We recently had our back yard terraced, clearing out a lot of the plants up there and then mulched the top of the top terrace in March. Since then, green leafy, stalky plants have been coming up. They grow super quick, I don't remember anything looking like this before we terraced but its becoming a infestation. The stems are very fragile you could just step on them to snap them but they keep growing back. The roots are crazy and we haven't been able to dig them out. The insides of the stem are white translucent inside with a hollow in the middle for the bigger ones. They have gotten to at least 4 feet tall but that's when we cut them down.