Garden pests are easier to handle than houseplant pests, because outdoors you have allies. Birds, ground beetles, hoverflies and hedgehogs do most of the work if you let them. What follows is pest by pest: how to recognise it, what helps, and what to skip.
One note before the list. Barriers, traps, hand-picking and encouraging predators are the reliable methods, and they are what this article concentrates on. Homemade sprays are the weakest tool here, and on anything you intend to eat, an approved product used according to its label is the safer choice than an improvised mixture.
Aphids
Dense clusters of soft green, black or pink insects on the newest growth, on flower buds and on the undersides of young leaves. You often notice the sticky honeydew, or the black sooty mould growing on it, before the insects.
What works. Squash them. Run a stem between finger and thumb, or blast the plant with a hose. On a rose or a broad bean, two minutes of this every few days through the spring flush keeps numbers manageable without anything else.
A soap solution helps on a heavy outbreak: a teaspoon of plain soft soap per litre of water, sprayed until it runs off, undersides included. It kills only on contact and only while wet, so it needs repeating every five to seven days.
What works better than either. Ladybirds and hoverfly larvae eat aphids in quantity. Planting a strip of open flowers nearby, or leaving a patch of the garden unmown, brings them in. The catch is timing: predators arrive a couple of weeks after the aphids do. Gardens that spray at the first sign never get past that gap, and end up spraying all season.
Skip. Banana peels buried at the base, and most garlic sprays. Neither reduces an established colony.
Slugs and snails
Irregular holes chewed in the middle of leaves rather than at the edges, worst after rain, with silvery slime trails in the morning. Seedlings and hostas vanish overnight.
What works. Going out after dark with a torch. This is unglamorous and it is by far the most effective thing on this page. Two evenings of collecting during a wet week does more than a season of remedies.
Beer traps work, and the science behind them is unromantic: they drown in it. Sink a container so the rim sits a centimetre above soil level, which stops ground beetles falling in, and refill after rain.
Barriers help around individual plants. Copper tape on pots, or a wide band of something dry and sharp such as grit, crushed shell or wood ash. All of them stop working the moment they are wet, so they suit pots and raised beds more than open ground.
Watering in the morning rather than the evening leaves the surface drier at night, when slugs travel.
Skip. Half-buried eggshells. The pieces are not sharp enough at the scale a slug experiences, and this one has been tested repeatedly with no effect. Salt kills the slug and damages your soil.
Caterpillars on brassicas
Ragged holes in cabbage, kale or broccoli, with green or yellow-black caterpillars underneath and clusters of small yellow eggs on the leaf undersides.
What works. Fine mesh or insect netting over the crop from the day you plant it. The white butterflies cannot lay through it, and a crop that is never colonised needs no treatment at all. This is the whole solution, and everything else on this list is a repair.
If they are already there, pick them off, and rub out the egg clusters on the undersides while you are looking. Check twice a week during the flight season.
Skip. Companion planting as a defence against these specifically. Interplanting has real benefits for soil and for predators, but a cabbage white will find your cabbages regardless of what is next to them.
Ants
Ants are usually a symptom rather than the problem. A column running up a stem usually means aphids or scale above, because ants farm them for honeydew and will defend them from predators.
What works. Treat the aphids and the ants leave. A sticky band around the trunk of a fruit tree breaks the route. Ants nesting in a dry pot are telling you the compost has dried out beyond the point where roots are functioning, and the fix is a long soak.
Skip. Boiling water poured on nests in a border. It kills roots as readily as ants.
Whitefly
Tiny white moth-like insects that rise in a cloud when you brush the plant, then settle again. Common on tomatoes, cucumbers and anything under glass, where they breed all year. The scales and eggs on the leaf undersides are what matter; the adults you can see are the smallest part of the population.
What works. Yellow sticky traps hung at the height of the growing tips, which catch adults and tell you how bad the population is before the damage shows. A soap solution on the undersides, repeated every five days, handles the rest. In a greenhouse, the parasitic wasp Encarsia formosa is sold as a biological control and works well in warm conditions.
Skip. Spraying the tops of leaves. Every stage of a whitefly's life happens underneath.
Vine weevil
The one that catches people out, because you meet the damage before the insect. Neat semicircular notches bitten from leaf edges are the adults, which is cosmetic. The problem is underground: white C-shaped grubs eating the roots, most often in pots and containers. A plant that wilts despite moist compost and lifts out with almost no root left has vine weevil.
What works. Nematodes, watered into the compost in late summer or early autumn while the soil is still warm. They are a living biological control, sold by garden suppliers, and they are the only reliably effective option for the grubs.
Beyond that: check the rootball of anything new before it joins the collection, and go out with a torch on mild evenings, since the adults are nocturnal and flightless and can be collected by hand.
Skip. Anything applied to the leaves in the hope of reaching grubs in the compost.
The one about vinegar
Vinegar comes up constantly, so it is worth being clear. It burns the top growth of whatever it touches, including plants you want. It does not travel to the roots, so perennial weeds regrow within a fortnight. Repeated use lowers soil pH and harms soil life.
On a gravel path or between paving slabs, where nothing should grow and there is no soil to protect, it is a reasonable option. In a bed, among crops or on a lawn, it will cost you more than it solves. If you are unsure what your soil is doing already, testing your soil at home is a better starting point than adding acid to it.
Working out what you are looking at
Nearly all of the above depends on identifying the pest correctly, and outdoor damage is harder to read than indoor damage because several things chew leaves.
Photographing the damage in Gardenix gives you a name, a confidence level so you know how much weight to put on it, and a treatment plan scaled to the severity rather than a generic article. Alongside each plan the app suggests home remedies, the traditional household approaches, which is the same territory this article covers, matched to what is on your photo.
For pests indoors, where the balance of methods is different and there are no predators to recruit, see how to get rid of plant pests without chemicals and common houseplant pests.
Gardenix is free to download on Apple iOS and Android, and the first 3 scans are free for life.