How to Harden Off Seedlings

Getting ready to transplant your carefully tended seedlings can be a scary proposition. The tender shoots and leaves can scorch or freeze quickly without the right hardening process.

In most cases, the best way to harden off seedlings is to not have to do it in the first place! 

Thankfully, you only need a few weeks and some free time to help your planting season go off without a hitch. Today, we’re walking through how to harden off your seedlings.

Let’s dig in!

What is hardening?

Hardening is the process of gradually introducing seedlings to outdoor conditions. Trying to move plants outside too quickly can cause them to fail. So, hardening is necessary for transitioning from seedlings to fully-fledged plants.

Many seed packets have a note written on the package saying something along the lines of, “Start 6 weeks indoors before frost”. 

What is the purpose behind hardening?

When you’ve started seeds indoors, they first grow soft and succulent. While they’re young, they need the protection and warm, moist air of the indoors. But when your planting season is near, they also need time to transition into hardy, weather-resistant plants.

The purpose of hardening is to strengthen your seedlings, and get them ready to weather the outdoor elements. You don’t have to begin your sprouts indoors, but you do have to bring them inside if weather conditions get too harsh.

When should hardening happen?

Around one or two weeks before you plan to move them outdoors, you should start to harden off your seedlings. What time of year this takes place depends entirely on your hardiness zone. Generally, when temperatures begin to stay above 50 degrees overnight, you can start thinking about transitioning to an outdoor setting.

The hardening process

Hardening might seem like a simple process, but it’s time-intensive and complicated. There are a few different ways to go about hardening off your seedlings. 

Here’s our favorite method!

Determine when it is the best time to harden your seedlings

Planting outdoors requires that temperatures start to warm up from the winter chill. Give yourself two weeks after the last hard frost before you start hardening your seedlings. Because the process takes seven to fourteen days, you’ll want to pay attention to the calendar. You don’t have to time it perfectly, but improper hardening can lead to stunted plants and reduced harvests.

Not looking at the forecasted temperature of the soil outdoors is an all too common mistake from gardeners. It isn’t enough to just look at the temperature of the air!

Some cold-hardy plants can actually go out two to four weeks before the last frost date. If you’re planting this type of seedling, you can start hardening earlier.

Gradually start to expose your seedlings

Hardening seedlings is a gradual process. Over a few weeks, you’ll expose your seedlings to outdoor conditions to avoid transplant shock. Beginning with just an hour a day, put your seedlings out in a sheltered spot. We recommend starting in a place without full sun exposure. Morning sun is ideal during the early stages of the process.

After that, you’ll increase their time outdoors by an hour a day until they’re outside eight hours at a time. While indoors, the more you mimic outdoor conditions, the stronger your plants will grow to be. For example, a fan mimicking wind conditions can create hardier plants before you plant them outside.

Avoid direct sunlight as they can cause damage to the delicate seedlings. We also recommend keeping them off the ground so that animals and insects can’t access them easily.

Make sure to adjust to sunlight and temperature

Over the process, you can begin exposing your seedlings to more direct sunlight and a range of temperatures. Once they're acclimated to the warmer parts of the day, you can start to leave them out at night. They should be alright if temperatures don’t dip below 50 degrees. Make sure to keep them watered, though, as the outdoor air can dry them out much faster than indoors. Keep in mind that light exposure is much more important of a factor than temperature.

How to ensure that the hardening process is successful

Temperatures are unpredictable sometimes and may interrupt a well-planned hardening process. But we have a few tips and tricks to keep your seedlings safe from drastic changes. 

There are multiple sheltering techniques available

Using a cold frame is one way to keep plants safe during an unwelcome cold snap. Essentially a smaller version of a greenhouse, these movable boxes offer a more controlled environment. An open bottom and light-permeable lid make them ideal for various settings. 

Many gardeners use them exclusively during the hardening process. Paired with warming cables, even overnight exposure is usually safe. 

Row cover cloth is another way to protect seedlings if you need to leave them out overnight. With this light and water permeable fabric, you can protect seedlings without a prolonged hardening process.

Manage your water and moisture 

Keeping plants watered and warm during this time is essential. If you’re using a sheltered hardening technique, you’ll want to keep an eye on your box's temperature and moisture levels. A cold frame can easily overheat and dry out your plants if you’re not paying attention.

Even though they’re protected from extreme outdoor conditions, they can still get shocked.

Adapt to how the seedlings react

While most hardening off guides have a “one-hour-a-day” rule, there’s nothing set in stone. Pay attention to how your seedlings react to the process. If they seem to be thriving, you can probably increase their exposure. 

However, if they’re not you may need to slow it down for a while before transplanting them to your garden. We like to say that while hardening, we’re taking our plants for their daily walk!

Whitwam Organics is here to help the hardening process for your seedlings!

Whitwam Organics is your source for everything related to plants! Our experts can help even the least experienced garden planners find success. With decades of experience, we’re ready to help you and your plants thrive! 

Check out our website today for all of your plant and garden education needs.