Answer: Mushrooms often grow where decaying organic debris is in or just below the soil surface. It's likely the soil you're using came from beneath, or just at the surface of a plot of land that had decomposing tree roots and/or bark. While the mushrooms are not harmful to your plants, they may draw insects that wouldn't ordinarily visit your garden. As long as you allow them to grow, they'll send spores out, which will result in even more mushrooms. If it were my garden I'd rogue the mushrooms out, insects and all.
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