Why aphids find balconies so appealing

Balconies, window ledges and small courtyards make brilliant growing spaces, but they are also warm, sheltered and often screened from the wind. That suits aphids perfectly. They are soft-bodied insects that struggle in exposed, breezy positions, so a sunny balcony filled with tomatoes, chillies, broad beans or roses is close to paradise for them.

Container plants also tend to be more stressed than plants in open ground. Compost dries out quickly, nutrients leach away with every watering, and roots are confined to a pot that swings from sodden to bone dry. A stressed plant produces soft, sappy growth, and that is exactly what aphids seek out. The good news is that on a balcony you can inspect every plant in a few minutes, which makes early intervention genuinely practical rather than a chore.

Check leaf undersides every week

Aphids nearly always arrive on the underside of leaves and along the newest growing tips, well out of sight until numbers build. Set aside ten minutes a week — the same day each week, so it becomes a habit — and go plant by plant.

• Turn over the newest leaves and check the soft tips, where aphids gather first.
• Inspect flower buds, the stems just beneath them and the joints where leaves meet the stem.
• Look for sticky honeydew, small white cast skins, and new leaves that are curled or puckered.
• Follow any ants you see. They farm aphids for their honeydew and will lead you straight to a colony.
• Squash small clusters with your fingertips, or wipe them away with a damp tissue.

Caught at this stage, an infestation is a smear on one leaf rather than a sticky, wilting problem across the whole pot. If you find aphids on a plant that is badly covered, it is often kinder to remove that plant before the colony spreads to its neighbours.

Encourage ladybirds and other garden allies

The single best long-term defence on a balcony is a healthy population of predators. A ladybird larva — that strange, spiky, grey-black creature — can eat dozens of aphids a day, and an adult ladybird will keep working through the season. Hoverfly larvae, lacewing larvae and tiny parasitic wasps all do their bit too.

• Grow small-flowered plants nearby to feed the adults: calendula, cosmos, dill, fennel, yarrow, marigolds and poached egg plant are all excellent.
• Leave a few aphids in place. Predators cannot survive on a balcony with nothing to eat, so tolerate a light scattering on one or two plants.
• Skip insecticides entirely. Even those labelled as natural will knock back the very insects that are doing the work for you.
• Keep a shallow dish of water with a few pebbles in it so beneficial insects can drink safely.

Be patient. It usually takes a couple of weeks for predators to find a new colony, which is why weekly checks matter so much in the meantime.

Knock them off with a strong jet of water

This is the simplest physical control and it works remarkably well. Aphids cling on weakly, and a firm jet of water dislodges them from leaves and stems. Most will not climb back up, and the spray also washes away the sticky honeydew that attracts ants and encourages sooty mould.

Use a trigger sprayer set to a strong, narrow stream, or a hose with a jet attachment if you have an outdoor tap. Support the leaf with one hand while you spray the underside with the other, working from the top of the plant downwards. Do this every few days while numbers are high, and afterwards on a weekly basis. Spray in the morning rather than in blazing midday sun, so wet leaves have time to dry before evening and fungal problems are less likely.

Plant nasturtiums as a trap crop

Nasturtiums are the classic sacrificial plant. Aphids find them irresistible, often preferring them to your vegetables, so a few pots of nasturtiums can draw an infestation away from your crops.

Grow them in their own containers rather than mixing them into a tomato pot, so you can simply inspect the nasturtiums each week and deal with whatever has gathered there. If a plant becomes smothered, cut it back hard or bin it and sow another — nasturtiums grow fast enough from seed that you can keep a rolling supply going through the summer. Keep them slightly on the dry side, as rich, lush growth makes them even more attractive than you need.

Keep plants strong and add a few deterrents

Vigorous plants resist aphids far better than struggling ones, so get the basics right. Water consistently rather than in fits and starts, and feed containers regularly with a balanced feed. Avoid high-nitrogen feeds, which push out the soft, sappy growth aphids love.

• Underplant or interplant with strongly scented herbs: chives, garlic chives, coriander, mint and basil all help mask the scent of your crops.
• Lay a sheet of reflective foil or silver mulch around the base of pots to confuse incoming aphids.
• Remove weeds from pots and balcony corners, as many harbour aphids between crops.
• As a last resort, mist a small colony with a diluted soft soap spray, then rinse the plant the following day.

Combine these habits — weekly checks, a strong jet, a welcoming home for ladybirds and a pot of nasturtiums doing the sacrificing — and you will rarely lose a crop. Steady, observant gardening beats spraying every time.