There’s an important step between growing a seeding in the house and moving it to the garden. Before planting seedlings outside, they need to be hardened off.
Hardening off is the process of acclimatizing a plant to a new growing condition. When you start plants inside and grow them in either a sunny window or under lights, the plant is growing under very stable and controlled conditions. The light isn’t as strong as the sun; the water is constant; there is little or no physical movement of the plant like that caused by the wind. When you add all of these factors together, you end up with a plant that isn’t tough enough to face life outside of the house – they’re literally “hothouse flowers,” weak and easily damaged. The tissues of the plant are soft and the protective layer of waxy cuticle on the leaves is very thin.
In order to make the transition from inside to outside, the plants needs some time to adjust or “harden off.” This is done by slowly introducing the plant to the conditions it’ll face in the garden. It’s often suggested that you place the plant in the morning sun for one hour on day one, then two hours the next day, etc. While this process makes sense, it’s far too structured for my taste. Instead, I put my tray of plants on an open porch where they only get a few hours of morning sun. Here they’re exposed to sunlight and have some physical movement caused by the wind. After about a week the plants are more sturdy and I feel safe to plant them in the garden.
As I was checking a pot of herbs, I realized the my basil plants weren’t full hardened off before I planting them. While the plants are growing fine, there are some leaves that are chlorotic (yellow) with a little bit of burning. What happened is that the intensity of the sun has scorched the leaves. It’s often called sunburn but it’s a very different process from the sunburn that we might get on a sunny day.
For a plant, sunburn occurs when there’s too much sun and too little water. The basil plants in my herb pot hadn’t had enough time to develop a full protective cuticle layer so too much of the sun’s energy was getting into the tissue of the leaf. As newly transplanted seedlings, the roots of the plants weren’t very developed so the absorption of water was a little slower than it would be in an established plant. The result of too much sun and too little water is that the leaf tissue dried out and died.
In this case it’s hardly a problem – the only damage was a few burnt leaves. But I’ve had plants die from not being hardened off properly. It’s tempting to want to take a plant that’s been growing inside and put it directly into the garden, but the results can be disastrous! In the case of seedlings grown in the house, a little bit of time spent hardening off a plant is worth the effort.