Well, I've never tried the combo, but I have a plant/ and a /sonic seperately.
So, I can say that Sonic is very kind to slots, so you'll have a lot of slots to put in to plant, which is nice, because plant like slots. I'm not too sure about the AI on Plant's Pet, i skipped the pet personally, but if the pet goes in to melee range a lot, then Disruption field will give you a good boost.
if you're teaming a lot, get the ally shields, though, if not, your pet will make good use of them anyway.
Liquefy will help creepers hit more, so that'll be good, infact, I'd say you may even want to slot Liquefy with Def-debuff to enjoy this even more.
I've only seen one plant controller without Seeds of Confusion, so...I guess you can take your pick on whether you want that or not, but I wouldn't go without it.
Spirit tree plus res shields may make the Tree a bit more effective at healing damage, since less is being taken, that's a neat route to go.
I can't say I think this combo provides any real synergy or dis-synergy (I'm pretending that's a word) but, it may turn out to be fantastic!
Also, if it's really good, tell us here! maybe we'll all role one and it'll be a new awesome.
Anyone have experience with a plant/sonic?
Thoughts?