So, you’ve started your seeds indoors to get a jumpstart on the gardening season. Go you! Maybe you used seed trays, or maybe you used the soil blocking method like we did this year. No matter which method you used, the weather is warm enough and your young plants are ready and eager to go outside. But wait! Your plants are still so tender and have no idea how harsh the outside world can be. Before you transplant your seedlings outside, you need to toughen them up. Smack some reality on them before you send them out into the wide, wild world of your outdoor garden. Face it, your baby plants need some hardening off if they’re going to survive out there.

Why Harden Off Your Seedlings Before Transplanting

Let’s look at the big why. Essentially, hardening off is making sure your seedlings are able to handle the variable conditions of the real world. Indoors, your plants are protected from the greater elements of the outdoors. They don’t have to deal with how harsh the wind, rain, and sun can be.

Indoors, plants are spoiled little brats who expect the world to cater to their every whim. If they get droopy, you water them. Need nutrients? You fertilize them. If they need more or less light, you change their situation to accommodate their whims. Out there in the real world of your outdoor garden, the sun can burn, the wind can gust, and the rain can beat down hard. If your plants aren’t ready to handle it, best case, they’ll do the plant equivalent of ugly-crying for a while. Worst case? They’re won’t make it out there in the cruel world.

Hardening off gives your young seedlings the tools they need to combat the variable nature of the world. In one of my favorite exercise programs (for humans), weathering and succeeding in a natural, chaotic world is called being “antifragile”. Hardening off is a training montage worthy of Rocky that gets your plants ready for whatever fight comes their way.

What is Hardening Off?

Let’s get science-y here for a moment. Last paragraph aside, your plants aren’t going to pick up a jump rope and go to town, right? They need different exercises than us flesh and blood creatures. But, ultimately, the concept is the same.

When we exercise, we break down our muscles at a core level, ripping at the fibers and then taking in nutrients our bodies use to build our muscles back up. The weaker areas of the muscles repair harder, stronger, and more resilient than before. This allows us to take on harder and greater challenges and be more prepared for the unexpected.

Hardening off your seedlings gives your plants that same resilience. By exposing them to the natural environment, their young stems sway in the wind or droop in the rain, breaking down less-strong areas of their stem. Bringing them back indoors after this period of personal training gives them a much-needed period of rest. During this rest period, the plants heal those broken cells and make themselves stronger. Next time they’re out in the world, they won’t bend so far or break as often.

You’re not just defending against wind and rain when hardening off your seedlings. Introducing your plants to direct sunlight gradually allows their leaves to adjust to the sun’s heat and light. Essentially, you’re making sure they have a slight tan before you take them to the beach. There’s no sunscreen you can apply to plants, so they need to be ready to handle the world on their own.

What Happens If You Don’t Harden Off Plants?

Plants that aren’t hardened off may break their stems if a gust of wind comes along. A heavy rain may be more than their stems and leaves can bear and snap clean off. An afternoon of beautiful, golden sunlight may burn their unprepared leaves. Plants that aren’t ready for the chaos of the real world are in for some rude awakenings.

To be fair, not hardening off your plants may not be a death sentence. Your plants may recover and do just fine without hardening off. But it will take time for them to repair, adjust, and get back to the business of growing. That’s time out of your available growing season that your plants aren’t using to make food for your table or flowers for your vases. Game of Thrones jokes aside, winter is coming, and the prime growing season only lasts so long. Use the time wisely by having your plants ready to go at the very beginning.

Transplant Shock

If plants aren’t sufficiently hardened off, the first thing you’ll notice when you transplant them is a wilt in their stems. This is transplant shock. Transplant shock can be as short lived as a couple of hours, or it can kill your plants outright.

Mild transplant shock may simply be your plant’s equivalent of crying after leaving home for their freshman year at university. They’ll get over themselves, perk back up, and be kicking ass and taking names soon enough. A little water during this phase goes a long way to getting their roots settled in their new home.

