This backyard wildflower is about 18 inches high and the main stem has vertical streaks of purple. Thanks!

Ragwort?

I thought ragwort too.

Maybe the native Butterweed Packera glabella? Butterweed (Packera glabella)

Here are a couple of photos from our database (blue link above):

Thanks Lucy68 and porkpal. Plantladylin, thanks for the photos. I think Butterweed is what it is too. Same leaf shape, same type of flowers and the stalk (stem?) has the same purple "stripes".

Yes, I think Lin is right also. I've never seen one but the photos are certainly a match.