My geranium has been gorgeous and I would like to replant in the spring.
This article will help you understand how to overwinter your dahlia tubers: https://www.gardeningknowhow.com/ornamental/bulbs/dahlia/storing-dahlia-tubers.htm
Here's how to overwinter your geraniums: https://www.gardeningknowhow.com/ornamental/flowers/geranium/how-to-save-geraniums-over-the-winter.htm
I have used sand, peat moss and vermiculite over a decade for winterizing the dahlias. Would using shredded paper, not newsprint, also a good medium for winterizing. I live in Zone 8B.
The American Dahlia Society recommends storing tubers in vermiculite.
Since you are in zone 8b, you can leave your tubers in the ground and after frost kills the foliage, cut the plants back and cover with 3 to 4 inches of mulch.
https://www.gardeningknowhow.com/ornamental/bulbs/dahlia/dahlias-annual-or-perennial.htm
https://www.gardeningknowhow.com/ornamental/bulbs/dahlia/storing-dahlia-tubers.htm
I live in Massachusetts: must dig up bulbs in the fall. Wish to save for next year.
This can be done quite easily! This article will help:
https://www.gardeningknowhow.com/ornamental/bulbs/dahlia/storing-dahlia-tubers.htm
After planting dahlia tubers on my allotment. Even though they are in flower, some tubers are visible. Do I cover them over with some soil or leave well alone. Thank you
I would go ahead and cover the tubers with an inch of soil. Here is more:
https://www.gardeningknowhow.com/ornamental/bulbs/dahlia/tips-dahlia-planting.htm
I've grown dahlias many times but it was from already started plants. Never started from bulbs. I don't know if the clump of a bulb is 1 piece or if I can break apart to get more plants out of it.
They can be separated for planting! These articles will help:
https://www.gardeningknowhow.com/ornamental/bulbs/dahlia/dividing-dahlia-tubers.htm
https://www.gardeningknowhow.com/ornamental/bulbs/dahlia/tips-dahlia-planting.htm
https://www.gardeningknowhow.com/ornamental/bulbs/dahlia/dahlia-care-grow-dahlia-plant.htm
https://www.gardeningknowhow.com/ornamental/bulbs/dahlia/dahlia-companion-plants.htm
I am planning my 1st flower bed
Likely, you won't find many companions with Ivy. They tend to take over areas when not kept trimmed constantly.
There are many thing that can be paired with Dahlias, however. Here is an article that will help:
https://www.gardeningknowhow.com/ornamental/bulbs/dahlia/dahlia-companion-plants.htm
My dahlias are coming through soil but some rather than having one stem, have many stems coming up from the same tuber. I assume this is because there were more than one eye on the tuber. Should I get rid of extra stems, and if so, how? Or should I just let them grow as well?
Let them grow this year, since they are already growing. During dormancy, it may be a good idea to divide them up for next year!
This article will help:
https://www.gardeningknowhow.com/ornamental/bulbs/dahlia/dividing-dahlia-tubers.htm