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How to harden off plants

Potted geraniums on the rickety shelves
Potted geraniums on the rickety shelves

Blog: Plants under glass grow soft – how to toughen them up (gently) to cope with life outdoors…

We’ve had a real blip in this warm spring – or the weather went back to normal – take your pick!

One of the things you dread as a gardener in North-East England is the wind blowing from the North or East in winter or spring, bringing with it low temperatures and extremes like The Beast from the East.

And last week we had it – biting winds coupled with a night frost – read more about it here.

Like a scout, you have to be prepared, as we’ve hit the time when plants need to start hardening off. You’re looking at things like (more details for all of the highlighted plants):

Terracotta containers and chimney pots filled with agapanthus, fuchsia, geraniums and dahlias
Terracotta containers and chimney pots filled with Agapanthus, Fuchsia, geraniums and dahlias

In my garden, this is the second wave of plants to go out – first were the almost hardy succulents. This week is those that have been in the unheated greenhouse – geraniums, cannas, scented-leaved geraniums, agapanthus and dahlias. The door has been left open, so they’re well on their way.

Frost-free nights

According to the week’s weather forecast, we’re frost-free with mild nights, which looks pretty perfect.

Last (in June) will be runner beans, courgettes, tomatilloes, bananas, ginger and coleus.

Everything is grown in pots, as my soil is heavy clay. Container gardening also means you can arrange them to produce different effects (like hiding the car which is permanently on the drive now).

A good tip is to keep pots grouped together. It makes watering much easier and if there’s a late frost, just pop a large sheet of horticultural fleece or net over the top.

Even hardy plants get used to regular watering, still air, and stable temperatures. Putting them outside to survive in widely fluctuating temperatures, much stronger sunlight, and winds will lead to a check in growth, even death if they are caught by frost.

The effect of hardening off is to thicken and alter the plant’s leaf structure and increase leaf waxiness. It ensures new growth is sturdy although much slower.

Harden off gradually

You need to harden off gradually, over a couple of weeks, depending on the weather. On a mild day, start with 2-3 hours of sun in a sheltered location.

Protect seedlings from strong sun, wind, hard rain and cool temperatures. Hardy plants acclimatise faster than tender kinds.

Scented geraniums (Attar of Roses) in pots either side of the blue bench
Scented geraniums (Attar of Roses) in pots either side of the blue bench

If you don’t have a cold frame, place plants in a sheltered position in front of a south-facing wall or hedge and cover with fleece to prevent sun scorch and temperature shock.

For the first week, leave outside during the day, but bring in at night. In the second week, leave outside at night, but keep covered (unless there’s a frost forecast).

Towards the end of the fortnight, remove the fleece during the day. If the weather is suitable, leave the plants outside at night but ensure they are covered. After this, leave them uncovered before planting out.

Covering with an old curtain or extra fleece can protect from sudden sharp night frosts.

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Mandy Watson is a freelance journalist and an incurable plantaholic. MandyCanUDigIt grew from the tiny seed of a Twitter account into the rainforest of information you see before you. Gardening columnist for the Sunderland Echo, Shields Gazette and Hartlepool Mail and editor of the Teesdale Mercury Magazine. Attracted by anything rebellious, exotic and nerdy, even after all these years. Passionate about northern England and gardens everywhere. Falls over a lot.

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