Clumber Gardener: Protection is key for those tender new spring plants

Chris Margrave, head gardener at Clumber Park
Chris Margrave, head gardener at Clumber Park
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For most gardeners, May is the busiest month of the year.

The grass needs mowing, weeds are growing apace, there are seeds to sow and spring flowering shrubs have finished flowering and will either need their faded blooms cut off or, in the case of forsythias and flowering currants, need to be pruned.

May is also the month when many different kinds of plants are moved from a greenhouse to outside.

This applies to tender perennials, like dahlias and cannas, half hardy annuals, such as those grown for bedding displays, containers, and those for hanging baskets, such as trailing begonias, petunias, French marigolds or busy lizzies.

Vegetables from warmer climes, such as runner beans and sweet corn are also moved outside at this time of year.

All need to be acclimatised to outdoor conditions, a technique called hardening off.

The UK’s climate is classed as maritime, with cool summers and mild winters, as opposed to the continental climate of the European mainland, which has hot summers and cold winters.

The upside for gardeners is that we have the opportunity to grow a wide range of plants which will survive outside most winters, but if grown on the same northerly latitude in, say, Germany or Poland, would be killed by the winter cold.

The downside with our maritime climate is that the division between spring and summer isn’t clear.

This is especially marked in May, when night time temperatures can vary from 10C to below freezing.

So care needs to be taken when transferring plants from a warm greenhouse to the lower temperatures and breezes and winds of outdoors.

Hardening off is best done over two to three weeks and involves gradually exposing plants to cooler conditions.

For the first few days, put the plants in a cold frame, then progressively open the top of the frame overnight, then take plants out of the frame and cover them with horticultural fleece overnight, then leave them uncovered.

Obviously, if a sharp overnight frost is forecast, put the plants back in the greenhouse or the frame.

Hardening off will toughen up frost-sensitive plants, but it won’t make them hardy and frost-resistant.

If you’ve already planted them out, cover them with a double or triple layer of horticultural fleece.

Potatoes planted last month will have started to produce shoots and leaves above ground.

These are tender and may also be damaged by frosts.

They can be protected by ‘earthing up’ that is, by drawing soil over them, a process which should be done anyway, as it can help increase yields, or again, by covering the vulnerable growth overnight with horticultural fleece, sacking or newspapers.

The newspapers will need securing, with stones or soil, as they will blow away in the lightest of breezes.

This may sound like a lot of effort, but if you’ve spent time growing your own, or you’ve bought plants from a garden centre, it’s worth taking the trouble to do this.

A little time spend now will set you on the way to an early display of flowers, or to a good crop of potatoes, sweet corn or runner beans, from plants which haven’t been checked or damaged by night time frosts.

Now we are into May, it’s important to water new plantings in dry weather.

A thorough soaking to the soil around the plant is better than asuperficial spraying to leaves and the soil surface.

Watering in the evening will make best use of water and mulching the soil, that is, covering it with a layer of material, such as bark chips, well-rotted manure, leaf mould or mushroom compost, will help hold moisture in the soil.

On dry, sunny days keep weeds in check with a hoe.

Continue staking taller growing herbaceous perennials such as delphiniums.

Sow carrots, beetroot and salad leaves such as lettuce and rocket.

For best results, water the base of the seed drill before sowing your seeds.

Hardy annuals can be direct sown into prepared soil this month to provide easy colour.

Candytuft,pot marigolds (calendula), love-in- a-mist (nigella) are all suitable, as are red field poppies.