If your plants aren’t prepared at all, however, their roots may have issues integrating with the garden soil. Inability to take in nutrients in their new home, and being unprepared for the sun will cause leaves to yellow. With a little support from trellises or stakes, even plants with a greater than mild case of transplant shock might survive. But they’ll require a lot more work from you to get through this low point in their lives and get established enough to make it on their own.

How Long Does It Take to Harden Off Seedlings?

Here’s the neat thing: it doesn’t take long to harden off seedlings and get them ready for the real world. Done right, it can take less time than the month or more that college-bound teens spend studying for the ACTs and SATs. If you receive a lot of sunshine in your area of the world, hardening off can be done in as little as a week. Even without direct sunshine, plants can still harden off sufficiently in as little as two weeks.

How to Harden Off Seedlings

Hardening off seedlings is a case of when and where. Depending on when you live, you will start the hardening off process earlier or later in the year. Knowing the average temperatures for your area of the world is important, but it’s not everything.

Daily High Temperatures

Keep an eye on your 10 day weather forecast and check your daily high temperatures. When your daily temperatures start hitting 40 to 50 degrees Fahrenheit (5 to 10 Celsius), you can start bringing your plants out for an hour or two during the heat of the day.

Shade First Then Sun

When you first bring your seedlings outside, make sure to start them in the shade. The first element your plants need to get used to is the wind. Gentle winds are best. If you get gales in your part of the world, you’ll want to keep an eye on the wind forecast as well. The first couple days, let them spend some time outside in the shade but where they can receive a little tousling of wind through their leaves.

After a couple days of spending an hour or so outside in the shade, your plants will be ready to try a little direct sunlight. The first day of direct sunlight, split their couple hours between sun and shade. The next day, give them a taste of what two hours of direct sunlight feels like. After each session, bring them in to rest and recuperate.

What About Rain?

April showers bring May flowers, right? Face it, you’re going to get rain in Spring. Keep an eye on your weather forecast. If it’s going to rain, keep an eye on how hard it’s raining. A gentle rain during your hardening off period is a blessing in disguise. Not only will your plants appreciate the rainwater, they’ll thank you later for the extra training in holding their heads up in the rain.

Water is heavy, so on rainy days only give your plants as much as you feel they can take. They’re probably not ready for a downpour just yet. Even a soft rain might be too much early on. You might give them only 30 minutes outside in a gentle mist. They will benefit from whatever time you do give them.

Daily Low Temperatures

While you’re in the hardening off process, keep an eye on your overnight low temperatures. Once the temperatures at night stay above 45 to 50 degrees Fahrenheit (8 to 10 degrees Celsius) for the next 10 days, you’re ready to transplant your seedlings outdoors.

If the weather forecast does end up shifting cold after you’ve transplanted outside, you can always cover your plants with a frost cover to hold in the heat for an evening.

Recap: Hardening Off

Here’s a general timeline of how to harden off your plants in a week. You can absolutely skip a day if the weather in your area isn’t cooperating. Maybe you have high winds or heavy rain all day one day. March comes in like a lion, after all. It’s ok to allow your plants a skip day in their personal training. But keep on them and make sure they stick with their physical regimen to make them as antifragile as possible!

1. Start the hardening off process when the high temperatures of the 10 day weather forecast never drops below 40 to 50 degrees Fahrenheit (5 to 10 Celsius).
   • 1st Day: 1 hour in the shade. (less, if raining)
   • 2nd Day: 2 hours in the shade. (less, if raining)
2. Day 3 & 4: 1 hour in the sun, 1 hour in the shade.
3. Day 5: 2 hours in the sun.
4. Day 6: 2-4 hours outside, sun/shade/rain mix, if you can manage it and the weather cooperates.
5. Day 7: if 10 day forecast shows low temperatures higher than 45 to 50 degrees Fahrenheit (8 to 10 degrees Celsius) for the next 10 days, you can transplant them into your garden. Otherwise, continue Day 6 at your discretion until conditions outside improve.

It’s a Hard Knock Life

If you give your plants the tough love they need this spring, they’ll be ready to take on whatever life throws at them. A firm, strong foundation early on is key to making the most of your growing season. I hope you use these tips to set your garden up for success. Let us know if you tried it and how it worked for you!