How to Harden Off Plants for Success in the Garden

There’s one sure fire way to kill your entire vegetable garden before the season has even begun, that is to skip the process to harden off plants. Young, tender plants need to acclimate to the great outdoors , so make sure that you harden off plants for a successful garden season first.

What Does It Mean to “Harden Off Plants?”

We say that we need to “harden off plants” when we have started young plants from seed indoors, and we need to move them outside. Weather conditions such as direct sunlight, wind, and temperature swings will kill tender vegetation if it’s not accustomed to such elements.

Instead of taking all of those healthy, young seedlings away from their cozy, indoor growing conditions, and shocking them on their first day out, we slowly acclimate them.

To harden off plants means that we’re building them up, strengthening their stems so they are able to grow well after being planted in the outdoor elements.

When to Harden Off Plants

As your daytime temperatures begin to warm, you can start to work through the hardening off process. Overnight temperatures should be above 40°F before leaving frost tender plants outside overnight.

Typically, you can begin these next steps to harden off plants roughly a week or two before the danger of frost has passed. I like to check the ten day forecast, and gamble with a few of my plants to get a jump start on the Spring season.

How to Harden Off Plants

First, check your weather forecast. Do not bring your tender seedlings out to meet a storm or strong winds on their first day out. Once you’ve determined the weather to be mild, you can begin the hardening-off process.

Now, bring your indoor-grown plants outside for a short time. Placing them in dappled sunlight or choosing a cloudy day to begin will give them their best chance. 

If you choose to place them in direct sunshine, you’ll need to give them less time outside. Then bring them back in for protection. 

On the second day, increase the length of time that your plants spend outdoors in the sun. Continue this process until your plants can spend days and nights outside.

A Schedule to Harden Off Plants

​Play it by ear. Every season is different, but here’s a general guide to harden off plants for success.

• 1st Day: Bring plants out to full sun for about an hour.
• 2nd Day: Repeat, for two hours.
• 3rd Day: Bring plants out for 4 hours.
• 4th Day: Give plants 6 hours outdoors.
• Day 5-7: Bring plants out for most of the day.
• 8th Day: If night time temperatures allow, let plants stay outside overnight.
• 9th Day: Put your seedlings in the places that you intend to grow them in your garden. Do not plant them yet, just see how they tolerate that spot.
• 10th Day: Keep watch over your plants in their new home.
• 11th Day: If your plants seem to be enjoying their new environment, go ahead and transplant them in your garden beds.
• For the next few days: Watch for transplant shock

Offer Plants Partial Shade

Alternatively, you could bring them outside to stay and offer your baby plants some shade cloth. This will reduce light levels on your plants’ tender leaves in the beginning. Remove the shade cloth a little at a time, increasing the length of time they spend in the sun daily.

Adjusting to Outdoor Conditions

Pay close attention to the weather changes that may happen. Moving plants outdoors for any amount of time during extreme weather is not a good idea. If you’re on day 4 and expecting a storm, just leave them inside that day. They’ll be fine under the grow lights they’re already accustomed to. Pick up right where you left off on day 5.

When to Harden Off Plants for a Full Night

​Only allow your young plants to spend a full night outdoors if the low temperatures aren’t expected to harm them. Frost will likely kill your tomato plants (or other heat-loving plants).

Avoiding Transplant Shock

After you have gone through the process to harden off plants, make sure they continue to thrive in their new home. Here are some tips to help your vegetable seedling avoid transplant shock:

1. The first thing that you need to do is water plants well.
2. Amend your garden bed, and add some high quality potting soil in the hole where you’ll place the new plant.
3. Try not to disturb the root system of the plant when removing it from its pot.
4. Carefully bury the plant’s root ball with the fresh potting soil.
5. Water the plant well.
6. Use a light liquid fertilizer to help ensure their nutrient needs from the start.

What Happens if You Skip the Process to Harden Off Plants?

Speaking from personal experience, there’s no faster way to kill your plant than to skip the hardening process. Yes, new gardeners, I know you’re excited to get those plants in the ground at the first warm outdoor temperatures.

However, if you skip this transition period, and refuse to harden off plants, all of your hard work will be lost. Take the time to help your plants adjust to their final positions to keep healthy plants.

Do You Need to Harden Off Plants That You Didn’t Start Yourself?

​That depends. If you bought your plants from a nursery that started them under cover of a warm greenhouse, then yes. Do harden off plants as already outlined.

If you buy them from a space that already has its plants in outdoor growing conditions, then you can plant them right away. However, I’d place plants in a prospective permanent spot and see how they do first. The following day, you can fully transplant them if they still look happy.

Do I Need to Harden Off Plants in the Fall?

​Yes. Fall and Winter crops will also need to go through the same transition from indoors to outdoors. Give them time to adjust to the sunlight, but also to the lower temperatures. 

Of course, you’ll be planting cold-hardy plants during this time, and you can probably speed up the schedule a bit. Always watch for cold weather, especially overnight. Utilize row cover or cold frames to protect them from frost if needed.

​Conclusion of How to Harden Off Plants

The key to successfully transplanting your seedlings is to harden off plants first. Remember to start slowly around your last frost date, increase the time they spend outside on a sunny day, and add a couple of hours per day until they can stay outside full time. Before you know it, your indoor seedlings will become strong, thriving garden plants producing you a bountiful harvest.

Happy Growing!

Donna and her family have been homesteading for most of their 20+  years together in some shape or fashion. She currently lives on their 20 acre farm where they grow as much food as possible. What started as a just a few laying hens, has grown into large gardens, pastured poultry, pork, and lamb. They are continuously evolving their small farm to not suit their family’s needs, but also providing to their local community. Donna’s favorite part of the family farm is her self-built micro-dairy, where she gets to love on dairy cows while serving her local community. Milking, cheesemaking, and processing dairy have become the soul of their homestead and the center of their farm.