Japanese Maple

Being deciduous, this tree takes a dormant-season horticultural oil well - a thorough winter application on bare twigs is the easiest control there is. Test any oil on delicate cultivars first.

Rows of small brown, tan, black, white, or waxy bumps along stems, twigs, leaf midribs, and leaf undersides that do not brush off but can be scraped loose with a fingernail. A sticky shine on the leaves - and on the floor, table, or path beneath the plant - that later turns black with sooty mold. Ants running steadily up and down the stems, since they farm and defend scale for that honeydew; often the first sign anything is wrong. Yellowing leaves, weak thin growth, and dieback of individual branches or canes that are heavily encrusted. On fruit crops, scale covers and sooty mold directly on the fruit. Check the leaf undersides and the shaded inner wood, which is where colonies build up unseen.

Horticultural oil is the main control: it works by smothering, so it has to coat the insects thoroughly - stems, twigs, and leaf undersides - and it usually needs repeating, because the waxy covering shields them and no single pass reaches everything. On deciduous woody plants a dormant-season oil smothers the overwintering stage while the leaves are out of the way, and timing a growing-season spray to the crawler stage, when the young are mobile and uncovered, is far more effective than spraying settled adults. On a small plant or a light infestation, scrub or wipe the scales off by hand, or dab them with a cotton swab of rubbing alcohol. Prune out and destroy badly encrusted wood rather than trying to save it. Wash off the sooty mold once the scale is gone - it lives on the honeydew, not on the plant, and stops once the supply does. Anything you spray must be labeled for the plant, and oils can burn delicate foliage, so test first.

Inspect. Look over leaf undersides and stems regularly - on a crop you pick, every harvest is a scouting chance - and check every new plant thoroughly before you buy it, plant it, or set it beside anything else, because scale almost always arrives on a plant rather than flying in. Quarantine new houseplants for a few weeks. Keep plants unstressed and not over-fertilized, since soft, forced growth is what populations build on. Prune for airflow and light to open up the shaded inner growth where scale hides, and prune out heavily infested wood as routine maintenance - on anything that gets a hard annual pruning, that alone removes most of any buildup. Leave the lady beetles, lacewings, and parasitic wasps alone, since they normally keep scale down to nothing, and keep ants off the plant where you can, because ants protect the colonies from those predators. Indoors, avoid hot, dry, still winter air.

Scale insects are sap-suckers that spend most of their lives immobile, fixed to a stem, twig, or leaf underside beneath a waxy or shell-like cover. They look far more like bumps, blisters, or crusty encrustations than like insects, which is why an infestation is usually well established before anyone notices it. Soft scales excrete large amounts of sticky honeydew that drips onto everything below and grows a black sooty mold - and those blackened leaves are often the first thing a gardener actually sees. Armored scales produce no honeydew but sit under a hard shield that protects them from most sprays. They spread as tiny mobile crawlers in the weeks after hatching, which is the only stage that moves and the only stage that is easy to kill. Populations build on stressed plants, on crowded shaded growth, and above all on plants kept indoors in dry, still air over winter.

Mower and string-trimmer wounds at the base do the same job in summer, which is the other reason to keep a wide mulch ring instead of turf right up to the trunk.

Long vertical splits in the trunk or a main limb, usually on the south or southwest side, most often noticed in late winter. Sunken, discoloured, or loose strips of bark that peel away to show dead tissue underneath. A wound that reopens in the same place each winter, with rolled callus ridges building along both edges. New shoots and leaves blackening after a late spring freeze and then dying back to older wood. Dieback in one part of the canopy the following summer, directly above a damaged strip of trunk.

Leave the wound alone. Do not paint, seal, or tar it - wound dressings trap moisture and make decay worse, and a tree closes its own wounds far better than any product does. Trim away only the loose dead bark hanging from the edges, using a clean knife and cutting back to bark that is still firmly attached, then let callus roll in from both sides over the following seasons. Keep the tree watered, mulched, and unstressed so it can do that work. Prune out shoots killed by a late freeze once you can see clearly what is dead. A crack that reopens every winter means the exposure was never fixed, so shade and shelter for that trunk are the real remedy.

