Hardening Off Plants & Planting Outdoors

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Spring is here, the weather is getting consistently warmer every day, and our seedlings that were started several weeks ago are ready to be transplanted to the garden or a container. But wait—we can’t just take them out of their protected environment just yet. They must be hardened off first.

Hardening off is the process of gradually transitioning a plant from their indoor or greenhouse location to the outdoor conditions of fluctuating spring temperatures, wind, and sun exposure.

It’s important to have an idea of the last frost date for our area, which is generally the 1st of April for Eastern North Carolina. In addition, the Guide to Vegetable Gardening in Carteret County gives recommended planting dates for our area. Start the process of moving plants outdoors about two weeks before recommended planting times for the particular crop.

Photo of Hardening Off Plants

Photo credit: Stacey Luker/Hardening Off Plants

Let’s use tomatoes as an example: The guide states they can be planted April 5–July 30, so two weeks before April 5 if the temperatures are at least 45-50 degrees during the day, move plants outdoors to a shady, protected spot, for no more than 2-3 hours.

Gradually increase the amount of sunlight the plants receive, and time spent outdoors, over the two-week period. The last day or two the plants can spend 24 hours outside. If temperatures fall below 45 degrees bring them back in for the night. Reduce the amount of water the plants receive but do not allow them to wilt.

Again, the overall goal of hardening off is to slow growth of the plants to allow them to adjust to changing weather conditions. Once hardened, even tomatoes can withstand the occasional dip in spring temperatures.

If planting in a traditional garden or raised beds, it is recommended that a soil sample be taken and analyzed prior to transplanting. Please note that due to COVID-19 soil sample requests are only accepted in Raleigh if deemed essential for agricultural purposes only. We will provide an update when this service is available to the public.

If using containers for the plant, make sure they are suitably sized for the variety. This How-To Guide on Container Gardens will provide detailed information on container selection and other requirements. If possible, try to transplant on a cloudy day or early in the morning or later in the afternoon to preclude additional heat stress on the plant.

Photo of Hardening Off Tomato Plants

Photo credit: Stacey Luker/ Hardening Off Tomato Plants

Make sure the plant is thoroughly watered prior to removing from the container. Except for tomatoes, which can be planted slightly deeper, transplants should be planted in the ground at the same depth they were in the container. Water thoroughly, stake if necessary, and fertilize in accordance with Cooperative Extension recommendations.

Written by: Stacey Luker, Master Gardener℠ volunteer