The Earthmother is the goddess of nature and plants, and doesn't enjoy heroes cutting a safari-style path through her precious flora. The Earthmother is one of the most straightforward deities in the game to please, and in practice the one of the few which you can reliably go the entire game without ever incurring any penalty to piety. She offers strong combat boosts in the early game, good experience boosts throughout, and potentially a lot of mana refreshment for the late game. However, her boons spawn plants which can cause exploration to be difficult.
The Earthmother is uncovered in a subdungeon to the South. Her subdungeon is completely revealed, filled with plants and level 9 "Gardeners". Cut your way across the room to her altar to unlock her, or using the WEYTWUT glyph on a nearby Plant is even simpler.
The only two acts that anger the Earthmother are destroying plant life and using the life-steal ability. The only act that pleases her is using IMAWAL (you can also use the Plantation boon to gain piety). This makes her fairly easy to play; petrify stuff to gain piety, and leave her plants alone. Since most dungeons don't have any plants to begin with, and the Earthmother does not create very many, it's unlikely you will ever have serious issues before the time comes to convert out.
The Earthmother's best boon is Entanglement, slowing every monster on-screen. Slowed monsters lose their first strike benefits, and also grant 1 extra XP when killed. For a mere 5 piety, this is a very good boon and one that you should almost always take. It's cheap enough to be used multiple times as a finishing move against single higher level enemies.
Vine Form is quite useful for low-level characters, granting extra hit points and damage reduction. Against low-damage monsters such as the Meat Man this can allow you to fight enemies three or four levels higher than yourself. When fighting high damage enemies, Vine Form can also be used to boost your hit points to a level where you can take one hit. However, these benefits do not scale with your level and they will be largely meaningless by the time you're level 10. Try not to invest in more than you need.
Greenblood boon adds a flat amount of corrosion damage per hit and ignores resistances, and therefore works well when more attacks will be required to kill the enemy. It's strongest in the early game, but the added damage remains relevant throughout the game when combined with attacks that deal low damage, such as knockback, fireballs, and monk hits. For example, four uses of Greenblood is equivalent to having an extra character level for the purpose of casting fireballs. The curse removal can also occasionally come in handy.
Clearance restores 10 mana if there are enough plants on the map to be cleared, at a very affordable piety cost. This allows Earthmother followers a massive amount of extra spellcasting during the late game. If there aren't enough plants remaining on the map, the full amount of mana won't be restored. Entanglement can be used to spawn plants cheaply if needed.
The plantation boon can be used to generate large sums of piety, and is particularly useful when converting into the Earthmother during the end-game. Keep in mind that using this boon will effectively preclude a conversion into Dracul, since those blood pools will no longer be available for him. If you intend to make such a conversion at some point, avoid Plantation.
The Earthmother grants 5 piety for petrifying plants and other non-XP enemies, and 10 piety for petrifying XP-yielding enemies. The best targets for this are plants, high level monsters that you are unlikely to kill such as the Animated Armour, and low level monsters. Petrifying plants can be used as a renewable piety generator, since the piety gained from petrifying the plants can be used to buy more boons which will spawn more plants. In this way, the penalty for preparing Earthmother's altar can sometimes actually be beneficial, although you will need to be able to cope with having your path blocked sometimes.
Sometimes you may need to chop down a plant or two as a last resort if you get blocked and take the piety hit, so in cramped levels consider maintaining a small piety buffer. An early Clearance can also be used for this purpose, particularly if the mana restoration can be put to good use to kill a high level enemy.
Earthmother's boons are strongest during the leveling up phase of the game. A dedicated Earthmother worshipper is often able to kill enemies one level higher than otherwise, and the bonus XP from these kills will be further multiplied by IMAWAL's 50% boost. This translates to more Entangled popcorn left on the field, and makes it easier to to level up twice during the boss fight.
One strategy with the Earthmother is to use a variety of her boons at a low cost, then either use IMAWAL or plantation to build up to 100 piety, activate the entanglement boon, and convert to another god with 50 piety. On the flip side, converting into the Earthmother to use plantation and then spend your piety on clearance to get mana for the final boss is a great "last-ditch" move for characters dependent on spellcasting. The Earthmother also makes a good "escape deity" choice if you just want to avoid taking further piety punishments. If there are no plants on the field and you don't have a life-steal ability, she will never punish you.
The Earthmother also has the least harmful desecration penalty on her altar, causing a mere 5 layers of corrosion. You can cure this with a burn cure potion, and it's not going to be a huge deal for a higher-level character anyway. This makes her pretty much the only deity who is routinely desecrated immediately after converting out, and one of the most frequent victims of desecration of any deity.