Whether you pick up some seedlings from your local nursery or start some at home, you may want to plant them outside as soon as possible. But moving them from their cozy, protected home to a harsh outdoor environment causes stress. Hardening off seedlings allows them to acclimate to their new home so they can thrive.
Properly acclimating your seedlings requires patience and some know-how, but it’s an easy process. Simply put, it involves slowly increasing their time outside until they’re ready to transplant. However, knowing some helpful tips makes the process easier.
I’m going to share some hardening-off tips I’ve learned while preparing thousands of seedlings for their lives outdoors.
What Is Hardening Off?
Hardening off is gradually acclimating plants to new environments to prevent shock.
If you spend time at a nursery or vegetable farm, you’ll quickly become familiar with the term “harden off.” But what does it mean?
Hardening off is plant speak for slowly toughening plants to new environments. Gardeners typically harden off greenhouse-grown plants before planting them in their new outdoor homes. This process limits what gardeners call transplant shock and prevents issues like dropped leaves, stunted growth, and discoloration.
While working on vegetable farms, I quickly learned hardening off seedlings was crucial to the plants’ long-term success. You can start with perfect-looking seedlings, but they’ll often deteriorate after planting if you fail to harden them off.
You should harden off both seedlings you start indoors at home and those you purchase from a nursery or big box store. Unless the seedlings grow in an unprotected outdoor environment, they probably haven’t been hardened off.
What Happens During Hardening Off?
Hardening off offers advantages like boosting the production of leaf wax.
We know hardening off allows plants to acclimate to their new homes. But what’s happening inside the plant?
The stress of a new environment causes plant growth to slow. Plants also develop a thicker layer of wax or natural oils on their leaves, leading to decreased water loss. You’ll also notice plants become more rigid due to increased lignin in cell walls. Finally, the amount of water in cells decreases as the dry matter increases.
When you add all these factors together, you’ll find that your plants can better withstand environmental stressors like wind, direct light, and drought.
Tips for Hardening Off Seedlings
Now that you know what hardening off is, let’s review some tips to make the process easier.
Wait Until the Weather Warms
Wait until temperatures consistently exceed 50°F before starting the hardening-off process for seedlings.
While you may be itching to plant your seedlings in your garden, don’t view the first warm day as a call to move your plants outdoors. If you start the hardening-off process too soon, seedlings must deal with stressful cold temperatures.
I like to wait until the daytime temperatures are consistently above 50°F (10°C) before beginning the hardening-off process. I also keep the long-range forecast in mind.
For example, I don’t place my seedlings outdoors if I experience a warm spell in February. My area will still experience below-freezing temperatures that may stress the seedlings. Instead, I wait until the weather is warm and within a few weeks of the last predicted frost date. Waiting until this time means there’s a good chance the weather will be warm enough to plant seedlings outdoors once they’ve hardened off.
If you’re unsure when to start, wait until the last predicted frost date arrives. Even if an unseasonably late frost arrives, you can move your seedlings indoors.
2. Don’t Rush the Process
Gradually increase outdoor exposure for seedlings over 1-2 weeks for optimal acclimatization.
Remember, the whole point of hardening off seedlings is allowing them to adjust to their new environment. If you grab a seedling tray from a greenhouse, let it sit outside for a night, and then plant the seedlings in your garden, the plants will still experience lots of stress.
A better option is to slowly increase the time plants spend in their new home. I like to increase the seedlings’ time outdoors by a few hours each day. That means the whole hardening-off process takes between one and two weeks.
For example, I’ll place my seedlings in a protected outdoor environment for one or two hours on the first day. A cold frame is ideal, but you can also use a lightly shaded, protected area. A covered porch works well, as does an area protected from wind by a house or row of shrubs. After the plants are outside for a few hours, I return them to their protected indoor environment.
On the second day, I place the plants in the same location and let them stay outside for three or four hours. I follow this pattern until the plants spend all day and night outside (provided nighttime temperatures don’t dip below freezing).
At this point, I move the plants to a spot that mimics their end location. That typically means an open area with full sun. After the seedlings spend a few days in this location, they’re ready to plant!
