Leyendo los comentarios en wowhead se ve que no sólo proccea con instants sino que está directamente relacionado con la cantidad de targets afectados.

"By Prinbball (844 – 3·20) on 2009/12/24 (Patch 3.3.0)
Resto Druids:
You get 5 (6 with glyph) stacks of this buff when you cast Wild Growth.

Heyqt on 2010/05/23 (Patch 3.3.3)
A paladin's greater blessings also do this effect, 8 people get buffed, you get 8 stacks"