My wife pointed out that a weed had invaded a bed I had been neglecting and was crowding out the helianthus and salvias I had planted there several years before. I took a look, and it was indeed a jungle of 5 to 6 ft tall plants spreading from shallow roots. I could pull out the smaller ones, but am going to have to excavate the ones with 5/8" thick stems.
There is a picture of a sample below. It has alternate leaves, mostly 5-lobed, green on top and silvery green on the bottom. Straight stems to 5+ feet tall, no sign of branching, and no apparent flowers or even buds by July.
I'm going to memorize the leaves, since I have no doubt it will pop up again, but can anyone guess its identity just to satisfy my curiosity?