Squirrels usually dig in urban planter boxes for simple reasons: loose soil is easy to move, freshly planted bulbs and seeds look like food, and bare dirt is a convenient place to cache nuts. The goal is not to hurt them but to make your planter less inviting and to put a barrier between the squirrel and the soil.
First, clean up attractants. Pick up fallen seeds, nuts, fruit, and compost scraps. Keep bird feeders away from the planter or use a baffle, because spilled seed draws squirrels. If you use compost, bury it deeply in a closed bin rather than leaving it on the surface. When planting bulbs, place them in a wire basket or cover the area with mesh until roots establish.
A physical barrier is the most reliable solution. For bare soil, lay 1/2-inch hardware cloth just above the surface, cut small openings for existing plants, and anchor it with landscape staples or heavy stones. You can also cover the planter with a removable wire cloche or hooped netting; choose mesh small enough that squirrels cannot squeeze through, and secure the edges so they cannot lift it. For taller crops, build a simple frame covered with bird netting and clip it shut. Check the barrier regularly and adjust it as plants grow.
Change the soil surface. A layer of large, irregular river rocks or gravel over exposed soil makes digging difficult and less attractive. Avoid tiny pebbles that squirrels can easily push aside. Around plants, use a coarse mulch or stone mulch, but keep it a few inches away from stems to avoid rot. Some gardeners also press down the soil after planting and water gently to settle it; loose, fluffy soil is easier for squirrels to dig.
Use scent and taste repellents as a secondary layer, not a solo fix. Commercial squirrel repellents, garlic, citrus peel, and capsaicin-based sprays can help for a short time, but they need frequent reapplication after rain and they do not stop determined squirrels. Always follow label directions and avoid products that could harm pets, children, or beneficial wildlife. Do not use poison, glue traps, or sharp materials.
Motion-activated sprinklers can be effective in urban gardens if there is a water source and the planter is not in a high-traffic area. They startle squirrels without injuring them. Ultrasonic devices are generally unreliable. You can also try strong-smelling plants such as mint, lavender, or marigolds, but treat them as companions rather than guarantees.
Finally, make the planter less useful as a nut storage site. Keep the surface covered where possible, remove buried nuts when you find them, and use netting during peak planting seasons. If squirrels are digging in one particular spot, place a small wire basket or a heavy stone over that area until the plants are established. Combine two or three methods—clean-up, a secure barrier, and a changed soil surface—and you will usually see far less digging in an urban planter box.