Plant/?
/Earth picks up with the Mallets and Seismic Smash, but it never really gets any good AoE attacks (still better than Energy though).
Personally, I'd consider /Fire. Plant doesn't need mitigation from its secondary and /Fire will give you great ST and good AoE with a terrific utility power in Fiery Embrace.
I've taken a Plant/Icy to 50 and that was a lot of fun. Icy Assualt s good all-round set with both good ST and AoE, but it's not the monster that Fiery Assault is.
Plant/Fire is definitely a good combo. Throw out Seeds, start throwing out AoEs and you can really beat down spawns. Another good one is Plant/Thorns; effective and thematic.
Arc #40529 : The Furies of the Earth
I like /Psi on my Plant, if only because of Drain Psyche. Lets me avoid Stamina which is a Good Thing™ in my book.
I'm enjoying my plant/earth, but earth is more of a single target set for sure. I like it because I can throw out seeds and carrions to distract my enemies while I beat the crap out of them with my earth hammerz.
I'm beginning to get tired of playing my Brute all the time and I'd like to get back into playing a Dom again. My first redside 50 was my Plant/Psi Dom and the ride to 50 was incredibly fun, mostly due to my love of Plant Control's graphics(and also Seeds of Confusion is AMAZINGGGGG)
So, what else could I pair with Plant? I like AoEs and generally stray away from mostly ST sets, but if the set is good enough I can probably deal with it. I also don't plan on spending a ton of money in IOs on them so they can be a SUPERDOM, so something that can be good with minimal IO use/SOs would be ideal.
I just made a Plant/Earth and she's sort of 'meh', but most Doms are at level 4.