Sedum

Lean soil is better for sedum anyway, and it is the surest way to avoid the soft spring growth aphids colonize.

Dense clusters of tiny insects packed onto shoot tips, flower buds, and the undersides of young leaves. Curled, cupped, or distorted new growth that never opens properly. A sticky shine on the leaves - and on whatever sits under the plant - that later goes black with sooty mold. Tiny white cast skins left behind where a colony has been feeding. Ants running steadily up and down the stems, which is often the first thing you notice. Mottled, streaked, or misshapen leaves that do not grow out can mean a virus the aphids brought with them rather than the feeding itself.

Knock the colonies off with a hard jet of water and repeat every few days - on most plants that alone ends it. Pinch out or prune off the worst-infested tips rather than treating the whole plant. If colonies keep rebuilding, spray insecticidal soap or horticultural oil directly onto the insects, covering leaf undersides, and repeat weekly until they are gone; treat early or late in the day, and never spray a wilted or heat-stressed plant. Avoid broad-spectrum insecticides - they kill the ladybugs, lacewings, and parasitic wasps that were about to solve the problem for you, and aphid numbers usually come back worse afterward. Anything you spray must be labeled for the plant you are treating; follow that label and observe its pre-harvest interval. On anything you eat, wash leaves, heads, and buds well before use, since aphids hide down in crowns, folds, and tight growth.

Go easy on nitrogen: the soft, lush growth that heavy feeding produces is exactly what aphids breed on. Check new growth once a week through spring and early summer so you catch colonies while they are still small, and rub the first few out with your fingers. Leave flowering plants nearby and skip broad-spectrum sprays - ladybugs, lacewings, hoverflies, and parasitic wasps do most of this work for free, and they only stay if there is something for them to eat. Keep ants off the plant where you can, since they protect aphids from those predators. Water steadily so plants are not stressed, and check any new plant over before you bring it home or move it indoors, since aphids usually arrive on something else.

Aphids are small soft-bodied sap-suckers - green, black, gray, pink, or waxy white, and mostly wingless - that gather in dense colonies on the tenderest new growth: shoot tips, buds, and the undersides of young leaves. They breed extremely fast in mild weather, so a few can become hundreds inside a week. Their feeding curls and distorts soft growth, and the sticky honeydew they excrete coats everything below and turns black with sooty mold. Ants often farm and defend the colonies for that honeydew. A few aphids are normal on almost any plant and a healthy plant shrugs them off; only dense, persistent colonies do real damage, and some aphids also spread plant viruses as they feed.

Mostly a problem on low creeping sedums in cool, damp shade. Grown the way sedum actually wants it - full sun, sharp drainage, airflow - the damage largely disappears.

Irregular ragged holes chewed out of the middle of leaves and along the edges - not the clean-edged cuts a caterpillar leaves. Seedlings that vanish overnight, or that are cut off at soil level with nothing left behind. Silvery dried slime trails across leaves, soil, pots, and paving in the morning: this is the giveaway that tells you it was mollusks rather than an insect. Chewed pits in ripe fruit, especially where it rests on the ground. Damage that shows up on the lowest, dampest leaves first and gets worse after rain or evening watering. Turn over a board, an upturned pot, or a clump of dense mulch during the day and look for the animals themselves sheltering in the cool underneath.

Hand-picking works and costs nothing: go out an hour after dark with a headlamp, drop what you find into soapy water, and repeat nightly for a week or two - persistence is what wins, not any single product. Shallow beer traps sunk to soil level catch them too, but need emptying and refilling daily to be worth anything. Iron-phosphate bait is the standard control and is far lower-risk around pets and wildlife than the old metaldehyde products, but it is not harmless: many formulations include EDTA, and a dog that eats a quantity of pellets can develop iron toxicosis. Scatter it thinly per the label, never in piles, and keep dogs off a baited bed. Clear fallen foliage and debris in autumn, where the eggs are laid. In a wet climate this is a population you manage each season rather than one you eradicate - and on most established plants light grazing is cosmetic and needs nothing at all.

Take away the daytime shelter first: lift boards and upturned pots, clear piles of debris, keep dense mulch pulled back from crowns and stems, and give crowded plantings enough airflow to dry out. Water in the morning rather than the evening so the surface is dry by the time they come out. Copper tape around containers and raised beds gives them a shock they will not cross, and it is the one barrier that reliably works. Get plants past the vulnerable stage quickly - start seedlings in pots and set them out at a decent size rather than direct-sowing into a cold wet bed, since most plants stop being worth defending once the leaves toughen. Leave the ground beetles, frogs, toads, and ground-feeding birds alone, because they eat mollusks all night for free and broad-spectrum slug pellets kill them too. Where thick-leaved or resistant varieties exist for what you are growing, they take far less damage.

Slugs and snails are nocturnal mollusks that rasp ragged holes in soft leaves, shear off seedlings at ground level, and chew into ripe fruit that touches the soil. They feed at night and in the early morning and spend the day hidden under boards, pots, dense mulch, leaf litter, and low foliage, becoming active in exactly the cool, damp, shaded conditions that suit young plants. Damage is heaviest in wet spells and on tender spring growth: a single wet week can erase a row of seedlings while an established, tough-leaved plant of the same species is barely marked. They lay clusters of small pearly eggs in damp soil and under debris, so a garden with plenty of cover carries a resident population year after year rather than being invaded from outside.

A cosmetic nuisance more than a real threat on garden sedum in the open - it is stressed and container plants that suffer. Full sun and airflow prevent most of it.

White cottony masses tucked into leaf axils, crevices and stem joints, which smear rather than brush off. A sticky shine on the leaves and on the surface beneath the plant, later blackened with sooty mold. Ants running steadily up and down the stems - often the first thing anyone notices. Yellowing, stunted or distorted growth with no other obvious cause. On a plant that has been treated repeatedly and keeps declining, check the roots, because root-dwelling mealybugs are easy to miss entirely.

Deal with small colonies mechanically: dab them with a cotton bud dipped in rubbing alcohol, wipe them off with a damp cloth, or wash the plant down with a firm jet of water aimed into the crevices where they hide. For anything larger, spray insecticidal soap or horticultural oil directly onto the insects, which means getting into the axils and undersides where they actually live, and repeat every seven to ten days for at least three rounds, because the waxy coat protects them and contact sprays do not kill the eggs. Cut out badly infested growth rather than nursing it, deal with the ants at the same time or the colony rebuilds, and test any oil first, since it can burn foliage in heat.

Inspect new plants thoroughly before you buy, and quarantine anything you bring indoors for a few weeks - mealybugs almost always arrive on another plant rather than flying in. Check leaf axils and crevices every few weeks on anything grown under cover, since a colony found early is trivially dealt with. Control the ants that protect and spread them; a sticky barrier on a pot rim or a stem does a lot of work. Keep plants unstressed, avoid heavy nitrogen feeding, and do not leave them standing in hot, dry, stagnant air.

Soft, slow-moving sap-suckers that coat themselves and their egg masses in a white waxy fluff, so a colony looks like scattered specks of cotton wool rather than like insects. They wedge into the places hardest to see and hardest to spray - leaf axils, crevices, stem joints, leaf undersides, the base of the plant, and sometimes the roots - and feed there for weeks, weakening growth and excreting sticky honeydew that blackens with sooty mold. Ants farm them for that honeydew, carry them from plant to plant, and drive off the predators that would otherwise control them. Populations explode in warm, dry, still air, which is why indoor and greenhouse plants suffer worst.