Gardening: Hardening off plants should be a gradual process

Plants hardening off on greenhouse staging.
Plants hardening off on greenhouse staging.
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One of the problems with starting plants off under glass is that eventually, they need to be hardened off.

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

Hardening off by a warm western-facing wall.

Hardening off by a warm western-facing wall.

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.

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.

A home-made shelter made of bubble wrap and metal poles.

A home-made shelter made of bubble wrap and metal poles.

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

Don’t plant out tender plants before the date of the last frost, which is usually the end of May/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-facing wall or hedge and cover with fleece to prevent sun scorch and temperature shock.

I make a small lean-to out of poles and bubble wrap – it’s 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).

Towards the end of the fortnight, remove the bubble wrap during the day (it has plently of airflow).

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.


Sow cauliflowers, purple sprouting broccoli and winter bedding plants, such as violas and pansies.

Spreading and trailing plants such as Alyssum and Aubrieta can become tatty, so trim them back after flowering.

Put supports in place for herbaceous plants like peonies that produce heavy blooms and can be battered by winds and rain. Keep an eye on tall-growing plants like hollyhocks and delphiniums – better to stake them early then not at all.

Lift clumps of forget-me-not once the display wanes, and before too many seeds are released.

Pinch out leading shoots on plants such as Chrysanthemum and Helianthus to encourage bushy plants.

Pot on root-bound tender plants, such as geraniums.

Tie in climbing and rambling roses as near to horizontal as possible, to restrict sap flow, causing more side-shoots to grow along the stem, producing more flowers.

Put netting in place to protect all soft fruit from birds.

Earth up potatoes when the shoots are 23cm (9in) high, in order to prevent the new tubers going green, leaving 5cm (2in) of shoot uncovered so that the plant has enough foliage to continue growing.


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