The simplest humane way to keep cats off a garden is to lay chicken wire, supported by bricks, over a cherished plot. Or scatter old holly leaves, pea sticks or bramble stems. Where this is impossible, plant rue around the bed. Cats hate it. (Alas, so does every other plant. A beneficial companion, rue is not.)
It's said that cats also loathe garlic, chamomile and tagetes. And I'm reliably told that spiny restharrow (Ononis spinosa) deters cats. That tip is totally useless, of course, for those of us who would not know it from a bath brush.
However, citronella is also a proven organic cat repellent, in my experience. Spray the edges of seed beds with 100 drops citronella per 1 litre water. Re-apply daily till your cats are re-educated. Orange, grapefruit or lemon peel is easier to obtain and I've found it effective, too.
Cordons of chilli oil, curry powder, garam masala and the like also repel cats. The next idea is a little controversial. Nor is it organic. So, if you are a cat lover, I must tread carefully (unlike my neighbour's cats who once used my seedbed as a jogging track).
I surrounded my plot with small plastic milk bottles, sunk in the ground with their tops removed and each containing a few teaspoonfuls of ammonia. It did the cats no harm because none came within ten feet of those fierce-smelling bottles.
A powerful cat repellent
Another idea, if you are not fastidious, is to put dog faeces into sealed margarine tubs, well perforated at the sides. Set them around your vegetable bed and cats will not come near. Nor will dogs. They have a keen sense of territory and will not invade another dog's 'land'.
Do not put dog, cat or human faeces – or the faeces of any other carnivores – on the soil itself, even around non-edible flowers. The residue persists in the soil and can be toxic. Children have become blind after wiping faeces- contaminated soil in their eyes.
Clay pellets impregnated with lion or tiger urine are now widely on sale. Said to be almost odourless to humans, they drive away cats, dogs, foxes, wolves, possums, yetis, and bears. But they too are best shielded from enquiring fingers in perforated pots.
If your bird boxes are assaulted by cats, grow roses and brambles up the trees or posts that supports the boxes. Or beg from your local fast-food outlet a large empty drum that once held cooking oil and slice it into a metal collar. Two collars will girdle even a large tree. Wire it around the support four foot above ground, shiny surface outermost, and such a girdle will protect bird boxes from cats, squirrels and small boys. Put lower down, it will stop rabbits gnawing trees.
Practical uses for pets in an organic garden
Small pets do sometimes have practical uses. For example, hair brushed from cat or dog blankets (or from horses or any hirsute animal) can be laid in bean trenches to add slow-release nitrogen.
Indeed, human hair swept from barber's shops makes a marvellous additive to the compost bin, if you're not fastidious. Hung in net bags around orchards, it will also repel deer and wild boar as efficiently as rotten eggs. (The hydrogen sulphide emitted by poultry egg debris was, in laboratory trials, even more effective in deterring four-legged pests than proprietary repellents.)
Culinary ways to drive cats away
Grow red hot chili peppers in your greenhouse – not to eat, because only Lucifer could relish them – but to grind up and soak in vegetable oil over winter. Swab this noxious paste on cardboard strips in spring and lay them around your seedlings, or any other plant you want to protect. Not only will cats scat, but the fiery odour will also repel many insect pests.
You can also blend such an organic (but humane) nerve gas 1: 5 with water mixed with washing up liquid, strain and spray on plants infected with aphids, caterpillars, and all things that creep, twitch or fly. It kills or deters almost all of them.
A more elegant cat repellent …
is made from a discarded plastic cola or washing up liquid bottle. Remove the cap. Push in several old nylon socks, some glass or rockwool insulation, or even the plastic foam interior of an unemployed teddy bear. This makes a wick. Make sure the wick fits tightly and protrudes from the top.
Fill the bottle one third full with your fiercest humane nerve gas (see above) and ensure the wick is soaked all the way to the top. Sink the bottle beside your choicest plants. The wick will then diffuse your cat repellent into the air.
Several of these bottles in a seedbed, regularly replenished, should deter even Tom & Jerry.
Do note, that the pest repellents above will not harm beneficial insects, birds, or any of your pets. Unless they eat them, which they will not. Just keep little children away.