I got some plants that were growing in a neighbors yard. A few of them had their entire root system cut off when they were dug up. I planted them anyway. My question is, will the plants regrow their roots and thrive or will they die?

I have soaked fans with crown but no roots in a solution of seaweed/SuperThrive, and had them grow enough roots to be planted, so yes, the roots can grow back. But I've never tried planting them right in the ground, so maybe someone else has experience with that.

@Minicopilot,

I see you are a new member, so welcome to the forum. I am not sure how familiar you are with the parts of a daylily. When you say the whole root system was cut off, do you know if any of the crown was left with the foliage? If not, then the plant will die. If part of the crown was left and maybe some small part of the root the plant has a chance, but you will probably be lucky if it lives.

A daylily can grow new roots as long as the crown is still attached, as Larry noted. But for the best chance of survival you will need to shorten the leaves if you haven't already, and keep the plants shaded.

I have planted fans that had the roots cut off, but still had part of the crown. These were planted directly in the ground. As Sooby suggested, I shortened the leaves, kept them well watered. They did survive.