Enkianthus Problems: Yellow Leaves, Scorched Margins and Root Rot

Most problems are not pests

This is a notably trouble-free shrub. Insects rarely do serious damage to it, and where a plant looks poor the cause is almost always a condition rather than an organism: soil that is not acid enough, a root run that dried out in summer, ground that stayed wet all winter, or wind that scorched the leaves. Reaching for a spray is usually the wrong first move.

The pattern of damage separates the causes cleanly. Yellow between green veins on the newest leaves is a pH problem. Brown, dry, crisp margins on otherwise green leaves is water or wind. Whole shoots wilting and dying while the rest of the plant looks fine is a root problem. Small pale stipples on the upper leaf surface with dark specks underneath is one of the few genuine pests. Work through those four before considering anything else.

Check the plant when you water it in summer, which is the season when almost everything shows, and turn a few leaves over each time.

Chlorosis, the commonest complaint

Yellow leaves with sharp green veins mean the plant cannot take up iron, and on this shrub that nearly always means the soil is too alkaline rather than that iron is absent. It shows on the youngest leaves first because iron does not move around inside the plant once it has been deposited, so new growth is served last and suffers first.

Confirm it with a pH test rather than by assumption, and take the sample from root depth at the plant, not from the surface elsewhere in the bed. If the reading is above 6.0, work elemental sulfur into the top few inches over the root spread, mulch with composted bark or pine needles, and switch to rainwater if your tap water is hard. Expect the reading to move slowly and the new growth, rather than the existing yellow leaves, to be the evidence that it worked.

A foliar spray of chelated iron greens up the affected leaves within a fortnight and is useful when a plant is badly weakened, but treat it as first aid. It does nothing to the soil, and if you have to repeat it every year the underlying pH has not been addressed. Look also for hidden sources of lime: mortar in a nearby wall, buried building rubble, or a concrete path whose edge runs through the root zone.

Drought scorch and root rot, the two water faults

The root system is fine, fibrous and shallow, sitting in the top six to eight inches (15 to 20 cm) of soil, so it is exposed to both extremes. Too little water and the leaf margins go brown and crisp, then leaves drop from the inside of the canopy outward, and shoot tips carrying next year's flower buds die back. Too much and the same fine roots suffocate and rot.

Drought scorch is easy to distinguish from disease because it is worst on the side of the plant facing the sun or the wind, and the damage is dry rather than soft. It cannot be reversed on leaves that already have it, but a deep soak and a thick mulch stops it spreading. Do not respond by watering little and often, which only wets the surface and makes the plant even shallower rooted.

Root rot shows as individual shoots wilting on a plant with damp soil, then whole branches dying back, and the roots when you scrape at them are dark and soft rather than pale and firm. Once it is established there is little to be done chemically. Improve the drainage, cut out dead wood back into sound tissue, and stop watering. On heavy ground the long-term answer is to replant on a raised mound or in a bed built up above the surrounding level, where the roots stay moist without ever standing in water.

Winter is the dangerous season for this. A shrub in heavy soil that is fine all summer can be lost to weeks of standing water between autumn and spring, so check where water sits in your garden after heavy winter rain before deciding the position is good.

A plant that wilts on hot afternoons and recovers by morning is showing you the shallow root system, not a fault. A plant still wilted at dawn is dry at the root and needs a slow, deep soak rather than another light sprinkle.

The pests that do turn up

Lace bug is the one insect that occasionally does real damage, and it is shared with rhododendrons and pieris, so it is more likely if you grow those nearby. It feeds on the underside of the leaf, producing fine pale stippling on the upper surface and hard dark varnish-like specks of excrement below. Damage is worst on plants in hot, dry, sunny positions with poor air movement, which is one more reason to shade the root run and keep the plant properly watered.

Spider mite appears under the same conditions, and looks similar from above. Turn the leaf over: lace bug leaves flat dark specks and the insects themselves are visible and flat-winged, while mites leave fine webbing in the leaf axils and the specks move if you watch closely. Both are treated by wetting the leaf undersides thoroughly, repeatedly, and by fixing the dryness that encouraged them.

Aphids sometimes cluster on soft new shoots in spring, distorting the tips. On an established shrub they rarely need action, since predators clear them within a fortnight, and a hard jet of water deals with a bad patch. Vine weevil is a genuine risk on container plants, where the grubs eat the roots over winter and the first symptom is a plant that collapses in spring; the adults leave characteristic semicircular notches bitten out of the leaf margins in summer, which is your warning to act before the grubs do the damage.

Deer and rabbits will browse young plants in some areas, and on a slow shrub a single winter of browsing sets it back years. Protect new plantings with a guard for the first few seasons if you have either.

Note the date you first see any of these, every year. Lace bug and mite both build up over the summer, and knowing the week they usually appear in your garden lets you act at the point when a single treatment still works.