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Hi Missy01, for Dahlias to stay good for many years and help to increase your plants, it is best to leave the plants outside in situation and watch for the foliage turn black or brown (frosted) at this point, lift the dalias either in their pots or remove from the ground, for digging up from the ground, use a garden fork, place the fork a good bit away from the plants stems as you cant see the size of the tuber now under ground and is a season, they can grow huge depending on type of Dahlia, gently lift all the plant AND soil place this onto a newspaper that you have laid on a tray, barrow or whatever you can use to take your tuber away for preparing to store over winter.
When you have removed the tuber AND foliage, use strong Secateures with clean blades to cat off the blackened/frosted Foliage, CUT about 4-6 inches from tuber and leave this 4-6 inches alone but remove the other greenery. gently shake away a little of the soil too, you may need to gently use fingers to remove some more, BUT dont have BARE tuber showing just yet.
Set the now prepared tubers UPSIDE DOWN so you have the tuber sitting like it has legs (the short stems are legs) I leave my tubers like this for about a week or so, this allows all the moisture to be removed from the hollow stems as to leave this alone, your tubers will rot over the storage period.
Next, go back and by this time the remaining soil will be able to be removed better without too much damage to the tuber. the dried soil will be easy to crumble away by gently using fingers to help, I also have to gently use maybe a plant label or pencil to help get some soil from in-between the fat tuber as while it has been growing, it will have grown new fat tubers under the soil to build it's self uf for the following year.
At this point you need to check the tubers to discard any really damaged parts, the stems you left (4-6 Inches) should be dried and woody by now. you may also have to cut away any wet/ mushy areas using a very sharp knife (I like a craft knife as you can change blades and also because the blades are sharp making for a clean cut reducing the chance of disease setting in,
Now you have 2 choices of storage, I like to go to garden store and buy a powder called (Flowers of Sulphur) this turns yellow when on the tubers, use it by pouring into polly bag, then into the bag, place the tuber (separate plants at a time) give the bag a gentle shake Roll about and this coats the tuber, once this has been done, either place each tuber into a wooden shallow box or plant tray, put a little compost in the bottom, gently place the now prepared tubers into box and try spread out the tubers, cut away any damaged roots etc, then pour or hand lay some compost around the tubers, you are not replanting so dont cover the tubers with compost, all you are doing is making sure the tubers dont rot, dry out to the stage where theu completely shrivel up, AND you want to be able to check them so buried under lots of soil eliminates that.
IF the soil gets too dry, mist it to keep JUST damp in no more.
Come next spring, bring the tubers in there boxes / trays into more light and look for very tiny new shoots, (these appear very close to the stems you left (4-6 ) inches, dont let water get into the hollow stems or rot will get to tubers.
The other way to store the tubers in to clean them as before, pack them into boxes of horticultural sand and keep in dark but dry area, always keep checking tubers every few weeks as insects like them, slugs / snails, earwigs etc all like over wintering in the soil /stems etc and rabbits, squirrels both love the tubers to eat so maybe you will have to lay chicken wire over the boxes if you have the latter 2 pests.
The best results have always been to use the first method I gave but a lot of people manage without using the fungicide of Flowers of Sulphur but there are other fungicide on the market now so ask at the garden store.
Hope all this helps you out and next year you have a great show of Dahlias and pos be able to cut the tubers up with a little bud attached for making new plants.
Best of luck. WeeNel.