There are four things to consider when hardening off:
Let's take a look at each of these:
It's far from ideal to take seedlings that have been growing in a wind free, warm, indoor location and plant them outside, or even under fleece or a polytunnel. It's much better to gradually increase the stress that your seedlings experience. useful techniques include:
The basic idea is to gradually condition your seedlings to the environment that they will 'enjoy' once planted.
Seedlings grown inside, in a tunnel or behind glass on a windowsill are often un-prepared for the full intensity of the sun in late spring, summer and early autumn, especially when combined with wind. Gradually exposing them to full sun is a good idea, even better - I think - is to lay some shade cloth over them initially. This also helps with the wind and water loss.
Mesh tunnels and cold-frame lids act as a great shade cloth, an extra layer of mesh laid over the top can also be used
When I plant on the allotment, it's often into a bed that - in winter - had a low tunnel or cold-frame lid on it. I have a few mesh lids that fit on these beds too, they protect from birds, insects and sunlight for the rest of the year. I generally only keep these mesh lids on for a few weeks, by then the plants are well established.
Many plants suffer more from wind damage than cold. The wind chill can desiccate and burn young leaves. Fleece is often the best way to solve this issue and we use it all the time in spring for lettuces, spinach, chard, beetroot etc. We also use it to wrap newly planted beans, to protect them from the summer gales.
Spinach, peas and beetroot under fleece in early spring