Awhile back someone asked how I save my Dahlias from one year to the next. I have been very busy and forgot to post a reply.
For almost forty years I have followed the book and waited until they are hit by the first frost. If a frost is forecast but does not affect them, I leave them in. When they have been hit by a frost they will look all saggy and droopy. This is the time to cut off all the growth leaving about ten inches of stem/stalk. Dig them up after loosening the soil all around the plant. Without causing too much damage, wash all the soil away from the tubers with a hose pipe and store upside down in the greennhouse for a week. After a week any exess water will have drained out and you can now cut the stalks down to about three inches, cut off any damaged tubers with a sharp knife and wrap each set of roots/tubers in several thicknesses of newspaper with an open top, rather like a loose fitting bird nest. Alternatively, you can sit them on a bed of DRY compost in a deep seed tray or similar. Then sprinkle more dry compost around the tubers until they are 3/4 covered, making sure you get some compost into the spaces between the tubers. During the winter do not provide too much heat. They only need to be kept frost free, not warm. I check mine after about a month to make sure any damage has not started to rot the tubers. If I do find anything I cut it off with a sharp knife. You can also dust them with a yellow powder but I forget what it is called. In late Feb early April start to dampen them with tepid water and after awhile shoots start to emerge from around the stalk bases. When these shoots are about two inches long the clump of tubers should be cut into as many pieces as possible making sure each cut leaves you with one or two shoots on a whole or part tuber. I have in the past turnd one clump into ten new plants. These pieces should be potted up in three inch pots and when big enough transfered into four - five inch pots. Plant out in late May if you can protect them from late frost, otherwise plant out in early June. In recent years, because of climate chang it is possible to leave them in the ground in some areas. They will start to shoot then you should dig them up, cut them up and re-plant.
When I plant mine in the ground I first dig a hole about the same circumference as a bucket. I use pelleted fowl manure on my geggies and when a bucket is empty I cut the bottom off so I have a tube about eight inches long. I put this into the hole so the the top rim is standing about two inches proud of the soil. On a decent day in March, I half fill each bucket with compost and I trowel a handful of bone meal into it. When I plant the new plants I cover them so the soil in the bucket is two inches below the rim i/e the same level as the garden. This is because although I put four canes around each bucket, if a further one is needed, as long as you place it outside the bucket you will not damage the roots. Another bonus is when watering, this is required often in hot weather because dahlias do like water, I can give the plants a good drink and leave each one with a two inch puddle around it. This puddle can only soak downwards ensuring the plants get all they need.
If you choose to leave them in the ground, they should be covered with a couple of layers of heavy polythene and a large pot or bucket with soil or a brick on to stop it blowing away. When cutting them up, take a shoot with a small heel of tuber, dip it in rooting compound and plant into a three inch pot. It will have stunted growth but will be o/k. At the end of the summer it will only be a very small plant. Keep it frost free for the winter and it should start into growth again next year when you can take next years cuttings and put the year old plant into the ground whence it will mature like the original.
I hope this is useful to the person who asked.