A 16-inch bed over a small area (13 ft diameter) on a 30-year-old maple is safe. Roots extend 2.5–7x the dripline; covering a tiny fraction won't harm it.
Local bulk supplier should have a clay/pine bark/sand/compost blend. For perennials/annuals: ~25% clay. For shrubs: ~50% clay. Clay provides water/nutrient retention in elevated beds.
The tree will take most water. Monitor closely.
Protecting Tree Trunks When Building a Raised Bed
Creating Well-Draining Soil for a Raised Perennial Bed
Revitalizing Old Raised Bed Soil for Vegetables and Flowers
Raised Bed Against a Cedar Fence: Rot Prevention in the Pacific Northwest
Diagnosing Raised Bed Problems: Root Rot, Drainage, and Early Blight
Raised Bed Gardening Benefits: Better Soil, Drainage & Yields
One Grape Vine Thrives, Neighbor Stagnates: Soil or Cultivar Difference?
Building a Tree Bed Retaining Wall: Back Side Materials When Hidden by Fence
Saving a Declining Cottonwood: Root Issues, Fertilization, and Stability
Adding Soil Around Sycamore: 2 Inches Max, Off Trunk
Soil Piled Against Tree Trunks: Hidden Root Damage
Newly Planted Myrtle Tree Browning: Transplant Shock, Not Death
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