Monday 19 February 2018

Gardening with Diarmuid Gavin: Dahlia dilemma

After a summer of floral delight, should you move your dahlias to avoid frost or risk leaving them in the ground?

It's Dahlia time for dedicated gardeners.
It's Dahlia time for dedicated gardeners.
Vase of dahlias
Climbing roses

Diarmuid Gavin

Despite our recent delicious, late spell of sunshine, there's a chill in the air. The high pressure lines on the weather chart means clear open skies and lack of cloud cover.

With no aerial blanket, mist, fog and (soon) frost are creeping in. In the coming weeks, tonnes of leaves will fall or be blown from trees and shrubs.

For some dedicated gardeners it's dahlia time. This species has delighted us in the dying days of summer with its range of startlingly bright and exotically-shaped flowers. But, now, it's decision time - should you lift the tubers and store them indoors or leave them in the ground to fend for themselves against potentially frozen soil?

Most of the colour we use in our gardens tends to come from warmer climates and survives here because of our temperate weather. Dahlias came from Mexico so it's understandable that they may not love our sometimes soggy soil.

An experienced gardener recently told me about his prized collection of 15 dahlias. Years ago, he used to take them out and store them, carefully wrapped in old newspaper, in an airing cupboard. His wife dumped the lot and so he took the decision to risk overwintering them in the beds.

Now, rather than digging them up for winter, he covers them liberally with a bracken mulch. He's only lost one to low temperatures over the last five years - and we've had some less-than-friendly winters in that time.

So, what should we do with our tuberous dahlias?

Even on our small island, climatic conditions vary. In milder areas, such as gardens near the coast, you may follow my friend's lead and use a mulch of bracken, manure, leaf mulch or bark on the dahlias. In the midlands, however, it may be necessary to lift before a freeze.

If you are lifting, here's how to go about it. First, cut down the foliage leaving just a couple of inches of stem. Wait until the foliage has been blackened by frost so the tuber will feel a jolt of autumnal cold.

Fork around the base of the plant to loosen the tubers, avoiding stabbing them. Lift from the ground and shake the soil from around the roots.

Next, hose them with water to completely clean them. Let them dry before storing (any dampness can lead to rot). Do this by hanging them upside down, and when dry but not shrivelled you can place the tubers in crates wrapped in fleece, newspaper or some old clothes.

An alternative is to store them in boxes and cover with sand or slightly damp peat. They can be dusted with a yellow sulphur fungicide and kept somewhere cool, dry and dark, where they stay dormant until around Saint Patrick's Day, 2016.

Take an occasional look at them through the storage period so that you can whip out any which seem dead or diseased. They won't go into the ground in mid-March - start them off under some heated glass in preparation for planting out in May.

Dahlia merckii has delicate pink flowers on tall stems. A great hardy dahlia, this can be left in the ground with a warming mulch cover.

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