Hardening off – how to acclimatise your plants to the outdoors.

The most hopeful gardener would believe that March was the end of the winter, and the start of spring to come – the first of March almost epitomising the true beginning of the gardening year. The weather, however, has other plans. Much of the UK has been carpeted in thick white snow in the former half of this month, diminishing all hope of being able to start gardening for longer periods in fewer layers! Therefore, it is important to be mindful of conditions this month, moving all your young seedlings outdoors on the first sunny day of the year is optimistic, but March is deceptive – many a gardener has been caught out by unexpected frosts and late snowfalls. They do say it is more likely to snow at Easter than Christmas, and you certainly wouldn’t be leaving young cosmos seedlings outside over the festive period!


The RHS defines “hardening off” as the process by which ‘plants raised indoors or in a greenhouse need to be acclimatised to cooler temperatures… for about two to three weeks before they are planted outdoors.’ It is frequently described as ‘toughening up’ – the easiest way in which a gardener can relate is through the comparison to layering! Instead of going out in just a t-shirt on the first day of spring, begin with a jumper and a coat also, and then shed these layers as you warm up. Thereby, you aren’t given a cold shock, but still become accustomed to the temperature of the outside air. Another analogy is the difference between plunging yourself into cold water or easing yourself gently into it. These may be somewhat far-fetched, but the principle is identical, giving plants short periods of exposure outdoors, and thereby allowing them to grow used to the conditions.


This may seem all very well, but how do you really know when the plant has adapted, and do you just put the seedlings straight from the propagator into the ground? The primary thing to remember is to do it in stages. If a plant has been sown on a warm windowsill or in a propagator, then ensure that hardening off begins as soon as germination has occurred. This will take the form of moving your plants into a heated greenhouse (or frost-free equivalent) during the day, and then bringing them back indoors at night. For seedlings newly germinated, this will stop them becoming too leggy – whereby the stem becomes pale and elongated due to excessive exposure to heat and light, weakening the plant. After a week or two (depending on the outside temperatures, more time is needed hardening off the colder it gets) your seeds are ready to graduate to the greenhouse full time! If you don’t have a greenhouse, don’t worry, as this stage can be skipped to move directly to a cold frame or a zippy, more hardening off time is just needed.


From your heated greenhouse, if you are so blessed with one, the seedlings can be pricked out if they have not been so already. After a couple of weeks of growth and acclimatisation, they can then be moved into a cold frame, zippy, porch, or very sheltered spot outdoors: preferably undercover and/or by a warm wall. Again, move them out for the day and back to the greenhouse/windowsill at night. If you have a cold frame – open the door during the day to allow air circulation, but close at night to avoid frosts. It may seem back and forth (a bit like seedling ping pong!) but night-time temperatures can drop unexpectedly, and it is not worth leaving anything vulnerable defenceless in the cold.


If your seedlings are a hardy variety, then they can be hardened off in late winter or early spring, regardless of frosts. Examples of hardy annuals include cornflowers, sweet peas and ammi majus. Of course, it is better to avoid extremely low temperatures or snowfall, as small and flimsy plants can easily become covered. However, if the variety is not hardy, then hardening off must only finish when night-time temperatures consistently are above freezing – this usually occurs between the end of March and the end of April, depending on your location. Tender annuals include cosmos, zinnias and nasturtiums.


Hardening off can seem technical and applied, but truly it is accessible to every gardener, and should eliminate every risk of frost damage to newly sown, tender seedlings. The main thing to remember is if it is so cold that you wouldn’t want to be outside overnight, then neither do your seedlings! Another thing to bear in mind is that a big greenhouse or fancy cold frame do little to aid the hardening off process, apart from allowing you to start slightly earlier in the year when sowing and growing. Otherwise, by using any space you have, be that a windowsill, sheltered patio corner or porch, seedlings can be slowly introduced to the outside world. And if you do plant too early, all is not lost; any tender plants can always be covered with a warm blanket of horticultural fleece in an emergency.


I will end with a final warning about the ways of the weather in March – don’t trust the sunshine!

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