Spring is the season of new beginnings and the rebirth of fresh vegetation. For most gardeners, spring is the time to start their seeds and to plan the layout of the upcoming garden. Some horticulturists get a jumpstart on the new gardening season by starting their seedlings indoors or in a greenhouse environment.

After all, a warm, controlled environment is the perfect place to germinate and cultivate vegetable seedlings. After a couple of weeks indoors, the seedlings will have grown tall in stature and outgrown their planting containers. The young plants are seemingly ready to be transplanted into the soil for the duration of the season.

It is very important that horticulturists hold back their desires to race to get these young plants in the ground. The transition from a protected environment, such as an indoor garden or greenhouse, to the comparatively harsh conditions of the outdoors is enough to stunt or destroy young seedlings before they even get a chance to properly establish.

To reduce stress caused by the transition from indoors to outdoors, growers should implement an acclimation technique known as hardening off.

Hardening off is the process of slowly acclimating young plants or seedlings to a new environment. In other words, hardening off is a way for gardeners to gradually condition their plants to the comparatively harsh conditions of the outdoors.

Experienced horticulturists understand that the “harsh conditions of the outdoors” is a relative term. Plants are supposed to grow and thrive outdoors. However, when a plant is used to the cozy conditions of an indoor garden or greenhouse, the conditions of the outdoors are relatively harsh and can literally shock a plant to death.

Take the Leap and Get Your Seedlings Outside

To make the hardening off process go as smoothly as possible, seedlings can be preconditioned for the transition while still indoors. A small fan that circulates the air around the seedlings will help strengthen stems and better prepare the plant for the wind outdoors. However, too much air movement can be counterproductive.

If there is too much air movement, the seedlings could dry out and/or become damaged. A fan added to the seedling area should not be pointed directly at the seedlings. The fan’s purpose is to move air around the room, not to blow directly on the seedlings. Consistent moisture for seedlings is very important for establishing strong, healthy roots.

Watering seedlings with a spray bottle is a great way to evenly distribute moisture to the seedbed. For most seeds, consistent moisture is imperative for germination and early leaf development.

Resist the temptation to fertilize the seedlings. Most plant varieties will not require fertilization until they have been transitioned to the outdoor environment. In fact, waiting to fertilize until the seedlings are in the ground can actually accelerate the time it takes for the plants to establish themselves outside.

The Process of Hardening Off Seeds

The three biggest factors affecting a plant’s transition from an indoor environment to an outdoor environment are direct sunlight, wind, and temperature. Each of these three factors can adversely affect seedlings and young plants when transitioning from an indoor environment to an outdoor environment.

Is it OK to Expose my Seedlings to Direct Sunlight?

Many gardeners do not realize how much more intense the sun is than an artificial light source. Even plants grown under high intensity discharge lighting will have a hard time transitioning to the intensity of the sun’s rays. Tender seedlings and young plants grown indoors on a window sill or under a grow light must be gradually transitioned into the sunlight. Always start by placing the tender seedlings in a shaded area.

After a couple of days in the shade, the seedlings can be introduced to direct sunlight. Remember, gradual increases are the key to success in the hardening off process. Each day the grower can increase the amount of time the seedlings receive direct sunlight.

Depending on the plant variety, the grower can usually increase the duration of time in direct sunlight by one to two hours per day. Typically, seedlings should be acclimated to the direct sunlight in about a week. Any sign of burning or wilting is an indicator that the transition is happening too quickly and should be slowed down.

Don't Expose Seeds to Wind & Dry Air

Many seedlings and young plants get accustomed to a high humidity environment. When placed outdoors, the wind and dry air can quickly dry out the medium and/or the seedlings themselves. If there is a strong wind present when a gardener is hardening off his or her plants, it is a good idea to create some sort of wind block for the seedlings. Even with a wind block the plants will still become acclimated to the ambient humidity and some air movement. Cold frames or mini hoop houses are valuable tools for the hardening off process and can offer additional wind protection for transitioning plants.

How to Handle Cold Nights When Hardening Off Seedlings

Many gardeners start the hardening off process when the night temperatures are still too cold for the seedlings. It may be necessary to bring the seedlings back inside each night until the overnight temperature increase or the plants become acclimated to the colder temperatures. Again, cold frames or other temporary structures which offer cover can be used to protect the seedlings from the lower nighttime temperatures.

Usually a week or so is enough time to acclimate the seedlings not only to the sunlight and wind, but also cooler temperatures. That being said, the gardener has to have some awareness of his or her hardiness zone and his or her geographical area’s average last frost date.

This information will give a horticulturist a good indication of when to start the hardening off process. Here is a list of the hardiness zones and the corresponding average last frost dates:

  • Zone 1 - June 15
  • Zone 2 - May 15
  • Zone 3 - May 15
  • Zone 4 - May 15
  • Zone 5 - April 15
  • Zone 6 - April 15
  • Zone 7 - April 15
  • Zone 8 - March 15
  • Zone 9 - February 15

When to Start Hardening Off Seeds

The average last frost date in a gardener’s geographical location is a good guideline for indicating when plants can safely be planted without the risk of frost. However, depending on the type of crops being grown and their sensitivity to temperature, there may be some further considerations. It is good to organize the crops into different categories which will determine when they can be safely hardened off.

Hardy plants, such as broccoli, brussels sprouts, cabbage, onions, leeks, parsley, and kale, should be hardened off when the average minimum temperature is 40°F or above. Half-hardy plant varieties, such as celery, chinese cabbage, and most lettuce varieties, can start the hardening off process when the average temperature is 45°F or above.

For tender plant varieties, such as squash, pumpkin, sweet corn, cucumber and muskmelon, an average temperature of 55°F or higher should be achieved before attempting the acclimation process. Some tender varieties will even require higher temperatures for an optimal transition. Basil, tomatoes, and most pepper varieties establish best when the average temperature has exceeded 65°F.

Timing Irrigation for Final Transplant

One way to help ease the transition from seedling tray to soil is to allow the seedlings to dry up a little right before planting into the soil. Do not allow the seedlings to dry out to the point of wilting or irreversible damage may be done. However, a slightly drier medium can make it not only easier to physically separate the seedlings for transplanting, but can also speed up the overall establishment process. If possible, transplant seedlings into the ground right before a heavy rain.

There is no substitute for a good spring rain to help welcome new seedlings into the garden. If rain is not in the forecast, seedlings should be watered with a diluted fertilizer (preferably one with vitamin B1, as found in most rooting hormone gels or powders, for example) as soon as they are planted in the ground. A diluted fertilizer with B1 will help decrease the initial shock of being transplanted.

After the plants are in the ground, make sure to keep the seedlings moist (not saturated) for the first week or so until the seedling’s roots have established deeper in the soil.

Every spring, garden hobbyists get the itch to get their hands dirty. Starting seeds indoors and then transitioning them outdoors is the best way to get a head start on establishing a flourishing vegetable garden.

Like many other gardeners, I have gotten the gardening itch way too early and have ended up with a jungle of seedlings that were very overgrown by the time they could be properly transitioned to an outdoor environment.

As with most things in life, timing is very important when it comes to transitioning seedlings into an outdoor garden. Growers who account for their geographical location’s average last frost date and the minimum temperatures suitable for their crops will be able to seamlessly transition indoor plants into an outdoor garden.

Proper timing combined with gradual increases in exposure to direct sunlight and wind will ensure not only a smooth transition, but also the best possible beginning to a new gardening season.

Read More: Flying the Nest: How to Harden Off Seedlings