To enhance plagued ability which is already extremely powerful on its own, you use blight and druidic combo too.
Getting as many blight 100% possible is obviously a good priority if possible and maxing the regen of mp and hp to its cap. With druidic, when you can max the cap of both mp and hp regen, you can get poison dot x15 stronger.
That very strong ×15 stronger dot is multiplied by poison cloud 400% dmg . That 400% is multiplied by 600% blight for example meaning 5×7 = 3500%. The actual poison cloud now would deal 3500% dmg of the ×15 stronger dot.
3500% × 15 = 36×15 = 54000% dmg.
The poison cloud could theortically deal 54000% dmg and it is multiplied by 125% per enemy by plagued.
Lets say we have 20 enemies in the poison cloud.
Basically: 54100% × (2.25 ×20) dmg
= 541 ×45
=24345% damage poison cloud with plagued and 20 enemies
That's how damaging a poison cloud can really be but that is just the dmg before crit dmg is applied and all the other multipliers factored in such as adventurer, equality and crit dmg. This is just a theory as well but it's more accurate to try this with numbers than % but the dmg is huge for a very well built Green Garden build.
Also crit dmg multiplies the damage after its been multiplied by set affixes, legend affixes and epic affixes. Essentially,a huge after damage. Even if this is incorrect by a few small ways, I think you already get the idea about how powerful it is to have druidic, high blight and plagued.