Picture this: It’s a beautiful Spring morning. The sun’s shining and your seedlings, lovingly grown under the perfect conditions for weeks, are ready to make the move outside. You transplant them into the perfect spot in your garden, water generously, and walk away to let mother nature do her thing.

Only to be crushed to find weak or dying plants at the end of the day. And with them your dreams of Spring salads and fresh Summer tomatoes.

We’ve all been there. The hardening off step was skipped and the plants in our garden paid the price.

The thing is, hardening off can feel so fiddly and unimportant. So easy to skip. They’re plants for goodness sake, surely they’ll benefit from a sunny day outside, right?

Nope. Hardening off, as annoying as it can be, is a true requirement for any plant grown under controlled conditions like those on an indoor seedling rack or in a greenhouse.

And it makes sense too when we look at the numbers. I just compared DLI values (a measurement of light hitting the plants over a designated time period) from my indoor seedling setup with strong LED lights to the overcast Winter day outside where I have a number of plants currently hardening off. 12 for indoors compared to 22 outdoors!

Just imagine how much greater the disparity would be on a clear Spring afternoon. How can any plant be expected to withstand that kind of jump without any help?

What is Hardening Off?

Hardening off is a step in the growing process where plants grown under controlled conditions are gradually and intentionally acclimated to the environmental factors outdoors prior to transplant. Factors like strong sunlight, wind, wider temperature bands, and fluctuations in temperature and humidity levels. It is achieved by slowly increasing the duration and intensity of exposure to these factors over a period of multiple days.

Hardening off is an absolute requirement as it allows seedlings to build their defenses in a manner that doesn’t stunt or entirely halt long term growth (although growth is impacted in the short term). If you’d like to learn more about the biology going on during hardening off, check out this short blog by the University of Indiana Urbana-Champaign extension office.

How to Harden Off Plants Prior to Transplant

The traditional method for hardening off plants is simple: place your seedling trays outside for a short window of time during the day and increase the duration until plants are ready for transplant. Usually that translates to a schedule with linearly increasing exposure durations like this:

Day 0: Acknowledge that your seedlings need to be moved outside or risk becoming rootbound

Day 1: 1 hour outdoors in a protected location like the porch

Day 2: 2 hours outdoors in a protected location like the porch

Day 3: 3 hours outdoors in a protected location like the porch

Day 4: 4 hours outdoors in a dappled sun location

Day 5: 5 hours outdoors in a dappled sun location

Day 6: 6 hours outdoors in a dappled sun location

Day 7: 7 hours outdoors in conditions matching their transplant location

Day 8: 8 hours outdoors in conditions matching their transplant location

Day 9: 9 hours outdoors in conditions matching their transplant location

Day 10: Extend exposure into cooler night hours

Day 11: Extend exposure overnight

Day 12: Transplant

If you’re looking at that schedule and thinking that’s too tedious and untenable for anyone with a job… you’re not wrong. It really is annoying, especially when you’re dealing with eight full seedling trays and 40 individual pots of tomatoes.

Luckily, there are some ways the adventurous gardener can make hardening off much less painful.

Strategies to Shorten Hardening Off Stage

  1. Wait for an overcast day. The best way to shortcut the whole process is to wait for a few days of overcast weather (not too hot, not too cold) and leave the plants outside for multiple hours from the start.
  2. Elevate the plants slightly to reduce cold shock. Got a potting table or similar handy? Put it to good use by elevating the seedling trays off the cold ground outside to start.
  3. Protect seedlings with gardening shade cloth. If sun scald is more of a concern than cold temperatures, use shade cloth for a day or two as an intermediary step between indoor conditions and full sun exposure.
  4. Introduce a cold frame. If cold is the bigger worry and you have a raised bed available, consider constructing a cold frame with greenhouse plastic to mitigate the temperature swings your seedlings experience.

Tips for a Smooth Hardening Off Phase

Regardless of whether you opt for a more traditional method or an abbreviated one, there are lots of ways to make hardening off safer and less onerous.

  • Continue bottom watering plants in their trays indoors (at least to start). Watering them from above during hardening off can result in moisture on the leaves and subsequently worse sun scalding.
  • Keep an eye out for whitening or silvery discoloration on the leaves. This is a sign the sun is beginning to scald your plants and you’re moving too fast.
  • Wait until the danger of frost has passed. Similarly, don’t start hardening off on an unusually hot day.
  • Take advantage of morning and evenings when the sun is relatively weak.
  • Even cold weather plants like lettuce must be hardened off.
  • If possible, transplant into the ground on an overcast or mild-weather day.

Finally, try to take a breath and resist the urge to just get it over with. It really sucks to lose plants you put a lot of time and effort into due to transplant shock (or impatience).

How Do You Know When Plants are Hardened Off?

The most surefire way to know your plants are ready is to follow the more conservative (read: slow) schedule outlined above. This is particularly true if it’s your first year transplanting seedlings outdoors.

Once you have some experience hardening off seedlings, try feeling your plants for significantly thickened leaves that are ready to withstand the harsher conditions in the garden.