We have steady 8-10oC night temps in London now, so mine are out and growing like rockets. Just a matter of finding enough big pots. I have gone dahlia crazy this year. I have spilt and given away about 20 to neighbours.
I'm in NW England and I've put mine outside. When I was weeding this week I noticed some sprouting in the garden from last year which had been optimistically left in the ground over winter and they're about 10cm high and looking healthy so I figured it was time to plant out the ones I'd bothered lifting.
I’ve planted some of mine out too. I live on the Wirral. I’ve put gravel round the base to deter slugs and snails. This is my first attempt at growing them from seed. They all look very healthy at the moment 😊
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Keeping an eye on weather and would bring back in if frost predicted.
However, it's up to you if you want to risk them. Bear in mind that it isn't just temperature that can be a problem.
I live in west central Scotland - not where that photo is...