How to Harden Off Seedlings Quickly: 5 Tips for Growing Healthy Plants

You’ve done the hard work of starting seeds and have tray after tray of beautiful seedlings waiting to go out to the garden. But now what? 

You and your seedlings must jump through one more hurdle to make it to their final home in the garden. In the gardening world, we call it hardening off. 

Hardening off is exactly what it sounds like—it’s the process of toughening up your seedlings so that they’ll be properly adjusted to life outdoors. 

Although not a difficult process, hardening off involves foresight and planning. You don’t want to start the process too early, or you run the risk of cold temperatures damaging your plants. Wait too late, and your plants might still suffer from being in pots too long. 

Keep reading for an explanation of why hardening off is an essential step and our top tips for transitioning healthy seedlings to happy transplants. 

What is hardening off, and why is it important?

Hardening off is the process of acclimating indoor-grown plants to outdoor conditions. Although it is most typically discussed in preparing seedlings for transplanting, hardening off is essential for any potted plant transitioning from indoors to outdoors.

The difference in temperature between indoor and outdoor spaces is the primary, but not the only, reason that hardening off is essential. 

Wind is another natural phenomenon that seedlings need to be introduced to, as well as full-spectrum sunlight. Humidity levels also differ drastically between indoor and outdoor spaces.

According to the National Center for Biotechnology Information, one essential plant organ is the cuticle, which is a thick, waxy outer layer that protects plants from harmful UV light. The waterproof cuticle also helps plants maintain their internal moisture levels. 

Indoor-grown plants don’t naturally develop cuticles since they are protected from the get-go. The process of hardening off encourages seedlings to grow cuticles so they will survive their time outside.

What are the consequences of skipping the hardening-off process?

Seedlings that don’t go through the process of hardening off won’t be as healthy as those that are. Without proper hardening off, seedlings may experience stunted growth and premature bolting from temperature stress. 

Even if temperatures don’t dip below freezing, seedlings can still suffer damage from dropping temperatures and the cool, wet conditions typical of spring evenings. Without gradual acclimation to natural sunlight, seedling leaves may become sunburned and discolored.  

If seedlings are planted outside before they are ready, they may not be strong enough to survive exposure to cold temperatures and drought. Weak seedlings will have less immunity than healthy plants and may be more prone to suffer from disease and pest pressure.

Rather than shock your seedlings with an abrupt transition from inside to out, it’s far better to gradually introduce any plant—but especially young seedlings—to their new home. 

5 tips to successfully harden off seedlings

Transplanting seedlings from an indoor environment to an outdoor environment is a necessary step for growing healthy plants. But, making this transition can be stressful for seedlings adjusting to the changes. Fortunately, there are some simple steps you can take to ensure your seedlings make a successful transition. 

1. The right timing

Give your plants at least a week—ideally two—to make the transition from indoors to outdoors. Find out when it’s safe to transplant seedlings in your area—this can be a week before your average last frost date or a few weeks after, depending on whether your seedlings are frost-hardy or not. Once you have that date in mind, start hardening off seedlings one to two weeks before transplanting them outside. 

An article by the University of Maryland explains that hardening off momentarily suppresses plant growth, so transplants can focus their energy on establishing themselves in the ground before returning their focus to vegetative growth. Of course, care must be taken not to stress plant growth too much, or the plants might sustain too much damage. 

2. A protected location

Animals, insects, and birds threaten young seedlings, so extra care must be taken to protect vulnerable plants from outside predators.

Choose a location to set out your seedlings. Your seedlings will be safe on their first excursions outside. Ideally, you’ll want to set your seed trays on a table or other elevated surface, so the plants are out of reach of slugs and other animals. 

3. Dappled sunlight

While the point is to show your seedlings some “tough love” and literally force the young plants to grow a thicker skin, you don’t want to do too much too soon.

Choose a location that offers protection from the harsh afternoon sun—dappled sun or partial shade is best for the first few days. Towards the end of the first week or the beginning of the second week, you can begin introducing your seedlings to direct sunlight. 

4. Gentle breeze

Prepare your seedlings for natural breezes by running a fan in the greenhouse or seed-starting room early on. You can also periodically brush the seedlings with your clean hand, bearing in mind that some plants—including tomatoes—don’t like to be touched. 

Seedlings need to acclimate to wind and airflow but don’t put new seedlings outside on a particularly gusty day, as it could do more damage than necessary. If there are high winds in the forecast, leave the seedlings inside for the day or protect them with a wind block. 

5. Reduced watering

While you don’t want to stop watering plants completely while hardening them off, you will want to reduce the watering frequency to slow plant growth before transplanting. 

