What does “Hardening Off” Mean?

What does “Hardening Off” Mean?

If you are new to gardening, you probably know that getting some seeds started indoors is a great way to ensure your plants get a healthy start where disease, weather, and pests will not hinder their beginnings. While this is true, there is a real lack of explanation out there about the next step in gardening, which is planting those seedlings in your harden, and the work that needs to go into their transition to make sure they actually survive!

Why do we Need to Harden Off?

All those cute seedlings under lights or in a sunny windowsill in your house are positively pampered compared to the plants that start life outside. Inside your home, those seedlings can expect a nice 65-70 degrees, day or night, no wind, no bugs, and consistent moisture. There is a HUGE difference in conditions between inside and outside, and you need to prepare your plants for that move as slowly and deliberately as possible.

Beginner gardeners will often just transplant indoor plants outside, and then wonder why they shrivel in the sun, snap in half on a windy day, or just fail to thrive outside. Plants grown in ideal conditions indoors must ALWAYS be hardened off before being transplanted outside.

Hardening Off

Hardening off is the process, usually taking 10 to 14 days, by which you slowly introduce outdoor conditions to indoor-grown plants.

Yes, hardening off is tedious.

Yes, it’s a pain in the patootie to keep moving your plants outside and then back in.

Yes, you have to do it, or your plants will suffer.

First, Start them Off Stronger

To aid in the later hardening off process, there are things you can do while the seedlings are still very young and living indoors. 

You can point a fan to blow air across them, to get them used to wind and to force them to grow more sturdy, thicker stems. In Brisbane, where we get very string wind as the fog rolls over the mountain in to San Francisco, this is a great way to get plants strong ahead of time. Leggy plants + wind = broken plants.

Letting established seedlings, like those with a few sets of true leaves, dry out a bit between waterings is another way to prepare your indoor plants for a very hot day in the sun outside, when they have to hold off until water comes again.

Next, Make a Hardening Off Schedule

When you’re almost ready to plant outside, such as when you know based on the Bay Area Planting Calendar that it will be optimal timing in about 2 weeks, you should start hardening off your plants. This means moving them outside into the shade or the sun, depending on what they are ready for.

Next, on a piece of paper or on your computer, start your Hardening Off Schedule. You can do it two ways:     

  1. Create a schedule ahead of time that you will keep to
  2. Record what you do to harden off your plants each day, and use your plants’ condition as a guide as to what to do next.

When I do a schedule ahead of time, it looks something like this:
Day 1: 2 Hours in Shade
Day 2: 4 Hours in Shade
Day 3: 1 Hour in Sun, 3 in Shade
Day 4: 2 Hours in Sun, 4 in Shade
Day 5: 4 Hours in Sun, 6 in Shade
Day 6: 6 Hours in Sun
Day 7: 8 Hours in Sun
Day 8: 12 Hours in Sun
Day 9: All Day in Sun
Day 10: All Day in Sun, stay outside overnight

What the make-ahead schedule does not account for is weather conditions. A foggy day is great if you’re on day 2, but unhelpful on day 7. Likewise, if the wind is blowing at hurricane speeds and you’re on day 3, you need to bring the plants in early and try again tomorrow. You may need to adjust your schedule if you don’t get the best conditions for the steps.

Finally, Stick to your Schedule

You can’t give up halfway and start from there a week later, because your plants will acclimate to their indoor digs again. The process needs to continue once you start, or you lose progress.

Take your plants out every day, placing them in the correct conditions for the given day, making sure they are sufficiently watered before or after they go outside. I take mine out in their trays to make it easier to move them (as you can see from the photo at the top of this post).


What is nice is that once you’ve hardened off your plants and they are fully accustomed to life outside, they can stay outside forever. If they are not root-bound, you can put off transplanting until you and the garden are ready to do it. You can transplant them, and, as long as you keep tending them with care and water, they should be happy in their new garden spot.

What if I Can’t Spend 2 Weeks as a Plant Chauffeur?

Yes, this schedule is rough if you travel to a job all day and cannot spend all your time shuttling plants around, but you also don’t want to come home to find crispy plants. While it’s a bit tricky, it’s not impossible to adjust the schedule to work for you.

Days 1 & 2: Find a great shady, secluded spot that gets NO direct sun all day and leave your plants there. Yes, less time outside would be ideal at the beginning, but shade is hardly the worst thing your plant will experience.

Days 2-5: These are trickier, as you need to know your outside space well to know where you have some morning sun but afternoon shade. If you can calculate it by watching some spots before you start the process, you could successfully leave your plants outside in a sunny spot that, an hour or several hours later, is in shade. another option is to aim for the bulk of these days to fall on a weekend so it’s easier to manage the tricky amounts of sun.

Days 6-8: Like days 2-5, you can find those spots that will work to give you the sunlight your plants need and not too much more.

Days 9-10: These should be easy to work around a normal schedule.

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