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Hardening Off

Explanation of how to harden off indoor grown plants


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Jump to:- What is Hardening Off?  | Why Sow Seeds Indoors?  | Why is Hardening Off Necessary?  |  How to Harden Off  | Sample list of indoor raised plants

If you've heard the term "harden off" and wondered what it means or how to do it, this article will explain everything you need to know. If you are planning on growing a variety of fruit, vegetables or herbs, it is quite likely that you will use the hardening off process for at least a couple of the plants you intend growing.

This article also includes a sample list of plants which are likely to need hardening off, but you are advised to check the full growing instructions on this site for each type of crop you will be growing. These can be found by clicking the general vegetable, fruit or herb section green buttons at the top of this page.

What is Hardening Off?

Hardening off is the term given to acclimatising indoor sown plants to outdoor growing conditions.

Why so some seeds need to be sown indoors?

There are 2 main reasons for sowing seeds indoors.

Firstly, there are many crops we grow which aren't actually native to our countries and which in their native countries, would naturally have a longer growing season in order to mature and produce a good crop. Furthermore, their natural growing climate is often warmer.

In order to prolong their growing season or protect them from cold weather, starting them off indoors is the best method thus giving them a better chance a) of survival and b) of reaching maturity and producing a good crop before the weather gets too cold. 

Secondly, but not so importantly, germination rates can sometimes be increased by sowing seeds indoors under more controlled conditions and/or in sterile compost or soil. This has the added benefit of lessening the possibility of  weeds germinating at the same time which will compete for food and water.



Why is it necessary to harden off plants?

It may seem an obvious statement, but plants are living organisms and, like all living organisms, are capable of being uncomfortable if suddenly subjected to different conditions. Under normal circumstances, us humans wouldn't leave a nice warm centrally heated house and go out in the snow for a lengthy period of time with no warm clothing on. So it is with plants. The shock of leaving warm conditions, be it a house or a cold frame, can be enough to kill them, so it's best to introduce them to cooler conditions gradually.

How to Harden Off  Indoor Grown Plants

Hardening off indoor raised plants is done by placing the plants outside in a sheltered position during the day once the daytime outside temperature is no less than 16C/61F, and then bringing them back indoors. This should be done over a period of a few weeks,  gradually lengthening the amount of time the plants remain outside from 2-3 hours when you first start, to all day after 2-3 weeks or so. The plants can then be moved outdoors permanently so long as night frosts are no longer a threat. You can try shortening the above process, however growth may be severely checked resulting in much delayed cropping....and if you get it very wrong DEATH.  

Even if you've sown plants in an unheated greenhouse, cold frame or under cloches, the same principle applies except rather than removing the plants altogether, you can just increase ventilation by opening the doors or lids for increasing amount of time as above.

Sample List of seeds commonly sown indoors

There are many seeds which can be sown indoors or under glass to give them extra protection, however this list shows only those which it would be almost impossible to sow outdoors in cooler climates such as the UK in order to ensure a good germination and a decent crop.

Aubergine/Eggplant Bell Peppers/Capsicum Cape Gooseberry Celeriac Celery
Chillies Cucumber Courgettes/Zucchini Kiwi Fruit Melon
Olives Strawberries Sweetcorn Tomatoes

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