The safest way to move a really heavy urban planter box by yourself is to avoid lifting it at all. For urban planter moving, use leverage, wheels, and short controlled movements instead of your back. First, check the weight and route. If possible, remove plants, loose soil, gravel, or drainage stones to make the planter much lighter. Clear the path, measure doorways, and look for thresholds, steps, or uneven pavement. If the planter is extremely heavy or the route includes stairs, stop and get help; no solo technique is worth a back injury.
Use a heavy-duty planter dolly or furniture dolly. Tip the planter just enough with a pry bar or sturdy lever, place wooden wedges under one edge, and slide the dolly underneath. Strap the planter securely to the dolly. Push with your legs and keep the load close to your body; do not pull with your back or twist while moving it. For low, flat planter boxes, use pipe rollers. Lift one edge only a few inches with a pry bar and blocks, slide a piece of plywood or cardboard underneath, then place pipe rollers under the planter. Move it slowly, repositioning the rollers as you go.
If you must lift a corner, use safe body mechanics for lifting without back pain: stand close, hinge at your hips, bend your knees, tighten your core, keep your spine neutral, and exhale as you lift. Lift only a corner and only a few inches. Never deadlift a heavy planter box alone. Wear gloves and closed-toe shoes, protect floors with plywood, and use ramps for small ledges. If the planter still feels too heavy, the route is unsafe, or you feel any strain, stop and get help. Moving in stages with a planter dolly and pipe rollers is far safer than trying to be a hero with your back.