Answer: I understand your desire to plant something under your maple but there are several considerations before you continue your project. First, maple roots are shallow and sometimes even grow above the surface of the ground. The roots will compete with anything you plant for moisture and nutrients, the the tree roots usually win. Secondly, adding soil or weed barrier over the roots can compromise their health, even the breathable landscape fabric can cause problems. Third, while there are some groundcover type plants that will grow under the circumstances, there are very few from which to choose. So, I would strongly suggest you not place any topsoil over the roots. Since the weed fabric is already in place, you can leave it there and cover over it with just an inch or so of shredded bark or bark chips to camoflage the weed barrier. Then cut holes in the fabric to plant your groundcovering plants (or flowering annuals and perennials). Pachysandra, vinca minor and other dry soil, shade loving groundcovers will grow beneath maples, even in shallow soils.
Now that you have some background information about planting beneath trees you can finish your project without compromising the health of your tree. Best wishes with your garden!
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