its a mystery dave, but looking at the leaves, it may well be a cedar seedling, leaves look about right, reddish stems ?, if so dave start pulling them up, pull them and pot them till you know for sure.
Is it cottonwood? We had trees like that in the yard when I was a kid. They popped up everywhere and had fuzzy seed things on them like caterpillars. (doesn't that really sound like a professional nurseryman to you?)
Cedar: Sometimes the most obvious answers are often overlooked. Of course these are cherry seedlings. The foliage is identical to the parent and, under the cherry tree, there are literally hundreds of these plants.