I’d treat it as a two-part job: get rid of what’s already there, then make the gap a bad place for weeds to settle. Start by pulling or scraping out the weed while it’s small. A putty knife, old screwdriver, or stiff brush works well for bench slats and table edges. Try to get the root, not just the leaves, and vacuum or sweep out the dirt, leaf litter, and grit that collects in the gap. That debris is often the “soil” the weeds are living in.
If the furniture is slatted wood, be careful about sealing every gap. Wood needs to move and shed water, and caulking can trap moisture and cause rot. Clean the gaps, improve airflow, and if weeds keep coming back, fit a weed-barrier membrane or gravel bed beneath the furniture rather than filling the slats. For metal or concrete furniture with non-moving gaps, clean thoroughly and use a small amount of flexible outdoor sealant only where it won’t block drainage or create a water trap.
On paved surfaces under benches and tables, keep joint sand or gravel topped up and repair cracks; weeds love cracked paving and soil-filled joints. If the furniture can be moved, lift it and put down a proper weed barrier under a gravel or paving base. Avoid loose soil directly under the furniture.
Regular sweeping is boring but effective. Remove leaves and grit before they become a seed bed, and pull weeds early. In public or pet-heavy spaces, skip harsh herbicides; hand removal, boiling water, or careful targeted vinegar can work, but don’t stain or damage nearby surfaces. The real fix is starving the weeds: no soil, no light, no standing moisture, and no missed seedlings.