Birch

A native beetle whose larvae tunnel in the layer just under the bark, cutting the tissue that carries food down the trunk. It is the single reason most landscape birches die. The crucial fact is that it cannot succeed on a vigorous tree: a well-watered, mulched birch fights off the larvae, while a drought-stressed one in hot mown lawn is colonized and killed. Adults fly from late spring into summer and are drawn to stressed trees. Attack always begins in the sunlit top of the crown and works downward, so by the time whole limbs die the tree is usually beyond saving. European white birch is the most susceptible species by far and river birch the most resistant.

Watch for: Thin, sparse, or bare twigs at the very TOP of the crown while the lower canopy still looks healthy - the earliest and most reliable sign, and often the only one for a year or more. Progressive dieback moving downward season by season. Raised, winding, snake-like ridges under the bark where larvae have tunneled. Small D-shaped exit holes, about an eighth of an inch across with one flat side, in the bark of affected limbs. Rusty staining or weeping from the bark over old galleries. Clusters of small shoots erupting below a dying section as the tree tries to compensate.

Prevent: This is entirely a matter of keeping the tree unstressed, and it works. Plant a resistant species - river birch throughout most of the country, paper or yellow birch only where summers are genuinely cool - and never plant European white birch. Site it with the canopy in sun but the root zone cool and shaded, and mulch three to four inches deep out to the branch tips instead of growing lawn under it. Water deeply through every dry spell for the life of the tree, not just while it is young. Avoid wounding the trunk, and prune in late summer or fall rather than spring, since fresh bleeding wounds attract the adults. Remove and destroy infested wood rather than stacking it near healthy birches.

Treat: There is no home remedy and no spray that reaches larvae under the bark. A tree with only the top few feet affected can sometimes be saved by an arborist using a systemic insecticide alongside aggressive watering and mulching, but that is a real trade-off on a tree that feeds hundreds of caterpillar species, and it is worth it only on a valuable specimen. Prune out dying limbs well below the damage. Once dieback has reached the main scaffold branches the tree is lost - take it down before it becomes a hazard and replant with a resistant species in a properly mulched, watered site. Prevention is genuinely the whole story here.

The defining problem of birch in American landscapes. Species choice and a mulched, watered root zone prevent it; nothing cures it. Any birch declining from the top down should be assumed to have it.

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.

Watch for: 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.

Prevent: 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.

Treat: 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.

Birch is a preferred host and the top of the crown can be laced within days in July. On a mature tree ride it out - it is cosmetic, and a systemic treatment would kill the caterpillars this tree exists to feed. Hand-pick reachable branches on a young tree, and treat heavy defoliation as a reason to water deeply, since the drought stress that follows is what bronze birch borer waits for.

Leaf scorch is not a disease and there is nothing to spray for it. It is what happens when a leaf loses water faster than the roots can replace it: the tissue farthest from the veins - the margins and the tips - dries out first and turns brown and papery, and the browning works inward while the rest of the leaf stays green. Hot sun, dry soil, drying wind, heat reflected off paving or a wall, a small or damaged root system, and a plant sited somewhere sunnier or drier than it wants are the usual causes, singly or together. It also happens in winter on evergreens, when bright sun and wind pull water out of the leaves while the ground is frozen and the roots cannot resupply. Shade and woodland plants, shallow-rooted plants, and anything recently planted scorch most easily.

Watch for: Browning that starts at the leaf edges and tips and spreads inward, with a fairly sharp line between the crisp brown tissue and the green that is still healthy. Leaves and fronds going dry, papery, and crisp rather than soft, yellow, or spotted - that is what separates this from a fungal problem. Damage worst on the side of the plant facing the sun or the prevailing wind and on the most exposed upper leaves, while the shaded interior stays fine. Symptoms appearing sharply after a heat wave, a dry spell, or a bright windy week in winter. Whole leaves or fronds shed early as the plant cuts its losses.

Prevent: Site the plant where it actually wants to be. This is a placement problem far more often than a watering problem, and a shade plant in hot afternoon sun will scorch every summer however much you water it. Give shade and woodland plants part-to-full shade and shelter from hot afternoon sun and drying wind. Keep the root zone consistently moist and mulch it well with leaf litter or another organic mulch, which holds water and keeps the roots cool - the single most useful routine step. Water deeply and thoroughly during dry spells rather than little and often, so moisture reaches the whole root zone. Pay particular attention through the first year after planting, when the root system is small and cannot keep up with the leaves. On evergreens in cold regions, make sure the plant goes into winter well watered, and keep it out of bright, windy, exposed positions.

Treat: Water deeply, mulch, and improve the shade - that is the whole treatment, because there is nothing here to kill. Leaves that are already scorched will not recover, so trim off badly browned foliage or fronds for appearance if you want to; the crown and roots are usually unharmed and push out fresh growth once conditions improve. On a plant that scorches badly every year the answer is to move it rather than to keep watering harder, because the siting is what is wrong. Some plants respond well to being cut back hard, which brings a clean flush of new foliage. And on plants that naturally die back or brown off late in the season, scorching at the end of a dry summer is normal and needs nothing at all.

Take this more seriously on birch than on most trees. Browning margins and August leaf drop mean the shallow roots are running dry, and that same drought stress is precisely what lets the bronze birch borer in. Widen the mulch ring, take the lawn out from under the canopy, and soak the root zone deeply through every dry spell.

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.

Watch for: 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.

Prevent: 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.

Treat: 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.

Birch aphids are usually only noticed by whatever is parked beneath the tree, which ends up sticky with honeydew and then sooty black. They do the tree no real harm and predators clear them by midsummer - so treat this as a siting question rather than a spraying one.

The larvae of small sawflies feed inside the leaf between its upper and lower surfaces, hollowing out blotches that start as pale green blisters and dry to papery brown. The first generation in May is the heaviest and the most conspicuous, and a badly mined tree can look scorched by early summer. Despite the appearance it is a minor pest: healthy birches tolerate it year after year, and introduced parasitic wasps have brought populations down sharply across much of the Northeast and Canada since the 1990s. It matters mainly because repeated heavy mining adds to the stress that lets the bronze birch borer in.

Watch for: Pale translucent blotches inside new leaves in May, spreading and turning papery brown, with the leaf surface intact above and below the damage. Whole leaves going brown and crisp on the outer, sunniest growth while the inner canopy stays green. Tiny dark larvae visible if you hold a mined leaf up to the light. Damage concentrated on the first flush of spring leaves, with later growth much cleaner. Gray birch and paper birch mined much more heavily than river birch.

Prevent: Choose river birch, which is far less affected than paper, gray, or European white birch. Beyond that, keep the tree vigorous with mulch and deep watering, which is what actually determines whether mining matters. Leave the parasitic wasps that now control this pest alone by not spraying, and skip preventive systemic treatments, which kill the caterpillars a birch exists to feed for the sake of a cosmetic problem.

Treat: None is warranted on an established tree. The damage is inside the leaf, so contact sprays do not reach it, and the leaves are already mined by the time you notice. Rake and remove heavily mined leaves if you like, water deeply through the summer, and let the tree push its second flush. On a small, newly planted, or already struggling birch, an arborist can time a treatment to the first generation - but on a healthy tree the honest answer is that this looks much worse than it is.

Cosmetic on a healthy tree and now much reduced by introduced parasitic wasps. Treat it as a signal to check your watering and mulching rather than as something to spray.