Answer: Any plants will compete for nutrients, yes. Since Japanese maples are shallow rooted, in my experience it is better to leave the area beneath them mulched rather than planting there. The mulch not only helps conserve soil moisture and holds down weeds, it also helps feed the soil on an ongoing basis as it breaks down over time. This is important because these maples need an evenly moist, organic and humusy soil. However a smaller hosta cultivar might be satisfactory as long as you maintain a steady mulch layer and are able to leave the hostas in place without constantly digging and dividing it as this would damage the maple roots.
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