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Moving Seedlings Outside

In my winter, garden-deprived state, I boldly set goals for my flower gardens (if you missed my resolutions, check them out here). Like most resolutions, the first one didn’t happen. My cool-season garden is not full of the cool-season annuals that I would like to see there. The sweet peas are still seeds in their packages. The best I can offer for a cool-season garden are the violas that volunteered and the pansies I managed to put in the ground very late last fall. I’ll gladly accept their cheer any day!

Tomato and pepper seedlings under a grow light

Tomato and pepper seedlings under a grow light

Please do not write me off as a gardener — not yet, anyway! I have managed to raise a fair number of seedlings for the vegetable garden. They are at the stage where I need to begin transitioning them from under the grow light in my kitchen for the vastly different environment outside! This process is also known as “hardening off.” It is a critical step any time you are propagating plants, whether by seeds or cuttings. (You may even need to do this when you buy plants directly from a greenhouse.  Early in the season, plants may not yet be hardened off when we impulsively purchase them!!) We have to prepare our new plants for the drier, brighter conditions outside of the propagation area. If we take this step, our new plants will have a much better chance of surviving the transplant process and establishing in our gardens.

There are several ways to prepare new plants for the garden conditions. I can harden off the plants by reducing fertilization. I have been fertilizing my seedlings since their true leaves have emerged. I don’t want to stop fertilization altogether because I do not want the plants to suffer nutrient deficiencies. However, I do want to slow down the rapid, lush new growth just enough to prepare the small plants for the transplanting process. So, I can reduce the fertilization process by diluting the application or by skipping every other application.

IMG_6823Another way to harden off plants is to gradually lower temperatures. Assuming that the temperature where the seedlings or plants have been growing is relatively warm (about 75F), you can begin gradually lowering the temperature. Dropping the temperature to 65F and then to 55F can help toughen plant tissues. We’re actually stressing the plants just enough to prepare them for external conditions.

This can be challenging inside of a house. If you have a cold frame, you can move seedlings there for this process. The unheated frame can provide cooler temperatures and protection from drying winds, but be prepared for challenges. Cold frames can actually heat up during warm, sunny days to the point that your plants are killed. A thermostat-controlled mechanism can help open and close the coldframe according to temperature. The automated process helps if you are away from the house, such as for work.


I brought my first flat of tomato seedlings outside in the late afternoon. The east-facing end of the carport provided a somewhat sheltered location to begin the transition process. I brought the seedlings back in at dark. I’ll repeat the process again each day for the next several days.

Since I do not have a cold frame, I will use what I do have: an east-facing carport. For you, it may be a similar location that is somewhat shaded or protected from bright light, heat build-up, and drying winds or breezes. I find that the morning light is enough to help toughen the plants, along with the drier conditions, but the spot is shaded enough from mid-day on that the plants do not get too hot in the afternoon.

The trick to hardening off is the gradualness of the process. If I take my seedlings out of my growing area (under grow lights in my kitchen) and put them outside all day long, the plants are going to dry out and die. However, if I bring them outside for an hour at a time for several days, then gradually lengthen the time they are outside, the plants will gradually toughen enough to survive the transplanting process. This is challenging since I work during the day, but I can also bring the plants out of the house in the late afternoon and use the evening hours with less-direct sunlight or take advantage of an overcast day to begin hardening off the plants.

An overcast sky was helpful on the first day that I brought out tomato seedlings.

An overcast sky was helpful on the first day that I brought out tomato seedlings.

I’ll keep you posted on the process. I am anxious to harden off the first batch of seedlings under the grow light! If I can transition my first vegetable seedlings to grow outside, then I will have room under the grow light to start my flower seedlings and a few more herbs. I also want to make some cuttings to propagate other plants in the garden. So, here goes!

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About sdorn

Sheri is the State Coordinator for the Georgia Master Gardener Extension Volunteer Program and Extension Specialist for Consumer Ornamentals. When she is not traveling about the state of Georgia admiring the work of Georgia Master Gardener Extension Volunteers, she spends time in her own (real and virtual) gardens.