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There are five things to consider when hardening off:

  1. Where your seedlings were growing prior to planting
  2. Sun intensity
  3. Wind
  4. Cold
  5. Rain, hail, snow

Let's take a look at each of these:

Seedling location

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:

  1. using a fan to provide a gentle breeze over the leaves
  2. putting seedlings close to an open window/door
  3. moving seedlings from an indoor location to a greenhouse, cold-frame or polytunnel and gradually moving closer to an opening

The basic idea is to gradually condition your seedlings to the environment that they will 'enjoy' once planted.

Sun intensity

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 for at least a few days after planting. 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

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.

Wind

Many plants suffer more from wind damage than cold. The wind chill can desiccate or ‘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.