Siting matters most: a Japanese maple with afternoon shade and shelter from winter wind suffers far less trunk injury than one standing alone in an open lawn. Keep the lower branches on a young tree rather than limbing it up, because their shade protects the trunk. Where the trunk is exposed, a light-coloured trunk wrap or white latex trunk paint through winter reflects the low sun that causes sunscald - remove wraps in spring so nothing girdles the trunk or harbours pests. Mulch the root zone to even out soil temperature, water well into autumn so the tree does not enter winter dry, and stop feeding by midsummer so no soft growth is pushed late in the season.

Japanese maples carry thin, smooth bark on a trunk that is often fully exposed on a small, low-branched tree, and that combination invites two cold-weather injuries. Sunscald happens on bright winter days when low sun warms the south or southwest face of the trunk, that tissue comes out of dormancy, and the temperature then crashes after sunset, killing a strip of bark that later sinks, cracks, and peels. Frost cracks are long vertical splits that open on very cold nights, sometimes audibly, as the outer wood contracts faster than the wood inside. A late spring freeze after leaf-out does the same sort of damage to young shoots. None of it is an infection, but an open wound is a doorway for borers and canker fungi.

Damage is cosmetic on an established tree and the leaves are replaced the following spring; a small or newly planted one is worth hand-picking or netting through the flight.

Clusters of shiny metallic beetles massed together on the upper, sunniest foliage and down inside open blooms, heaviest on warm sunny afternoons from around late June. Leaves reduced to lace, with the veins left intact and the tissue between them gone - unmistakable, and quite different from the ragged chewing of a caterpillar. Ragged, chewed petals and blooms that never open properly. Beetles that let go and drop straight to the ground when you disturb the plant. Feeding damage on ripening fruit, on the plants they also take fruit from. Whole branches going brown and lacy at the top of a plant while the shaded lower growth stays untouched.

Persistent morning hand-picking into soapy water is the most effective home control there is, and on a vigorous established plant the damage is usually cosmetic and needs nothing else. Neem deters feeding rather than killing, and has to be reapplied. For heavy pressure, spinosad or a targeted insecticide labeled for the plant will work - but time any spray away from bloom and apply it at dusk to protect bees, and observe the pre-harvest interval on anything you eat. On flowering plants, some gardeners simply shear the blooms off for a couple of weeks and let the plant rebloom once the wave has passed, which removes the lure as well as the damage. Young plants and young trees are the ones worth defending: a mature tree or an established vine tolerates several weeks of leaf loss without lasting harm.

Hand-picking is genuinely the main defense in a home garden. Go out in the cool of the morning while the beetles are sluggish, hold a tub of soapy water underneath, and knock them in - they drop when disturbed, so most fall straight into it. Do it daily through the flight: because the beetles call each other in, taking them off early stops a plant becoming a beacon. Do not put pheromone or lure traps anywhere near the garden - they demonstrably pull in far more beetles than they catch. And do not count on lawn grub control: milky spore has performed poorly and inconsistently in field trials across most of the US, and treating your own turf does not reduce the adults feeding on your plants, because they fly in from surrounding properties. Where it is practical, row cover or netting through the peak flight excludes them outright. Otherwise riding out the six-to-eight-week wave is a legitimate strategy, since most established plants absorb the damage.

Japanese beetles are metallic green-and-copper beetles about half an inch long that emerge in early summer and feed for six to eight weeks. They feed in aggregating masses - one beetle's feeding releases a scent that draws in more - so a plant goes from untouched to covered in a couple of days. They chew the soft tissue between the leaf veins and leave the veins standing, so the leaf ends up as a brown lace skeleton, and they shred open flowers and sometimes ripening fruit as well. They work the sunniest upper parts of a plant on warm afternoons. The white C-shaped grubs spend the rest of the year in turf and pasture soil eating grass roots, and the adults then fly in from wherever they hatched, which can be some distance away.