Gadenin日本語Get the appRule out the environment before you blame a pest
Brown, crisp leaf margins on a broadleaf evergreen are far more often physical damage than infection. Cold drying wind on frozen ground is the classic cause: the leaves keep losing water, the frozen roots cannot resupply them, and the tissue at the edges dies. The pattern gives it away, because the damage is concentrated on the face of the plant that meets the wind and stops abruptly where a wall or another plant provides shelter.
Wet roots produce a different pattern: dull, slightly yellowed foliage across the whole plant, leaves dropping from the interior, and a complete lack of new growth in spring. Look at whether the damage is one-sided or general before you spend money on anything.
Sun scorch on a plant recently moved from shelter into full sun is a third possibility, and it shows as pale bleached patches in the middle of leaf blades rather than browning at the margins. All three of these are corrected by changing the growing conditions, not by spraying.
| Symptom | Likely cause | First action |
|---|---|---|
| Brown crisp leaf edges on one side only | Cold drying wind | Windbreak screen on the exposed face, thick mulch |
| Dull yellowing leaves, interior leaf drop, no new growth | Roots standing in water | Check drainage, stop watering, replant on a mound if needed |
| Sticky shiny leaves, black coating over them | Scale insects and the sooty mold that follows | Find and treat the scale; the mold is a symptom |
| Small bumps on stems and leaf undersides | Scale insects | Physical removal, then a horticultural oil at crawler stage |
| Pale bleached blotches in the middle of leaves | Sun scorch after a sudden move | Shade for a week, reintroduce sun gradually |
| Sooty black leaves with ants running the stems | Ants farming scale for honeydew | Deal with the scale; the ants leave with it |
| Small round leaf spots, no other symptoms | Minor leaf spot fungus | Remove worst leaves, improve airflow, no spray needed |
| Sudden wilt of a whole branch | Root or collar damage, or a rotted stem base | Excavate to check depth and drainage at the stem |
Scale insects, and the mold that gives them away
Scale is the pest that actually matters on this plant, and the density of the foliage is why. The insects sit on stems and the undersides of leaves deep inside the canopy, protected under waxy shells that shrug off contact sprays and that most people mistake for part of the bark. They look like small oval or round bumps, brown or gray, and they scrape off with a thumbnail.
You will usually notice the consequence before the cause. Scale excretes honeydew, honeydew coats the leaves below in a sticky film, and a black sooty mold grows on the film. The mold itself does not attack the plant, but a thick black coating blocks light from the leaf and a heavy infestation drains enough sap to stop growth and stop flowering. Ants running purposefully up and down the stems are a reliable early clue, because they farm the honeydew and defend the colony.
Treat it in two stages. Physically remove what you can find by wiping stems with a cloth or scrubbing gently with a soft brush and soapy water, and cut out any badly encrusted twigs entirely. Then time a horticultural oil or insecticidal soap for when the young crawlers are active and mobile, which is the only stage that is genuinely vulnerable; adults under their shells are largely protected. Two applications a couple of weeks apart work far better than one. Wash the sooty mold off with plain soapy water once the insects are dealt with, or simply let rain and new growth take care of it.
Problems that come from the site rather than the plant
A surprising share of calls for help on this shrub are planting faults that only show up years later. Work through them in order before treating anything.
- Check the planting depth Scrape soil and mulch away from the base and find the point where the stem flares out into roots. That flare should be at or just above soil level. If the stem disappears straight into the ground like a post, it is buried too deep, the bark stays wet and rots, and the plant declines slowly. Pull the soil and mulch back to expose the flare.
- Test the drainage Dig a hole a foot (30 cm) deep near the plant, fill it with water, let it drain, and fill it again. If the second fill is still standing the following morning, the roots are living in water for part of every year. Divert the water, raise the bed, or move the plant.
- Look for circling roots On a plant that was pot bound when planted and has never thrived, dig gently at the edge of the original root ball. Roots wrapping in a tight circle mean the plant is slowly strangling itself. On a young plant it is worth lifting, cutting the circling roots and replanting; on an old one, all you can do is keep it well fed and watered.
- Clear competition at the base Turf and weeds growing up to the stem compete directly with a slow shrub and hide string trimmer damage. Clear a circle at least three feet (90 cm) across and mulch it, keeping the mulch clear of the stem itself.
When something goes wrong, write down the date, which side of the plant it appeared on, the weather in the preceding fortnight, and what you did about it. Next season the same problem will look familiar instead of new, and one-sided damage recorded twice in the same place is proof of a wind or drainage fault rather than bad luck.
When to treat and when to leave it alone
Most cosmetic damage on this shrub is not worth a spray. Minor leaf spots, a few chewed leaves, a scatter of scale on one branch and a season's wind scorch all get grown out of by a healthy plant. Reach for treatment when growth has actually stopped, when the sooty coating covers most of the canopy, or when a problem is spreading measurably from month to month.
Damaged leaves never repair themselves. Once a margin has browned, that leaf stays browned until it is shed, so judging recovery by looking at old foliage will always tell you the plant is still ill. Judge by the new growth instead: clean, full-sized, properly colored new leaves mean the problem is behind you, whatever the old leaves look like.
Know when to give up on a plant as well. A shrub that has sat in wet ground for several years, lost most of its interior and failed to push meaningful new growth after a hard cut is not going to come back on this site. Replacing it on a raised, better-drained position gives you a better plant in three years than nursing the old one for ten.