They are certainly winter hardy in zone 8, so digging is not a requirement. However unless drainage is very sharp, you do run the risk of the tubers rotting over winter in damp soil conditions. Most of the professional growers do dig and store their tubers over winter so as not to run the risk of losing valuable stock plants and to allow for dividing, etc. Personally, I prefer to grow dahlias in containers rather than in the ground for the following reasons:
1) Winter is not an issue. Containers can be easily stored out of the weather to avoid over saturation. FWIW, there doesn't seem to be any greater risk for cold damage to the tubers in containers than in the ground, at least in my rather mild area.
2) Soils in containers warm up faster than ground soil does. Growth begins earlier and you have a larger, more robust plant earlier in the season than one that has been left in the ground over winter or freshly planted in late spring.
3) Since dahlias in the ground are late to appear in spring, one runs the risk of damaging buried tubers with other gardening activities, specially if not clearly labeled. Not a problem with containerized plants.
4) I also find it easier to monitor for slugs with container grown plants and slugs can do considerable damage to emerging dahlia growth. I can also regulate fertilization better with the containers.

gardengal---how big and deep of a container do you use?
Duane

I leave my dahlia tubers in the ground. I will put a few inches of new compost down to help insulate them. I do this mainly because I'm a bit lazy when it comes to digging/storing/digging/replanting. I have not had a problem...yet

Dahlias, Hedychium, Canna, even Brugmansia I leave in the ground. For me, if it doesn't over winter in the ground, it isn't worth having.

Cheers, Barrie.

Duane - usually a 3G black nursery pot, sometimes a squat 5G. Depends on the ultimate size of the dahlia I'm growing.

And I should have added that another reason (and my biggie) for growing them in containers is that I can move them around easily. Some dahlias can get to be big, foliar monsters and not necessarily something I want on a long term basis in my pretty-crowded-already planting beds. Nothing like being able to grow these plants to size and bloom-readiness in another, less crowded location and then slip the pots into a vacancy created by the cutting back or dormancy of an earlier season plant.

gardengal--I like the way you think. I'm also overcrowded due to lack of enough sunshine in my back yard. I cram alot of sun loving plants in one small area. I think I may try the container method with a few of them this coming year. Thanks for everyones input!!

Gardengal,

Can you dig them, divide them, and pot them up...and still have them overwinter well? I have bishop's children and two of the plants got to be almost three feet wide, but the tubers are in the ground right now...but they take up too much room in my tiny flower bed...can I pot them up now and have them make it, or should I dig, divide, and store...then pot them up in the spring?
Netta

I planted a few varieties of Dahlias back in 1988 when I first moved to my present house. Only one survived the first winter: Procyon. I had added a lot of sand to the soil, and Procyon was in a very sandy spot. Believe it or not, it is STILL alive, and grew over 6 feet tall this year. I fertilize it every year, and it gets watered once a week, along with everything else. It gets naturally mulched with leaves in the Fall.

I leave them but they don't bloom as early as ones in pots. One year I dug a tuber and forgot it sitting uncovered in a pot under my deck all winter. I found it in spring and planted it in the pot until it was a foot tall, then put it in the ground. It bloomed much earlier than tubers in the ground and made an enormous bushel basket sized clump of tubers by fall. That same dahlia, undug several years, was my earliest and biggest bloomer this year, planted at the edge of my driveway where maybe it gets some additional warmth. Several undug dahlias didn't make it to bloom. I think if they had been started in pots they would probably have bloomed. So I think it depends somewhat on the dahlia and how early it blooms.

A friend who digs hers, and plants them in very good soil in raised beds with a lot of fertilizer has plants that get to 6' tall and are loaded with flowers.

Interesting point about the bloom time. My Procyon also starts blooming very late, like late August.

I'm normally lazy and have taken a darwinist approach to my dahlias (survival of the fittest). The cheap-o ones I bought at costco have survived without digging much better than the fancy ones I bought at the garden show.

This year I *may* dig some because I want to move them next year -- we'll see if that happens or not.

I'm considering digging my favorites just to get the effect of starting them early in pots next year. I'm also looking at Park's and Thompson & Morgan's dahlias to grow from seeds. On the dahlia forum people rave about Park's Stargazer, a dwarf cactus-flowered one, and T & M carries some species ones that may be better at overwintering, but they are single. I also want to try some with dark foliage and dwarves- Bishop's Children and Harlequin (collette). I have a large new area to landscape and I'm thinking of filling it with dahlias from seed, nasturtiums, 4 o'clocks, and maybe some grasses.

Is it the cold or the moisture that keeps Dahlias from overwintering? I have some in pots that I'd like to move into an uninsulated garage for the winter. Will the cold kill them? Should I water them during that time? I have limited room to bring them all inside.

I live in a rental, and the landlady told me to leave the dahlias in the ground. They're in a raised bed and get good drainage. They didn't bloom well this year though and probably need dividing. I have my own little bedding dahlias in that same raised bed and they don't get dug either.

They are late to come up and late to bloom because it takes time for the soil to warm up in spring. But, dahlias are supposed to be late bloomers. I have plenty of spring blooming stuff already and the dahlias are for late summer when everything else has quit.