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Hardening off seedlings

We start seedlings in our basement under lights. Their leaves are beautiful, dark green and tender, and they just look gorgeous. But they are tender and frail, and need to be acclimated by gradual exposure to outdoor conditions, before being planted. Gardeners often call this conditioning process "hardening off." Most of the seedlings that we are distributing now have been hardened, but you may want to do a little more before planting them outside in their final outdoor growing spot, whether that's in the ground or in a container. Here is a nice article about hardening plants from Penn State Extension, and here is another from

Some growers even expose seeds to stresses prior to, or during, germination. Exposing tomato seeds to heat as they germinated was found to be helpful in preparing tomatoes for high-heat conditions in Saudi Arabia. The authors of the experiment found best results by soaking the seeds for 5 minutes, then drying them and putting them in 140-degree Fahrenheit temperatures for an hour or two. (Reference)

Scientists talk about this phenomenon of hardening as acquisition of "memory." Memory in plants means an initial exposure leads to a more efficient or rapid response to subsequent stress. In this sense, plants can remember drought, cold, excess light, salinity, and the presence of specific chemicals. It's even possible for plants to pass on a stress memory to their offspring!

So how do plants remember things? They don't have neurons and brains like we do. One mechanism is building up and retention of biological molecules (proteins and RNA) that are poised to carry out the stress response. By slowly exposing a plant to outdoor conditions, you are giving it time to turn on the appropriate biological circuits and build up the physiological tools it needs to flourish in those conditions. When conditions are great -- warm and sunny, with just the right amount of water and food -- plants will gradually reset and go back to happy, trustful growing. However, they may also remember the stress and be better prepared for next time - either through retaining some of the signaling molecules that are ready to turn on the stress response, or perhaps through longer-lived epigenetic mechanisms.

Epigenetic changes involve modification not of the DNA genome sequence, but of the chemical "tags" that can be attached either directly to the DNA, or to the structural proteins around which the DNA is wrapped. These modifications are not as durable as the DNA sequence itself, but can be inherited by daughter cells. Interestingly, plants are different than animals in that the germline cells (that make the next generation) are descended from somatic cells, and not directly from the previous generation's germline cells as in animals. This means that there is more opportunity to write (epigenetically) into the plant's germ cells information about the plant's experience during its lifetime. Moreover, the process of embryogenesis in animals erases most epigenetic markings, but plants retain more of their epigenetic marks during embryogenesis.

At any rate, keep an eye on your plants as you expose the to more sun, heat, cold and wind. How does its appearance change? Under what conditions does it appear to thrive?

Have fun!

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