I fail to see how calling someone an asparagus would count as an insult, however...
She's a flower youkai, so it might fits.
I'm not so sure, but I think, Iku says that the cause of the flowers are all bend is due to yesterday's rain. (I can only read a small bit of japanese, so I won't bother putting translation unless I can read the whole bubble)
Probably because of Yuuka's green hair again, similar how you might call a redhead "carrot-top". (It also might refer to the asparagus fern rather than the usual eating vegetable; it's not actually a fern, but it does look more like Yuuka's hair.)
Whaoh man,she called Yuuka ASPARAGUS! You're really in for it now..
Hothead!Stupid, stupid, stuuuupid! Stupid Yuuka!Greenie!Iku-sensei!That means...PantSiscon!I told you you had it all wrong!If it's those flowers you're talking about, they were bent in yesterday's downpour.People like you are really...The Truth and the ConclusionPant...I'm sor--Asparagus!