Could be over-fertilizing combined with over-watering. I hope there are drainage holes in the container.

Plants might be trying to conserve water due to low indoor humidity, maybe there has been increased use of your heating system during the dull period.

At any rate they are definitely spindly, due to indoor light levels. When you've done this before they haven't burned up after being moved out because of the major differential between indoor and outdoor sun exposure?

the heat definitely turned on when the weather turned. I'm in central Alberta, Canada. So we're still getting below freezing every night, and fully expect another good snow fall before spring truly arrives. its 12 degrees celcius today though, and beautifully sunny, so I've moved my dahlias outside (I have a south facing back yard)

in previous years they've had no difficulty switching from indoor to outdoor. i grow them in a south facing alcove window that gets direct sunlight all day (albeit through a double pane if glass?) my Dahlias have pumped out lots of great flowers and the foliage seemed to thrive - until this year. I really appreciate your input and am giving better sunlight a try. They certainly are spindly!

Some of my blooms from last year for posterity

Aphids can cause curling but usually a different look than that. Could it be some kind of sucking bugs?
Or if you fertilised with compost, if it was made from hay or manure from animals eating hay from fields sprayed with the aminopyralid class of herbicides, it could be that.

It could be herbicide residue in the potting so, too.

Looks like the start of a spider mite infestation. The curling back of the leaves is the earliest sign of mites. Once you see the webs, the population of mites are so high that you'll have a VERY hard time getting rid of them... so there is hope if you move quickly.

Washing every leaf and stem with soapy water can help at the early stages, along with a light soaking of mildly soapy water in the soil, then a horticultural oil spray on all growth after that.

If the leaves have started to become brittle, its too late for those leaves, and you need to pinch the growth back down to the tuber. No worries, it will regrow. Brittle leaves will never recover, and you need to cut your losses quickly to get the plant back on track. Be sure to put the debris in bags and throw out in your trash! Disinfect any tools in 10% bleach. Wash hands between plants. Don't let cats rub against plants, transferring pests. Eat lots of dark chocolate. Well, okay, I suppose the last suggestion is a personal preference when dealing with a mite outbreak.

I see you have a lot of different plant species started... the mites most likely jumped ship from one of them, so once you get the dahlias cleaned up, you might want to isolate them. Some plants are more sensitive to spider mites than others.

Good luck!

CiCi

i started getting this exact issue, in september with outside plants. the leaves are curling, growing darker green. the blooms are tiny and don't really open up. or if they do, they look old. it started in one plant and then another which is about 3 feet away. it's almost as if the plants are dehydrated but they are not. the leaves are rubbery/leathery like. even the new growth is like that. i can't seem to find anything on the internet about it.

broad mites

@Kat Crowley (Zone 3a-4a) I was wondering if the plants did fine once you planted them out? I’m experiencing the same issue now, and I’m planning to plant them out in a few days. Thank you!

@Kim (Toronto z6a/7a) This was my first year starting dahlias early inside, and I had the exact same leaf-curl problem. I thought it was because they got water-stressed (underwatered) at one point. I definitely forgot to water them, the soil got extremely dry and they seemed to curl after that point. With subsequent regular watering I did not notice any new leaf curling. It seemed like it just took that one time and several of the leaves curled and will not uncurl. I planted them outside last week and they are growing just fine now. New leaves are not curling. I cannot speak to the long-term health of the plant obviously because mine are still young and not anywhere close to flowering yet...

@Brian Wintermyer i have the same problem as you did. I can’t find anything. From the tuber to sprouting my leaves look whilted and tiny blooms. Any suggestions?