If you came to me and said that this was the character you wanted to play and how do we make this happen?
The poison stuff is easy enough with the Potions and Natural World skills. The other stuff we would have to hash out to find the best fit.
There are two basic approaches I would think. One option would be to go with the Unknown East magic system and give the magician the sphere of Earth and either the rune of Enhancement or the rune of Summoning. That would give you a wide latitude for defining magical effects that fit within the framework of your earth mage.
Alternatively, we could handle it with summonings and spells. There aren't a lot of earth based spells written up, but I think this is due in large part to the idea that magic in the Young Kingdoms is a sort of corruption of Chaos. Magic consists of tiny holes the Dukes of Entropy are poking through the manifest will of Law that is reality. GURPs Magic (2 ed) has some great earth and plant based spells that would be very easy to port over to the Elric! sorcery system. Also, there are some elementals stated up in the core book. If this were my game I would gladly let you stat up (for approval) some minor elementals you could summon.
A third option I would raise is being a friend of the plant lords. A lot of the stuff in your examples, extracting poison by brewing potions, the bramble blockade, sound to me to be in the realm of the Plant Lords rather than Grome's purview. I would suggest we took the work done on the Elemental Paths from Magic of the Young Kingdoms and make a Plant Lord path based on the same idea. Instead of using the GURPs spells as spells, the would become special abilities. Instead of using summoning spells with would put something together for your character to invoke the aid of the plant lords.
Rather than looking at it as having too many possibilities, I like to think of it as examples that provide guidance on how to institute unique ideas.