Yes—but only to a limited degree. An urban planter box can help take the edge off noise from a busy street below, especially when it is large, heavy, densely planted, and placed at the balcony or window edge. It works best as a soft acoustic buffer rather than a true soundproof wall.
The key issue is mass. A thin plastic box filled with lightweight potting mix will do very little. A deep planter with substantial soil volume, dense evergreen shrubs, and a solid back or integrated screen can absorb and scatter some mid- and high-frequency sound. It can also block the direct line of sight between the listener and the street, which often makes traffic noise feel less intrusive. This is why urban planter box noise reduction is usually modest but still worthwhile in real-world balconies and terraces.
However, low-frequency traffic rumble is much harder to stop. Sound from a busy street below can travel upward and around barriers, and any gaps around the planter will reduce performance. Plants alone are not a reliable planter box sound barrier. Leaves and branches may soften some high-pitched noise, but they do not replace mass, airtight construction, or a properly designed acoustic screen. For stronger street noise blocking, the planter should be combined with solid balustrades, sealed windows, thick curtains, acoustic panels, or double glazing.
Practical choices matter. Use the largest planter your structure can safely support, choose dense evergreens rather than sparse seasonal flowers, keep the soil moist and well settled, and place the planter as close to the noise path as possible. Line the full width of the balcony edge instead of using one isolated pot. Check building rules, drainage, waterproofing, wind load, and structural weight before installing a heavy balcony planter. Maintain dense growth, because gaps in foliage weaken the effect.
In short, acoustic landscaping with planter boxes can improve comfort and make a busy street feel less harsh, but it should be seen as one layer of a broader noise strategy. If you need major noise reduction, do not rely on a planter box alone.