There are five 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 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
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 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.
Spinach, peas and beetroot under fleece in early spring
Take care though, in late spring fleece laid directly on top of plants can scorch the leaves, a good example of this would be fleece laid on the big leaves of summer squash.
Used like this - for hardening off - fleece is only needed for at most a couple of weeks, but if it’s cold as well then longer is better.
I prefer to use Envirotect, rather than horticultural fleece as it has better airflow and drops less microplastic and it lasts longer, but fleece is fine for short periods.
There are a few very temperature sensitive plants. Tomatoes and French/runner beans need to be kept above freezing, peppers above 10c for example. The same is true of summer and winter squash. We use a combination of bubble wrap tends and fleece to reduce the risk of losing plants to the cold. Some plants, like peppers, survive below 10c but they won’t thrive.
Early beans and courgettes in bubble wrap tents, inside the polytunnel
I often find heavy rain, hail or snow to be the worst for young plants, so I generally protect most of my plants in winter, under low tunnels, or cold frames. If I used fleece I would support it on hoops or cans.
I have this useful guide to the hardiness of young plants
https://airtable.com/appZUuG0p5vkxzPmp/shrz60PM7kFMSuYt6
My successions database table, includes some advise for how to protect each succession
https://airtable.com/appZUuG0p5vkxzPmp/shrCdyoBm9gDM1HJ2
Personally I start my seedlings at home, either in my conservatory, under grow lights or in the greenhouse. I have a fairly powerful fan in the conservatory and I switch that on for a few hours a day (when I'm not at home). The conservatory is warm so the seedlings need to be conditioned to the cold, I do that initially by opening windows and the roof vent during the day. I then have a number of different environments that I might use for hardening off, as detailed in this table.
The mini greenhouse on the allotment
Except in summer I generally plant under-cover in a cold-frame, low tunnel or under fleece and I will choose a cloudy day, with no risk of a frost. I will plant in the afternoon and then close the tunnel/frame up so that the plants don't get too much wind. I will gradually increase the level of venting - hence wind - that the seedling experiences over the next week or more.
Cold-frames provide a lovely growing environment for young plants
I generally try to plant out young, typically a month old for most things, but sometimes 6 to 8 weeks for brassicas and tomatoes and even longer for slow growing plants like peppers. However most of my hardy plants only stay in a controlled environment at home for the first 3-4 weeks of their lives and then move to the polytunnel or the little greenhouse.
4 week old onions now in the polytunnel for 4-5 weeks before planting
I'm also used to have a little grow 'tent' in the back garden, which was very useful.
I found this incredibly useful for growing on seedlings in late spring, summer and early autumn when it's too warm in the conservatory. It provides shelter at night and during storms, but I generally have the door open all day long.