I found this out only recently myself. Also, if you happen to have ENDISWAL, this lets you destroy plants without angering the Earthmother (excluding the magic immune ones, of course).
i love the Earthmother. She's useful if found early (where Stone Skin and Greenblood make a significant difference), useful if found late (plantation + clearance for mana spikes), useful throughout the entire dungeon (mass slow for 5 Piety!), great for converting out (Plantation for easy 100 Piety, great for converting into anyone except Dracul), great for desecration (costs 1 burn salve) and not only gives you IMAWAL, but gives you ways to use it without having to sacrifice Popcorn! It's also very easy to avoid angering her. Oh, and acid casters. Basically, it's very rare that I feel the Earthmother to be a waste an altar spot. If only she weren't so painful to use in the vicious dungeons...
(Obviously the people who've posted in here didn't need me to tell them this...

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