Yes, they are very persistent! One of the first things we did when we moved in last summer was clear tallow trees from the pond edge, but not all of the stumps were removed. Of course they started resprouting and we're constantly battling them. RoundUp doesn't work, a harsher brush killer is needed but we haven't gone that far yet since we aren't keen to use many herbicides. Needless to say, we will likely always battle seedlings sprouting since the rest of the pond edge has tallow and unfortunately we don't own those properties to get in and work on cleaning them out.
I guess I'll let you know how tallow works when I get them installed.