put plants outdoors in a shaded area for two hours, and then move to a partial sun area for an additional 1-2 hours.

Day Four-Put plants outdoors in partial sun for four hours.

Day Five-Repeat Day Four.

Day Six-Put plants in partial sun for 6-8 hours.

Day Seven-Repeat Day six, but go for later in the day. Afternoon sun tends to be a bit stronger.

Day Eight-Leave plants out all day (not in direct sun!) and for a little bit into the evening hours. This allows them to feel some of the cooler evening temps.

Day Nine– Leave plants out all day and all night. Make sure your night temps aren’t too cold.

Day Ten-Repeat Day Nine and assess the weather for transplanting into the garden.

Finally, keep in mind that everyone has different climates, so it’s important to watch your seedlings and keep an eye out for stress. White spots on leaves or curling can mean that your seedlings are being shocked. If this happens, maybe repeat that day and adjust accordingly.

And that’s how we harden off our seedlings!