Hardening off plants helps ensure sturdy new growth

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

Friday, 10th May 2019, 4:45 pm
Sunflowers and other annuals hardening off in the sun.
Sunflowers and other annuals hardening off in the sun.

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 – as we have now – will lead to a check in growth, even death if they are caught by frost.

Runner beans and perennials ready to go outside.

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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 couple of weeks, depending on the 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.

Sweet peas hardening off in a sink under fleece.

Hardy plants acclimatise faster than 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 the North East of England – but have horticultural fleece ready.

If you don’t have a cold frame, place your 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 tend to 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.

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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Thin out direct sowings of hardy annuals and vegetables, in two or three stages at fortnightly intervals.

Remove faded wallflowers and spring bedding, to make space for summer plantings.

Divide clumps of herbaceous perennials. Bamboos and clumps of bulbs or rhizomes can be divided in the same way.

Cutting back clumps of spring-flowering perennials such as Pulmonaria and Doronicum can encourage a fresh flush of foliage.

Lift and divide overcrowded clumps of daffodils after flowering. Deadhead tulips and daffodils. Apply a liquid fertiliser to bulbs after they have flowered, to encourage a good show next year. Allow foliage to die down naturally.

Inspect lilies for red lily beetles – they’re extremely destructive and active now.

Protect carrots with insect-proof mesh to prevent carrot root fly.

Liquid feed fruit trees growing in pots with a balanced feed every fortnight.