If you are not looking for 6 piece bonus, roots is possibly the best place for the proc
Roots is a key damage power for a Plant controller - it would be hard to slot it for anything other than damage. The AT-IO set enhances control, other than the proc. I would consider putting 5 of the AT-IO set in Vines or, if you take it and slot it, Spore Burst. You need 6 if you are seeking the Ranged Defense bonus, but if you can give that up, then put the proc in Roots and the set maybe in Vines (unless you want to slot the purple set there.
Another option might be Strangler. I use Strangler as an important single target damage power in addition to the Hold, but you might get by with 5 of the AT-IO set and then a damage in the 6th slot.
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Thanks all. Any particular reason why put 5 in spore burst over seeds? Just curious I have both and neither is filled with purples at the moment. Sadly I'm on an orange budget. Really wish I could get a range in seeds but that's probably just being greedy on my end.
How about thunder clap? It's a fairly spammable aoe with no base damage (ie, you're not wasting the damage potential of roots) but actually has a longer base duration than vines. It's only mag 2 but it can stack with tornado and/or fissure to hold bosses.
You could also just put the proc in a different power from the rest of the set.
I haven't really seen a definite answer as to whether it will proc more than a regular/purple proc in aoe powers, even ones with long cooldowns (the formulas I've seen suggest no, the anecdotal evidence says it "feels" like it procs more). Someone is just going to have to go on the test server and find out.
The reason not to put it in a confuse power is that confuse sets give awesome bonuses, even the yellow/orange ones, so it's kind of a waste to slot the ato set instead.
I'm kind of curious as to how well the ato proc would work in carrion creepers in practice. In theory, there's the carrion creeper patch which would check every 10s for every defeated enemy. Each of this mini patches has a small radius and low target limit and so should give the ato a better proc rate than you'd see in a larger aoe.
So I ended up with an ato proc last night for my plant/storm/stone and my google searchfu seems to be failing me. I thought about trying to squeeze it into carrion but then since it affects control I thought it would only work on the immobilize like trap of the hunter and thus not all that great.
Then I thought about roots as my dom's ato proc works fine in his aoe immobilize but not sure on the controller end.
Any suggestions?