granted, this trinket is the worst for trees, the proper spell rotation to stack this most effectively i believe would be....

rejuv.... lifebloom... proc trinket... swiftmend... regrowth... bloom... lifebloom.... rejuv.... bloom... swiftmend.... (NOT 5x REGROWTH LOL)

you would get the 5 stack +30 healing bonus on one target after about 16 seconds

likewise, you could attempt to stack the buff on two targets, a bit less succesfully....
i will use 1 and 2 for each target

rejuv1, lifebloom1, lifebloom2, proc trinket, swiftmend1, regrowth1, regrowth2, bloom1, bloom2 lifebloom1, lifebloom2, rejuv1, rejuv2, bloom1, bloom2, swiftmend2

you would get 2 targets with 4 stack +30 healing with little or no time for error

NOTE: this +30 healing bonus does proc from tranquility's "ticks", so if i my examples didn't reflect a conservative attitude the spell rotation to stack this the best would be something like...

lifebloom1, lifebloom2, lifebloom3, proc trinket... tranquility... lifebloom4, lifebloom5

you could get a whole group to 5 stacks with minimal or no utility, woot! however, outside of simply stacking the trinkets buff the fastest, proc'n this b4 tranquility would be about the only time a druid could utilize this trinkets "use" (this still sucks, b/c you have a 2 minute cooldown to amplify a 10 minute cooldown)

FINALLY....
trees maximize healing by "rolling" lifeblooms, so allowing a lifebloom to bloom just to "boost raid healing" seems extremely counterproductive

QQ's about trinkets that suck for trees, this trinket rocks for chain heals