How to toughen up seedlings for planting in the ground

There is so much to be done at the moment: all those seeds that need to be scattered and the seedlings that will be rapidly outgrowing their pots. But if you are just opening your first seed packet, don’t panic. It’s always tempting, particularly when Instagram is taunting you, to think that you are behind in sowing. You are not. Don’t get distracted by false gods.

a close up of a flower garden in front of a window: Photograph: Herb Bendicks/Alamy

© Provided by The Guardian
Photograph: Herb Bendicks/Alamy

As my mother always reminds me, the hardier veg (beetroot, spring onions, lettuce, carrots, parsnips, basically anything other than chillies) sown later in March or early April grows at such a pace that it will quickly outstrip the early birds. I say this over and over again, in part to remind myself; but seeds sown in step with the season grow in time with the lengthening days, and are much more likely to take on the slugs, snails and other marauders out to get them.

However, if you are impatient like me, you are bound to be running out of space. There are trays of seedlings everywhere, and thus comes the inevitable juggle of hardening off.

Hardening off is a necessary process to toughen up that which was once warm and cosseted. As a rule of thumb, it takes two to three weeks and needs to be incremental – a little colder each day.

Indoor growth is often weak, a little spindly even; hardening off toughens the plant up. The leaves and stem structures will start to thicken and the surface will become waxier to take on the new conditions. If you don’t do it and there’s a sudden cold shock, growth will be checked and the plant weakened.

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Hardening off is best done in a cold frame or unheated greenhouse. But you don’t have to purchase either of these – just reuse old packaging materials. Polystyrene is brilliant; use an old fishmonger’s box, or make your own from sheets of the stuff. Being white it reflects the light, and yet also protects from frost. Put a sheet of glass over the top – I favour shelves from old fridges because they are so tough – and you have a very good cold frame for free. It won’t be pretty, but your seedlings will survive.

For the first week, keep the lid on day and night, then it comes off bit by bit until being completely lidless. On a very sunny day seedlings may wilt; if they are not dry, put netting or even fleece over the unlidded box to create a bit of shade as they adjust to new light levels. In a month’s time, hardening off may not be required if the nights are consistently above 10C. Right now it is a necessary dance.

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