The simple answer is: it depends entirely on the specific planter box. Not all urban planters are created equal, and their ability to support a small tree or shrub hinges on several critical factors.
First, consider construction and materials. A lightweight, thin plastic or flimsy wooden box will likely buckle under the sustained weight of soil, water, and a growing root system. For trees, you need heavy-duty materials like thick, marine-grade polymer, reinforced concrete, sturdy steel, or thick cedar/redwood. The design must include robust joinery and ample structural support.
Drainage is another weight-related factor. Proper drainage holes are non-negotiable to prevent waterlogging. Saturated soil can become extraordinarily heavy, adding significant stress to the planter and the structure beneath it (like a balcony or rooftop). Always use a high-quality, well-draining potting mix designed for containers, not dense garden soil.
Perhaps the most crucial consideration is where you place the planter. A heavy, tree-filled planter on a balcony or rooftop imposes a live load that the building must be engineered to handle. You must consult a structural engineer or your building management before placing any large planter in such locations. For ground-level installations, ensure the surface is stable and level.
To choose correctly, always check the manufacturer's stated weight capacity and dimensions. Select a planter that is both wide and deep enough to accommodate the tree's mature root ball, providing stability against wind. As a rule of thumb, the planter should be at least 2-3 times the volume of the tree's root ball.
In summary, yes, many urban planter boxes are designed to handle small trees, but you must deliberately select a product engineered for that purpose, use appropriate soil, and verify the installation site can bear the significant added weight for years to come.