Wild Growth (WG) - Places a hot on up to 5 party/raid members (unless glyphed), healing them once every second for 7 seconds. The amount of healing provided per tick decreases slightly over time.
Wild Growth is a very good raid healing spell that does a lot to mitigate the effects of AoE damage.
There are many situtions where you'd want to cast WG on cooldown, especially when raid healing.
In general, cast Wild Growth on the melee group (which is usually grouped up) as this is often the best way to insure it will hit the maximum number of targets. If you know that the ranged are somewhat stacked up then you may WG them as well, but melee is often your best bet.
Important: Wild Growth will always attempt to be cast on your current target, whether it be friendly or hostile. If you have an enemy targeted, WG will attempt to heal nearby friendly targets, if any exist.