Short answer: For most urban gardeners, the lightest planter box that is still sturdy enough for a small tree is a double-walled, rotomolded high-density polyethylene (HDPE) planter. Choose a 24–30 inch diameter and 24–30 inch depth, about 30–45 gallons. Empty, it should weigh roughly 25–45 pounds. A high-quality fiberglass planter can be a little lighter, but it often costs more and can crack under hard impacts. A fabric grow bag is lighter still, but it is not a rigid urban planter box and usually needs a frame and irrigation.
The key is to look at both empty weight and structural design. The best HDPE planters have UV inhibitors, internal ribs, reinforced bases, and molded drainage holes. That combination keeps the planter light while resisting sun, frost, and the sideways force of a small tree in wind. If the planter is too thin, it will bow when you add moist soil. If it is too heavy, it becomes hard to move on a balcony or rooftop.
Size matters more than the planter’s empty weight. A small tree needs enough root volume. A 30-gallon planter is usually the practical minimum for trees like dwarf citrus, Japanese maple, olive, bay, or crape myrtle. A 45-gallon planter gives more buffer. Remember that moist potting mix can weigh 150–250 pounds in a 30-gallon container, so the planter itself is only part of the load. Use a lightweight mix based on pine bark, coir, compost, and perlite instead of heavy garden soil.
Drainage and stability are also essential. Make sure there are drainage holes, add a screen to keep soil in, and raise the planter slightly so water can escape. If the planter is on a balcony or roof, anchor it or place it in a sheltered corner. Small trees act like sails. Locking casters make a light planter movable, but they do not replace wind protection. Stake the trunk loosely and prune for a balanced shape.
So the best buy is usually a 30–45 gallon, double-wall HDPE planter with UV protection, internal ribs, and good drainage. It is light enough to move when empty, sturdy enough to hold a small tree, and far easier to handle than concrete, wood, or composite. If you want the absolute lightest rigid option and budget is not a concern, compare it with a reinforced fiberglass planter of the same size.