Whether you start seedlings indoors or purchase young plants from your local nursery, come spring, it can be tempting to move them immediately into the garden. However, to ensure your tender seedlings grow into strong, healthy plants they need to be properly hardened off before they can be transplanted outdoors. This simple act of acclimation will help toughen up your delicate seedlings and get them ready for a successful transition into outdoor soil. We spoke with gardening experts to learn why it’s essential to harden off your seedlings, plus how to do it the right way.

Why You Need to Harden Off Plants

Indoor seedlings spend their short lives in a controlled environment that is vastly different from outdoor conditions. Introducing them to outside, harsher elements too quickly can result in transplant shock; this is where the plant becomes stunted or even dies. For example, if a plant goes from a warm-conditioned greenhouse (around 60 degrees Fahrenheit) to cool days and cold nights where frost is possible, transplant shock can absolutely happen. “This could stunt the growing process, and the plant may never grow to its potential, or there could be a total loss of the plant,” says Erika Ducharme, assistant grower at Ledgewood Gardens Greenhouse.

Hardening off your seedlings means gradually acclimating them to sunlight, wind, and temperature fluctuations by slowly increasing their time outdoors, which reduces the chances of transplant shock and can greatly increase the quality of your plant’s life. Thankfully, hardening off your plants is an easy process that even a beginner gardener can successfully achieve. 

How to Harden Off Plants

Hardening off plants isn’t hard. Here’s how to properly harden off your seedlings for a smooth transition into their outdoor home.

1. On a warm day when the temperature is above 45 degrees Fahrenheit, and the weather is calm, place your seedlings outside, out of direct sunlight, for one hour. (Dappled sunlight is recommended). After one hour, bring your seedlings back inside.
2. The following day, bring your plants outside again (in dappled sunlight) and extend their time outdoors to two hours.
3. Repeat this process over one to two weeks while gradually increasing the seedling’s exposure to sunlight and time outdoors. Each day, the seedlings will begin to tolerate more exposure to the elements.
4. When your seedlings have been outside for most of the day and have had proper exposure to the light levels required for their growing needs, the hardening process is complete. 

Keep in mind, even if your plant has acclimated successfully to the outdoors, it’s best to check your soil temperature before you transplant. “We don’t recommend that you plant your seedlings in the ground until it feels warm, approximately 70 degrees Fahrenheit,” says Karen Ramsey, master grower and owner of Ledgewood Gardens Greenhouses. You can do this with a soil thermometer commonly found in garden supply stores.

When to Harden Off Plants

The hardening-off process generally begins two weeks before your last spring frost date. This gives your seedlings enough time to acclimate to the outdoors while dodging any chance of frost. A gardening rule of thumb is most seedlings can be hardened off when daytime temperatures are between 45 degrees and 50 degrees Fahrenheit. Cold-hardy seedlings like Brassicas and pansies can be hardened off earlier.  

Expert Tips for Hardening Off Plants 

From unexpected cold snaps to when to cover your plants at night, these tips will make the hardening-off process a breeze.

Keep a Steady Eye on the Weather Forecast

Spring temperatures can quickly fluctuate. When temperatures outside reach 60 degrees Fahrenheit and there is low wind and low rainfall, you can leave seedlings outside all day. When nighttime temperatures are consistently above 45 degrees Fahrenheit, your seedling can safely remain outside through the night. 

Protect Seedlings From Too-Cold Temps or Steady Rain

If temperatures dip below 45 degrees Fahrenheit, or if there is a steady rainfall, it’s best to bring your plants back inside. If you have planted your seedlings in the ground and the forecast is a below 40-degree night, it’s best to cover them with a sheet or agribone, (a weather shielding material that protects outdoor plants from the elements). If you have planted your seedlings in pots, and the forecast is below 40 degrees, you can bring them inside or cover them with a sheet.

Make Transport Easy

If you have many plants that need hardening off, keeping them in a wheelbarrow or wagon will make for easy transport as they travel indoors and out.