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= an almost permanent Healing Rain with no cast time (other than a GCD) and a three second cooldown, a large triggerable AoE heal and 20% extra on your instant cast Swiftmend. Seems pretty good to me, and you're not exactly giving up much to do it.
Currently, (12-30-14) this glyph is bugged. It does allow you to place your Living Mushroom as stated, however it has two hidden bonus perk.
The first super secret bonus is that it actually INCREASES the mana cost of Living Mushroom from 6,912 to 8,640 mana. Hooray!
The second super secret bonus is that it brings back the old tool-tip information of Living Mushroom, back in the days when it could receive a healing increase from Rejuvenation overheals, as well as making references to using the ability Living Mushroom: Bloom which no longer exists.
My guess is that this glyph in someway triggers the old mana cost percentage of the spell, prior to it's changes in Warlords of Draenor.