I have plenty of earthworms in the ground against the house where we have a few shrubs and a couple of potted plants. It's all covered with cedar bark and separated from the lawn with decorative manufactured stone blocks. I have some worms in the garden bed as well, it's not raised, but it is covered with cedar bark.
I use no fertilizers, pesticides, or weedkillers in either part of our property.
The lawn guy does use antkiller in a liquid fertilizer to treat our lawn but the leaching into the garden is minimal because of barriers (we have 160-year-old bricks for the barrier in the front garden).
I can't say for sure but I'm thinking the cedar bark is the key. It has a naturally waxy surface that keeps the soil beneath it moist, and serves as insulation against excessive heat or cold.
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