Maple trees are large dense trees and unless you've thinned out your tree, you likely have a full shade area underneath it. Maples also have shallow roots which usually grow directly under the soil's surface. With these conditions, it is often difficult to successfully plant a thriving garden under a maple tree. Fortunately, with a little planning and careful planting, you can plant a garden beneath your tree that will look nice for years to come.
Choose perennial plants that grow well in the shade. Perennials will grow year to year, unlike annuals which need to be replanted yearly and will disturb your maple's roots more than necessary.
Consider the size of the plants when they reach maturity. Compare this to the distance between the ground and the lowest branches. Give your plants enough room to grow tall without growing into the tree itself.
Purchase the smallest containers for the plants you've chosen to limit the digging you will need to do.
Remove the grass if any under the tree with a garden fork and pull it up with your hands if necessary after loosening. Do not dig too deep--a maple's roots are shallow. This is easier done when the ground is slightly moist. If you do not get this step down in one day, keep the area well watered if you have exposed any roots to the air.
Consider using a herbicide with glyphosate as its active ingredient (e.g., Round Up) as an alternative to 4, especially for large patches of grass. The grass will die in approximately 3 days. Do not remove the grass--you can plant right in the dead grass. It will decompose and add valuable nutrients.
Plan out the space. Taller plants should be planted closer to the trunk and shorter once toward the outer edges. Give the plants space enough to grow to their mature size. Avoid the need to transplant later when overcrowding occurs.
Dig holes with a small trowel just as big as the container of your plants. If you come across a root that is more than 2 inches in diameter, move to a different spot. Then, sprinkle a little bit of nitrogen slow-releasing fertilizer into each hole. Follow manufacturer directions as each fertilizer is different.
Take the plants out of their containers and set them in the holes (soil and all). Backfill the soil to fill in any space and pack it down lightly.
Add 4 to 6 inches of organic mulch, such as bark or wood chips. Keep the mulch about 2 inches away from the tree trunk to prevent mold and other diseases from growing or spreading.
Water the planting area well with about 2 inches of water. Repeat each week until the plants are established (in about 1 to 2 months). Then, rainfall will usually suffice, unless dry conditions exist.