Short answer: yes, a street bench can be anchored into grass or dirt. You do not need an existing hard surface like concrete, but you do need a proper anchor point. A bench simply set on lawn or bare soil will sink, tip, or shift, especially in public areas.
For most grass and dirt installations, helical ground anchors or earth screws are the cleanest option. A ground screw for a bench looks like a large galvanized steel helix with a bracket at the top. You drive it into the soil with a bar or impact wrench until the plate is at grade, then bolt the bench legs to the bracket. This creates a solid connection without pouring a full concrete pad. In firm clay or compacted soil, this can be strong enough for a standard street bench.
If the soil is loose, sandy, or often wet, use longer anchors. Galvanized or powder-coated steel resists rust better in ground contact. In cold climates, try to anchor below the frost line, or choose an adjustable footing you can relevel after winter. Frost heave is one of the main reasons benches in grass become wobbly over time.
Another dependable method is to dig holes and pour small concrete footings at each bench leg. This is not the same as needing a concrete slab. The footing can be hidden just below the turf, with a post base or J-bolt set into the wet concrete. Once it cures, bolt the bench down. This approach works well for high-traffic street furniture, heavy benches, and areas where theft or vandalism is a concern.
Heavy-duty ground spikes can hold a light residential bench, but I would not rely on them for a public street bench. A treated timber or steel base frame staked into the ground is a useful middle option because it spreads the load and reduces sinking on soft grass.
Whatever method you choose for an outdoor bench installation, make sure the finished bench is level, does not rock, and has safe clearance around it. If the site is public property, call a utility locate service before digging or driving anchors.
In short: for a permanent street bench, helical piles or buried concrete footings are the professional choice. If you need to secure a park bench on grass or dirt for a backyard or temporary installation, long ground anchors can absolutely do the job without an existing concrete surface.