I think this might be Senecio vulgaris, Old-man-in-the-spring.

Here is a link that might be useful: Google image results

That is grayish-hairy, with a different leaf shape.

Thanks for the response. I checked Senecio vulgaris and it's definitely not that. The flowers aren't composite. The leaves are soft. It started in the spring as a rosette similar to a dandelion, but the leaves were thinner at that stage and more sharply toothed. I've tasted it and it's milder than dandelion.

Is it possible it's a wild lettuce like Lactuca canadensis?

Sisymbrium altissimum?

It looks a lot like the prickly lettuce in our side lot. It is called Lactuca but I not sure.

I don't think it's the lactuca - it may be the Sisymbrium altissimum, but I need to wait for the flowers to fully open to say for sure. Thanks all for trying to help - the mystery is really frustrating me!