I have a theory that Plant/Cold is a top notch combo. That's because of several things, but in particular the bottomless endurance bar that Heat Loss provides, which enables you to spam Roots and AoEs with no risk of dropping armor toggles. Seeds keeps enemies from striking back and the debuff in Sleet aids your damage. Based on what I've seen my Mind/Cold troller do to spawns I'm almost positive Plant/Cold after IOs should be able to rip through content. The only risk to the character is the lack of a self heal, but as I said my Mind/Cold character deals with that well enough. My Mind troller is a Mind/Cold/Primal, but if I went Plant I'd probably go Plant/Cold/Ice to take full advantage of the two AoE damage powers in Ice.
One thing about Cold is it sucks until level 35, gets pretty good at 38, and gets really incredible with IOing. The set provides the baseline you need to grab great recharge and ranged defense, and the endurance to run any toggle you want with no risk to your build. Once you play a character with perma endurance boosted Heat Loss you will see why Cold is often ranked alongside Radiation and Kinetics among the top sets. You just probably don't want to exemp with it too often. 
Another thread here asked about little used secondaries and several people mentioned Plant/Cold as a good combo. I have one and it saddens me that I just don't find her that fun to play. I love the concept and want to like her. Please tell me why I am wrong. Is it because I tend to solo?