GARDENING: Hardening off plants is a gradual process

One of the problems with starting plants off under glass is that eventually they need to be hardened off.

Friday, 29th April 2016, 12:39 pm
Updated Wednesday, 4th May 2016, 3:08 pm
Sweet peas planted out by the end of April last year.

Even hardy plants get used to a life of luxury – regular watering, still air and stable temperatures.

Putting them outside to survive in widely fluctuating spring temperatures, much stronger sunlight and winds will lead to a check in growth, even death if they are caught by frost.

By the end of May, the plants are tough enough to be in the open air.

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I haven’t started hardening plants off yet due to the bad weather – all these pictures were taken last April/May, so don’t worry.

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.

You need to harden off gradually, over a period of a couple of weeks, depending on weather. On a mild day, start with two to three hours of sun in a sheltered location.

Protect seedlings from strong sun, wind, hard rain and cool temperatures. Hardy plants acclimatise faster than half-hardy ones.

Heath Robinson-style hardening off frame made of aluminium poles and bubble wrap.

Don’t plant out tender plants before the date of the last frost, usually the beginning of June in North East England – but have horticultural fleece ready.

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

I made a small lean-to out of poles and bubble wrap – not pretty, but it does the job. It’s also sheltered against a west-facing wall, which slowly releases its heat at night.

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).

By the end of May, the plants are tough enough to be in the open air.

Towards the end of the fortnight, remove the bubble wrap 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.


Heath Robinson-style hardening off frame made of aluminium poles and bubble wrap.

Thanks to such a cold April, everything’s miles behind. Don’t work against the weather, go with it – we’re all in the same boat – I reckon three weeks behind the same time last year.

Cover emerging shoots of potatoes with soil. If the tubers are exposed to light they turn green because of chlorophyll, which makes them have a high level of glycoalkaloids toxins.

If you’re seduced by ready-planted hanging baskets and bedding plants at the garden centre, make sure they’re properly hardened off – always ask.

In the veg garden, sow quick-growing crops, such as salads, between longer-term residents, such as brassicas, while they’re establishing.

Mulching fruit crops will help them to retain moisture around the roots so you will use less water.

Take softwood cuttings of deciduous shrubs, including Forsythia, Fuchsia, Hydrangea macrophylla, Philadelphus and Spiraea – choose non-flowering stems.

Check roses for signs of black-spot, aphids and leaf-rolling sawfly damage, treat if required.

Trim winter flowering heathers with shears.

After flowering, dig up and divide Primulas.

Tall-growing plants such as Delphinium, lupins and monkshood need a framework of canes and string around then to help prevent them been damaged by winds. Do this now so the supports become unobtrusive as the plants grow through them.

Give shrubs, trees, and borders a mulch of compost, to preserve water and smother any weed seedlings.

Keep nipping off the dead flower heads of late-flowering daffodils and let foliage die down naturally.

For more on these topics, plus cook what you grow, traditional recipes, North East information, environmental news and more, log on to (now smartphone friendly),, follow me on Twitter @MandyCanUDigIt or you can like me on Facebook at Mandycanudigit