The most reliable way to protect an urban planter box from squirrels is to make the soil surface difficult to dig through and to remove easy reasons for them to visit. Start with a physical barrier. Cut hardware cloth or sturdy wire mesh to fit over the soil, leaving enough room around plant stems so the plants are not damaged. Anchor the mesh firmly at the edges with landscape staples, heavy stones, or bricks. This stops squirrels from digging while still allowing water and air to reach the soil. For newly planted seedlings, a temporary wire cloche or mesh cover works well until the plants are established.
A thick layer of heavier mulch can also help. Use large bark chips, gravel, or decorative stones rather than light, loose mulch that squirrels can push aside easily. If you use containers inside the planter box, line the bottom with mesh and cover the top with a fitted wire guard. This is especially useful for bulbs and fresh transplants, which squirrels often target.
Scent repellents can add another layer of protection. Capsicum-based sprays, garlic, peppermint oil, or commercial squirrel repellents may help, but they need to be reapplied after rain and should be used according to label instructions. Avoid anything that could harm pets, children, or local wildlife. A motion-activated sprinkler is another humane option for urban gardens, though it works best where a water source and drainage are available.
Finally, reduce attractants. Clean up fallen seeds and food scraps, secure compost and trash bins, and avoid leaving bare soil that looks like an easy place to bury nuts. You can also provide a small, separate digging tray away from the planter box to redirect squirrels. Combining a secure mesh barrier with heavier mulch, regular repellent application, and good garden hygiene gives your urban planter box the best chance of staying protected and keeping your plants rooted.