I'm so incredibly happy that they survived! I've been watching videos for how to divide them but I'm inclined to leave them as they are.

Lucky you! What a treat to find tubers you thought were gone are thriving! I would suggest splitting them - they get progressively more crowded and harder to split each year you leave them. They will get so crowded it will affect the vigor of your plants, and they bloom less. Especially the one in the top pic - it already looks pretty crowded.

I, too, got lazy in digging up my dahlias and all seemed to survive the winter. All cannas survived as well. Plus side of our extremely mild winter.

There were some survivors that I missed - they're five or six inches now. Can I still divide them?

Absolutely... just leave a sprout on each tuber/mini-clump you divide off.

If a sprout is knocked off, don't worry, a new sprout will grow right next to the base of the break point within a couple weeks. Growth tends to accelerate as the weather becomes nicer, so you won't have missed the window of bloom opportunity.

I was noticing that J Pru suggests splitting. I have a few questions since this is my first year with Dahlias. In mild winters (which we typically have where I live), do they expand like the Day Lilies do? And how do you split them? I read a reference that suggested to only do so if you know what you are doing, and if these things expand like you say, I'm going to be in serious trouble next year. Mine are near my sidewalk, and are already looking like a jungle on he entire third of the garden. Don't get me wrong, I love my Dahlias, but need to be prepared. I didn't know they expanded, but I did notice that one set of tubers often gave multiple plants.

I highly suggest you check out 'dividing dahlias' on YouTube, and watch several so you get a good idea of what you'll be finding in the fall. There are lots of good tutorials on YouTube. It's a tough thing to try to describe with words.

I absolutely LOVE dividing tubers in the fall. Its a challenge for me to get as many viable tubers off one clump as possible... a living puzzle! Of course, they overwinter better when you don't micro-divide them, so there's a balance that the over-ambitious grower has to find.

Update: The surprise survivors are doing really well! I'm so glad they survived because the newer dahlias I planted this year have grown tiny flowers by comparison. Here are some photos:

The first blooms are always so fun to watch. Thanks for sharing!