I have the same problem with wildflowers. I try flagging them too but it doesn't always work, rocks do though - that's one thing dh looks for when he's on his riding mower.
I'm planning on putting this one in my cottage garden, it might be a good one for you too, hardy down to zone 4, it typically gets about 3 feet tall and wide.
New Jersey Tea (Ceanothus americanus)
http://www.wildflower.org/plan...
