Hi Karen, 

Thank you for reaching out to UVM Extension with your question. 

This could possibly be wind whip. Do you remember any big storms occurring before you noticed this damage or perhaps when the seedlings were young? If they were a little leggy when transplanting, the wind can twist the plants and cause damage. Here's a bit of info on this:

Wind whip 

If you are planting in a windy area, you could build a temporary wind block for a little while until the plants are bigger and stronger. Also, ensuring a good hardening off process when transplanting seedlings from indoors will help prevent injuries to new plants as they transition to the outside environment. Here's a good article on that process and timeline:

Hardening Transplants

Please reach out if you have any other questions at all. I hope this helps you as you plant that next generation of dahlias!