i kind of prefer positron's chance for energy damage. chance for kd is silly in a power with kd protection, and the chance for hold is low magnitude and duration (8 sec, mag 2, 25%), which i don't think is worth it. I don't know if the proc is effected by domination though, if it is it may be worth it with permadom.
positron's chance for energy and trap of the hunter chance for lethal.
Thanks for the information 
Once you hit 47, make the investment and pick up the Gravitational Anchor: Chance for Hold. It's a purple recipe, which means it's not going to be cheap, but is well worth the money spent as it's a 25% chance to hold your target - in the case of Roots, it's a 25% chance to hold anywhere from 1 target to all of the group you cast it on.
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I have my Plant/Thorns Dom up to level 41 and have hit permdom status. For the next two levels I plan on slotting up Roots to increase my AoE control/Damage.
I really want to max this out for I16 when I can greatly increase the number of spawns on a mission.
What are the best IO "Chance for" to add into the Roots power?