I came across this same issue (with returning the bonsai bush to Leshy) and I REALLY wanted those 2 points for answering the riddle before just giving up and planting the bush at Erana's.

The key is to wait unitl Leshy disappears (after he's repeated the riddle) and THEN click the mouth on yourself to talk to him - even though he's "gone" he's still there. Just tell him about the bush in the goo. Success!!!

You need to use a certain inventory item on it (not the ritual, though). The "lines in the sand" description is kind of a hint about which item to use.