3. Pay Attention to the Wind
Avoid exposing seedlings to winds over five mph initially, and cover them if strong winds arise unexpectedly.
While ten-mile-per-hour winds may not seem like a big deal to you, they can be shocking to tender seedlings! Remember, these seedlings have lived a pampered life with limited exposure to elements like wind, cold temperatures, or heavy rains.
Keep an eye on the weather forecast, and avoid setting seedlings outside when winds are above five miles per hour. While seedlings can handle wind after a few days outside, avoiding wind during the first few days of the hardening-off process is best.
If unexpected high winds arrive a few days into the process, cover the plants with row cover to dampen the impact.
4. Keep Seedlings Well Watered
Proper watering during the hardening-off process minimizes stress for transitioning seedlings.
Transitioning seedlings to a new environment is stressful, and you want to limit stress as much as possible. Correctly watering the plants eliminates stress caused by dry or water-logged soil.
Remember the differences between an indoor and outdoor environment impact the amount of water your plants require. Therefore, you’ll likely need to alter your watering schedule after you move your seedlings outdoors. While you may have watered your indoor seedlings once a day, outdoor sun and wind may require you to water them two or three times per day.
The end goal is to keep the potting soil moist but not wet. Some dry soil toughens the plants, but completely dry soil causes too much stress.
5. Don’t Forget to Harden Off Summer Seedlings
Gradual acclimation helps summer seedlings smoothly transition from a greenhouse to an outdoor garden, reducing stress.
Greenhouses are great for planting seedlings. Most people realize a winter greenhouse’s cozy and warm environment differs from the windy and cooler environment outside. However, there are also differences between a summer greenhouse and an outdoor garden.
In the summer, greenhouses provide a bit of shade and lessen the light intensity. This is especially true if greenhouses or propagation tunnels are covered with shade cloth. Along with differences in light, you can expect differences in wind and rainfall.
If you remove summer seedlings from the greenhouse and immediately plant them in an outdoor garden, the plants will experience unnecessary stress. They may become sunburned due to increased light or wilt due to drying winds. Therefore, you should take the time to harden off summer seedlings like spring seedlings.
Slowly expose them to increased light and more time in a hot and windy environment. While it may seem unnecessary, hardening off increases the chances summer seedlings will thrive.
6. Consider Plant Type
Cool-weather crops handle lower temperatures well, while warm-weather ones need more care during hardening off.
While no seedlings should be exposed to below-freezing temperatures, some are more cold-tolerant than others. Cool-weather crops like lettuce, broccoli, spinach, and kale can tolerate cold temperatures better than warm-weather crops like tomatoes, cucumbers, and squash.
That means you should be more careful with warm-weather crops than with cool-weather crops. While kale seedlings can handle a 45°F (7°C) day a few days into hardening off, the same weather severely stresses a cucumber seedling.
The plant type also impacts when you should begin. I often harden off crops like kale, lettuce, green onions, and bok choy before the last frost arrives. As long as the seedlings are slowly acclimated to the outdoors, they can survive a frost with help from row cover. However, frosts stress and stunt tomatoes, eggplant, and peppers, even if you’ve properly hardened them off.
7. Protect Plants After Transplanting
Hardening off reduces transplant stress, but extra protection post-transplant may still be necessary.
Properly hardening off seedlings decreases the stress they’ll experience during transplanting. However, that doesn’t mean it completely eliminates transplant shock.
Even if you harden off your seedlings, you may still need extra protective measures after transplanting. For example, if you transplant bok choy or kale in the spring and an unexpected freeze arrives a few days later, you should protect your plants with row cover. This will allow the plants to continue to adjust to their new environment with the least stress possible.
I sometimes need to help summer plants adapt to their new hot homes. Since I plant fall crops like kale, broccoli, and lettuce in August, these cold-hardy crops often have to deal with high temperatures and intense sun. I cover the crops with shade cloth and irrigate daily if I deal with sunny, dry conditions and temperatures above 90°F (32°C). After a week in the ground, the plants can deal with these conditions.
Final Thoughts
Hardening off seedlings is an essential part of growing happy, healthy plants. If you acclimate your plants over a week or two, they’ll be ready to thrive in their outdoor home.