Don’t withhold water to the point of stressing seedlings, but get in the habit of watering deeply two to three times a week rather than daily. Allow the soil to just barely dry out between waterings, and you’ll encourage plants to grow stronger roots that will transplant better than seedlings with shallow root systems. 

Signs seedlings are ready to be hardened off

If you start seeds at the right time, it will be easy to tell when it’s time to begin hardening off your seedlings. Besides a break in the weather, there are three major signs that indicate it is time to start preparing your seedlings for transplanting. 

• Seedlings develop true leaves 

Seedlings are ready to be hardened off when they grow their second set of true leaves. The first leaves that seedlings produce are called cotyledons, and these tiny leaves look alike across the board. As seedlings mature, they begin producing true leaves—leaves that are typical of the species. Most seedlings develop true leaves about three to four weeks after germination, and having multiple sets of true leaves signals the plant is entering a different growth stage. 

• True leaves begin to yellow 

Although it’s common for cotyledons to yellow and fall off as a plant grows, it could signal a nutrient deficiency when true leaves turn yellow. This can be remedied by bumping the seedlings into bigger pots with fresh soil or transplanting them to their permanent home.  

• Seedlings become rootbound

Seedlings that are beginning to become rootbound in their seed trays or pots are nearing time to transplant outside. Rootbound is a term used to describe when a plant’s root system has almost exceeded the confines of its container. Gently lift the seedling out of its plug and inspect the roots. If there is no room for the roots to expand, it’s time to prep the seedling to get planted

Hardening off: a schedule that works

Now that you understand how and why hardening off is essential, here’s how exactly to do it. This sample schedule is designed to harden off any seedlings in just one week, but if you take two whole weeks, spend three or four days in each phase. 

Day 1–2

Start by placing seedling trays outside for a couple of hours during the warmest part of the day. Choose a location that gets indirect light or offers the seedlings some protection from direct rays via a shade screen. Bring the plants inside before evening temperatures drop. 

Day 3–5

As the week progresses, leave the seedlings outside for a little longer each day, working up to 6 hours of direct sunlight. Move the plants to a location that receives a light breeze.

Day 5–6 

Finish out the week by leaving the plants outside overnight. Check the forecast first—nighttime temps in the 50s are ideal for heat-loving plants, and 40℉ is about as cold as you want to risk it with cool-season crops.

Day 7 

Transplant seedlings to their permanent home. If possible, wait for an overcast day and plant early in the morning. Water seedlings thoroughly after transplanting. 

That’s it! You may not immediately notice any difference, but after a week of all this shuffling around, your seedlings will be much stronger and more than ready for transplanting. 

Tools that help with hardening off 

If you’re a little nervous about setting plants out too early, or even if your climate is somewhat unpredictable, these tools offer a little extra protection on unexpectedly cold spring nights. 

• Cold frame 

A cold frame is a great tool to use for hardening off plants. You can buy a cold frame online or at a garden supply store or make your own! All you need is enough wood to build a shallow box, a window or piece of greenhouse plastic for the top, and a couple of hinges to attach the top to the box.

The transparent lid will allow light to enter the box and retain heat. When you go to harden off seedlings, simply open the lid to vent the cold frame and allow the excess heat and moisture to escape.

Keep an outdoor thermometer in the cold frame to monitor temperatures carefully. Ideally, you want temperatures to stay between 50℉ and 80℉. Leave the cold frame open for increasing periods during the day, and be sure to close the frame at night or if temperatures dip below 40℉. 

• Cloche

Pronounced “closh,” a cloche is simply a cover that can be placed over plants to offer protection on colder nights. Cloches may be made of glass or translucent plastic and can be made to cover individual plants or rows.

This Season Starter is an example of a reusable cloche. Place around the transplants and fill the walls with water—the water will absorb heat during the day and release it at night when temperatures drop.  

• Frost cloth 

Like a cloche and a cold frame, a frost cloth insulates the plants underneath it by creating a pocket of warm air. A frost cloth can raise the air temperature by as much as five degrees, but it only works if it is at least an inch above the plants. If any part of the plant is in contact with the cloth, the plant could sustain frost damage at the point of contact. 

In summary

Hardening off seedlings can be a rewarding experience as you watch your plants thrive in their new home after properly transitioning them from their starter trays. Remember to adjust watering schedules and reduce the amount of water applied to seedlings while closely monitoring temperatures. 

Tools like cold frames and frost clothes can help you with the hardening-off process and protect your transplants from environmental conditions. If transplant shock does occur, act quickly to reduce the severity of the damage or replace the seedlings with a new batch.