Not automatically. Whether an urban planter box stays upright in strong wind depends less on whether it feels “heavy” in general and more on its weight-to-sail-area ratio, center of gravity, base width, soil moisture, plant height, and exposure. A wide, low, soil-filled concrete or timber planter can weigh hundreds of pounds and will usually stay put in typical city gusts. But a tall, narrow resin or plastic box filled with dry potting mix, topped with tomatoes or ornamental grasses, can act like a sail. Even if it has some weight, it may tip, slide, or rotate.
The key forces are wind pressure and leverage. Wind pushes on the box and on the plants. At 40 mph, wind pressure is about 4 pounds per square foot; at 60 mph, it is roughly 9 pounds per square foot. A 2-by-3-foot side can catch 24 to 54 pounds of force. Plants can double or triple that exposed area. If the force acts high above the base, it creates an overturning moment. The planter resists with its own weight times half the base width. So a 50-pound box with a 1.5-foot base has only about 37 pound-feet of resistance, while a 200-pound box with a 2-foot base has about 200 pound-feet. Tall, narrow, lightweight planters are the most vulnerable.
Wet soil helps. Dry potting mix is light and can blow out; saturated soil is much heavier. But wet soil also increases weight on balconies, so check load limits. A 20-gallon planter with wet soil can weigh 250 pounds or more. That is often enough to resist wind, but not if the planter is tall and the plants act like a sail.
To reduce risk, choose wide, low, heavy materials; fill with dense soil and water before storms; trim tall plants or move containers indoors; group planters together; use windbreaks; lock casters; and anchor boxes with straps, brackets, or weighted bases. On rooftops and balconies, secure planters to railings or walls only if permitted and structurally safe.
So, are urban planter boxes heavy enough to blow over in strong wind? The question is slightly reversed. Many are heavy enough not to blow over, but plenty are light, tall, or exposed enough to tip in a strong gust. Assume any planter can move in extreme wind, and anchor the ones that matter.