Storing dahlia tubers…
If you are one of the lucky few with free draining soil and mild winters, then storing dahlia tubers is easy - just leave them in the ground, mulch them to protect them over winter and then they’ll reappear in spring. Where I live in the Chilterns the soil is HEAVY clay and stays totally waterlogged from November to at least April. This means that if I want any hope of dahlias next year I have to lift the tubers and store them, otherwise they will just rot.
I tend to dig up the tubers in late October, or after the first frost, whichever comes first. I dig them up carefully with a fork (although inevitably there are always a couple of casualties where a tuber gets skewered while I’m digging). I cut the flowers stalks off, rinse off some of the excess soil (I don’t totally clean them) and let them dry out upside down (usually in the greenhouse but I have been known to take over the pantry to do this). I then divide them (more on this below) and stick them in a cardboard box to store overwinter in a cold part of the attic.
I cannot stress too much how important labelling is - you will not not remember what varieties these are once you’ve dug them up and stored them. I label the plants with tree tags while they are growing in the ground. Then label each tuber with a tag once I’ve dug them up, then write on each tuber in a sharpie marker. I also label the box. Might sounds excessive but works for me. I’ve tried many storage methods and in the past have wrapped them in newspaper, but these days I find it easier just to store them loose in a box and they do fine. The key is to keep them dry but not let them completely dry out and shrivel. I find a good temperature to store at is 5-10°c. If they shrivel a bit don’t panic - they will usually be ok when you plant them.
Dividing dahlias is how you propagate more plants of exactly the same variety - they will all be clones of each other, unlike growing from seed. If the clumps are small then I don’t tend to divide but with the bigger clumps I do this before storing, rather than the following spring, as I find it easier to see the eyes on the tuber. To grow into a new plant each tuber needs a healthy body, a strong neck and an eye. Eyes are always found at the top of the neck at the crown of the plant - tubers attached only to bottom of another tuber will not have eyes. A large clump could be divided into many individual tubers all of which will grow into a new plant if they each have a body, neck and eyes. There are many videos of how to do this including this great one from Moorfield Farm Flowers. I find that size does matter - tiny tubers won’t store well and will shrivel away to nothing but as long as the tuber is wider in diameter than my thumb then they normally store well. Don’t store anything soft tubers or anything with signs of rot or disease.
In late March you can pot up the tubers and start them off (just protect from frost and don’t overwater). I plant in slightly damp soil and don’t water them until they are showing signs of new growth.