The windowsill is territory, and pots are toys
Understand what the cat is actually doing before defending the plants. A windowsill combines elevation, warmth from the glass, and a live view of birds, passers-by and weather — the richest entertainment a home offers an indoor cat. A cat clearing that space is claiming a lookout, and the plants are obstacles on it. The knocking itself then becomes its own reward: cats test objects at edges with a paw from kittenhood, and a pot that tips, spills soil and crashes delivers movement, sound and a rushing-in owner, a full show for one small push.
Some of the behaviour is gardening, not vandalism. Cats dig in soil and chew leaves that resemble grass, so a plant on a sill is furniture, toy and salad bar at once. Any plant a cat can reach must be treated as one it will eventually mouth, which is why the safety check below is not optional.
Notice which sill and which hour. A cat who clears the kitchen sill at breakfast is claiming the room's social centre; one who works along every sill at dusk is on an evening patrol, and the plants are simply on the route.
Redesign the sill so both can stay
Give the cat a legal claim on the best sill: a suction-cup or bracket window perch, a cleared cat-length stretch with a washable mat, or a sturdy shelf at the same height immediately beside it. Cats accept substitutes at the same height and view far more readily than grander perches in worse locations, so do not expect a cat tree across the room to outbid the window.
Then make the plants boring to push. Group remaining pots tightly so none stands alone at an edge, choose heavier pots or set pots into a tray or window box arrangement that moves as one solid unit, and add a layer of large decorative stones over the soil to end digging and soil-scattering. Move fragile, tall or top-heavy plants to a sill the cat genuinely does not visit, a hanging planter, or a room the cat does not patrol.
Provide the legal version of the game nearby: light, unbreakable objects the cat may bat off a low shelf, and a pot of cat grass on the cat's own stretch of sill. A cat whose patting and chewing have sanctioned outlets tests the forbidden ones less often.
The safety check that comes with the sill
Verify every houseplant within the cat's reach against a reliable list of plants toxic to cats, and remove or relocate anything on it — several common windowsill plants, including many lilies grown indoors for flowers, are seriously dangerous to cats even in small amounts, and pollen or a chewed leaf can be enough. If you are unsure what a plant is, treat it as unsafe until identified, and keep cut flowers from mixed bouquets off cat-accessible sills for the same reason.
If your cat chews a plant you cannot identify, or shows drooling, vomiting, pawing at the mouth or unusual quietness after access to a new plant, contact a veterinarian promptly and take a photo or sample of the plant with you, because identification changes what the vet needs to do.
Keep fertiliser and pesticide use on cat-accessible plants to cat-safe minimums, and cover freshly treated soil, since a digging cat ingests what is on its paws at the next grooming session.
Keep the peace with routine
Refresh cat grass and wipe the sill stretch weekly so the cat's spot stays inviting. When a pot does fall, clear it up without theatre; a dramatic response is part of the show the cat may be replaying for. Most cats settle within a couple of weeks once the perch is occupied daily and the pots stop being movable.
Recording which plants were moved, which were checked as safe, and where the perch went keeps the arrangement from unravelling when someone replants the sill — and KinsLe Care Teams lets the household log those plant checks and perch jobs with timestamps so the windowsill stays shared on purpose.