HOW TO OVERWINTER DAHLIA TUBERS |The Garden of Eaden

With milder winters becoming ever more common, there are now two schools of thought when it comes to over-wintering Dahlia tubers. The traditional method is to lift them and then store in a cool dry, frost free position, while the second and slightly more risky way is to leave them where they are but with the addition of extra insulation.

LIFTING AND STORING DAHLIA TUBERS

The practice of removing Dahlia tubers from the ground for over-wintering goes back to at least a couple of hundred years so you know that it is definitely going to work. The time to do this is always going to be dependent on the weather so come the autumn you will need to keep a close eye on your plants.

As soon as the first good frost hits, the leaves on the Dahlia will blacken and the plant will naturally begin to move into its dormancy stage. However you will want to leave it a week or so before further preparation commences so that the plant can adjust to the seasonal change and absorb nutrients and carbohydrates from the stems back into the tubers.

Cut the stems to about 6 inches from the ground and then using a fork carefully lift the dahlia so that when removed from the soil the tubers remains intact. You will probably need to circle the root system with the fork first to help loosen the soil before lifting. About 1 ft from the stem should be suffice.

Once lifted, gently place the tuber clump onto the ground, then carefully remove as much soil as you can without breaking or cracking the ‘necks’ of the individual tubers. Unfortunately, a tuber with a cracked or broken neck will tend to rot and will not produce new growth next season.

Remove any diseased or damaged tubers and trim off any fibrous roots to reduce the incidence of fungal infections, then wash the rest of the soil off with water and allow to dry – upside down - for a couple of days in a cool, frost-free environment.

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Prepare a container such as a seed tray or shallow box with a covering of horticultural sand, peat or vermiculite at the bottom. Now place the tubers into the containers and cover them with slightly moistened horticultural sand, peat or vermiculite. The container can now be placed into storage in a frost-free position such as a garage or basement or anywhere that has an even winter temperature of around that 4 – 7 Degrees Celsius.

During this storage time you should be examining the tubers at least once a month, throwing away any which are showing signs of rotting. If the tubers appear to be drying out, then sprinkle the covering medium with a small amount of water. If they to be appear too wet then remove them from their container and allow to dry off on some old newspaper for a couple of days before placing back into storage.

Come the following spring - and just before the growing season - divide the tuberous roots into sections using a sharp blade making sure that each section has at least one prominent bud. Dust each cut section with a fungicidal powder and allow them to dry for a couple of days. That way the cut surfaces have a chance to callous over before planting.

These new root sections can be potted on in John Innes No.1 but unlike most other plants it is important NOT to water them in. Label them and place them back into a frost-free area moving them into a bright position. Do not move into direct sunlight until the foliage has a chance to harden off.

OVER-WINTER DAHLIA TUBERS IN THE GROUND

Recent trials have shown that it isn't always necessary to lift and store Dahlia tubers so long as the ground is suitably prepared before planting. However, wet and freezing winters may still kill Dahlia tubers when they left in the ground, so it can still be worth lifting a few plants for storage – just to be on the safe side.

The key to successfully over-winter Dahlia tubers in the ground is to make sure that they were planted into a free draining soil in the first place as this will reduce the tubers becoming waterlogged during this risky part of the year. Also, it is advisable to plant them deeper in the soil than would normally be the practice - about 8 inches or so deep is fine.

The tubers will require additional protection to avoid them from being damaged by hard ground frosts. This can be achieved by employing by simple mulch such as straw, peat or even more soil. However using a traditional ‘Clamp’ will be the most effective.

HOW TO MAKE A TRADITIONAL CLAMP

I know that these are not Dahlia tubers but it is a traditional clamp

Take some straw and cover over where the Dahlia tubers are under ground. Now position more straw - in a vertical fashion - so that it forms a raised mound above the tubers. When looking at it, the lengths of straw should now be sloping away from the top of the mound to the bottom of it so that it draws any water away from the center of the mound.

Next the straw mound is ‘earthed –up’ which is a bit like making a sand castle on the beach. You dig a moat around the outside and you throw the excavated soil on top of the straw mound. When you get to the top of the mound you will need to leave a little straw chimney. This allows the mound to ‘breath’ which helps to stop fungal rots from progressing inside. The last thing to do is to smooth over the soil sides so that if it does rain the water will run off down the sides rather than enter into the mound itself.

The 'clamp' can be removed once the threat of frost is over.

A WORD OF WARNING