Flowers aren't my 'thing' but, working as a lad in a Sussex nursery, the dahlias were lifted and stored in boxes over winter so that they could be divided to increase the following year's stock before re-planting. That could be an option if you want to add to your display?
They are still in full flower up here to! We dig them out knock off the soil and put them into cases which are kept in the room with the boiler over winter. (it's a pellet boiler and is in the barn so this is an area just above freezing)
They will thrive better, get into growth more readily, and grow better, earlier, if left in the ground, but you can't leave them if conditions don't suit. It's a simple choice
It's a place where beautiful isn't enough of a word....
I live in west central Scotland - not where that photo is...
Got my first dahlias this year. I am located in London, so rather mild, but it's the damp I am concerned about if the rains of last winter reoccur. I have lifted, cleaned and dried the tubers then placed them in a cardboard box lined with newspaper and covered them with dry compost. They will overwinter in the shed. That being said, I would have had to lift them anyway because I want to change their location in the garden.
Dahlias left in the ground overwinter will not start to grow again until the soil temperature reaches 55 to 60 degrees Fahrenheit. For many, this is not until April/early May. Commonly, dahlias will bloom about 8 to 10 weeks after starting into growth hence most dahlias will be in flower from at best, late June to July. Furthermore, you need to be vigilant in order to protect the emerging shoots from slugs and snails.
Dahlias which have been lifted can be set to grow earlier if you have somewhere suitable. The soil temperature of pots/trays of tubers in a conservatory/propagator will get to 55/60 degrees quicker than the ground. A rough rule of thumb is that dahlias in growth can be planted out at the same time as outdoor tomatoes, say mid to end of May. This confers some advantage on overwintered ones and they are also at a stage when they are more resistant to the slug/snail onslaught.
I always lift my dahlia collection waiting a week after they have been blackened by frosts. Leaving them after a frost is rumoured to help the eyes form. I store mine upside down in trays covered with dry spent compost. I turn them back the right way up in about February so as to make sure any early sprouts are growing in the right direction. I was late once - they got turned into cuttings!
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I live in west central Scotland - not where that photo is...
Dahlias which have been lifted can be set to grow earlier if you have somewhere suitable. The soil temperature of pots/trays of tubers in a conservatory/propagator will get to 55/60 degrees quicker than the ground. A rough rule of thumb is that dahlias in growth can be planted out at the same time as outdoor tomatoes, say mid to end of May. This confers some advantage on overwintered ones and they are also at a stage when they are more resistant to the slug/snail onslaught.