I've posted a link below to the Dahlia Forum since I think they are best qualified to answer your questions. I acquired some Dahlias for the first time in 9 years from a friend in early spring and decided to get back in the swing of things with them as they are deer resistant and just so beautiful. Needed some insight and the DF was very helpful and kind. I used to grow quite a few on another property until a back injury stopped me, but that is passé and I'm ready to get back in the game. :O) You do get snow in London so I would imagine that the foliage will die sooner or later. I did take mine out before the foliage went because I had to but it was just a matter of two weeks before they would have succumbed to a frost. You might be a bit different with your temperate weather. Whatever you do, leave at least a few inches of the stem or you may lose all the 'eyes' which will grow next year. I dig then hose the tubers clean, then place in my cool basement to dry. I did divide this fall because I'm too busy with winter sown seedlings in the spring. Whatever I can do now, I do. Here is a link that might be useful: Dahlia Forum.

To: portia, That was exactly the same circumastance I went through the last 2 years with dahlias. We got LOTS of rain last year from April through July, into August, which is odd considering it normally starts getting dry in June in the mid Atlantic. So even though I used a mix of potting soil with gardening soil and some native soil, it was waterlogged, so I had to replant twice last year. This year I assumed we wouldnt get all the rain we got last year, but in May we had a huge storm that dropped inches of rain and when I looked at them the next day, it was like digging a hole in the sand at the beach, when you dig deep enough you run into water. Well, all I had to do was dig down 3 inches around the dahlia tubers, and I was hitting water! So I dug them all up, buried the sopping wet soil back into the hole, but got rid of about an inch of it, then made a new mixture of about 50% sand, built mounds roughly 6-7" tall and buried the tubers inside the mounds at about 1" above normal ground level, so that in reality they werent deep enough to become covered in water again. They dont like being dug up & replanted after they have been in the ground a few weeks, and it caused them to stop growing for about 2 weeks to reestablish their roots. From now on I think mounds are the answer to planting dahlias at least til I find a better place to plant them.

I live in the Pacific Northwest. Some people say you can leave dahlias in the ground but to cover them with oil cloth to keep them from getting wet. It's not the cold that kills them; it's the soggy weather. I usually start with one tuber and in one season, it multiplies into a monstrous clump of tubers. I do dig mine up and use a method described on this line of wrapping them in Saran Wrap and storing them at temperatures of 40-55 degrees.

I just attended a discussion given by the Dahlia Society. The recommendation is to NOT dig Dahlias for storage, but to mulch for the winter, then AFTER they've sprouted in the garden, carefully dig and divide them, be certain to always keep a growing bud or sprout on the newly divided tuber, a section of the base of the previous year's stem. You can replant them immediately, or share the extras around. And a note... If a tuber breaks off and it doesn't have a bud or sprout, it will NEVER sprout. Another tip is to put a stake in the hole when you plant the new or newly divided tuber, that way you won't skewer the tuber. I grew Dahlias for years in California, and though it is admittedly a milder climate, culture is the same.