Harden-Off Your Transplants for a Great Start

By Carey Thornton, Adult Education Coordinator
Whether you’re growing seedlings indoors or planning to pick-up your summer veggie starts at our May Edible Plant Sale, make sure you harden them off before planting outdoors – it could be the key to your success!

Hardening-off is the process of slowly adjusting your plants to outdoor temperatures, wind, rain and sunshine. At this time of year, many of the plants we dig into our gardens come from inside nurseries, cozy greenhouses or our own sunny windowsills. If you take them directly outside and plant them straight into the ground, they would have a rude awakening once the bright sun, harsh wind, cold rain and chilly nights got a hold of them. This is often the cause of “transplant shock.”

Hardening-off is a pretty easy process. Give them 3-14 days transitioning to their final homes outside. For cool season crops like broccoli and cabbage that don’t mind too much cold, just a few days may be needed for this process. With heat-loving crops like tomatoes and peppers, more time (up to two weeks!) may be necessary.
Because the quality of light from a supplemental florescent bulb is very different from full sunshine, for the first few days you’ll want to pick a partly shaded site that is protected from wind. Bring your babies in at night during most of the hardening-off process. Each day, allow your transplants to be exposed to more and more sun, eventually leaving them out in the full sun at their ultimate destination. As you near the end of your hardening-off process, it is time to leave them out all night. Finally you can dig your little guys into the soil.
Use season extension tools to make this job easier. An opaque plastic (e.g., polyethylene) covered hoop house or cloche serve as a great transitional space for your transplants. Even a structure covered with floating row cover (spun poly-cloth you can purchase under the brand Reemay or Harvest Guard) can serve as the extra protection your plants need to become well-adjusted.

How do you know if plants starts need to be hardened-off? If you are purchasing from a garden center and you are choosing plants from the outside racks, they are probably already conditioned for outside. If you are choosing them from an indoor or sheltered area, they need to be hardened-off.

The summer veggies you buy at Tilth Alliance’s May Edible Plant Sale have gone through a few days of hardening off, but still need a little protection before planting them into the garden. Since nighttime temperatures often don’t get warm enough for heat-lovers like tomatoes, eggplants and peppers until late May, use those extra few weeks to give your starts a good transition by hardening them off.