Not sure if this is a bug or misleading documentation.
The verdant charms are described as Infusion Enchantments, as such I'd expect to be able to stack them with the escalating costs seen with other infusion enchantments. However I just sacrificed my Sustainer for a Lifegiver instead. =\
It is misleading information, but actually when you see the infusion crafting recipe, you can see that you can use either *Blank* or *Lifegiver* to make Sustainer. Well, they are too strong to be stacked anyway.