Japanese Anemone Problems: Holed Leaves, Brown Wedges and Scorched Margins

Read the pattern before you treat anything

Japanese anemone is a robust plant and genuinely trouble-free most years. The mistake people make is reaching for a treatment when the actual cause is cultural, usually soil that dried out at the wrong moment. Damage on this plant is very readable if you look at the shape and the position of it rather than just the color.

Work through the table below before doing anything. In particular, distinguish between damage that follows the leaf veins in straight-edged blocks and damage that spreads outward in rounded patches, because those are two completely different problems with completely different responses.

Slugs on the spring rosette

Slugs and snails are the main pest, and their window is narrow. They go for the soft new leaves as the clump comes up in spring, and they can shred a young plant badly. Once the foliage has hardened and the stems have started to extend, the plant becomes much less interesting to them and the damage effectively stops.

That means the whole job is protecting the emerging rosette for a few weeks in spring, not defending the plant all season. Put protection in place before you see the first leaves rather than after the first damage, because slugs feed at night and you will always be a week behind if you wait for evidence.

Clearing dead stems and fallen leaves off the crown in late winter removes the shelter they overwinter in, and is the single most effective thing you can do. Go out with a torch on a mild damp evening and remove what you find by hand, which is more effective than most people expect. On an established colony, accept some damage: a mature clump grows far faster than slugs can eat it, and holed leaves in spring are hidden by the growth that follows.

Foliar nematode: the one to act on

This is the only problem on Japanese anemone worth treating as urgent, and it is unmistakable once you know it. Microscopic worms live inside the leaf tissue and are confined by the leaf veins, so the dead areas they create are not rounded blotches but angular wedges and blocks with edges as straight as if they had been ruled. Held up to the light, the pattern is a mosaic of dead panels between green veins.

It starts on the lowest leaves, because the nematodes travel upward in a film of water on the plant surface, and moves up the plant through a wet season. Affected leaves go from pale green to brown to black and hang on the stem rather than dropping cleanly.

There is no spray for it in a garden. Management is entirely about breaking the cycle. Pick off and bag every affected leaf as soon as you see the pattern, and never put that material on the compost pile. Clear all fallen debris from around the plant in autumn, because the nematodes survive the winter in dead leaves and in the crown. Water at soil level rather than over the foliage, since the film of water on wet leaves is how they move. Thin overcrowded stems so air circulates and the leaves dry quickly.

A badly infested plant that has lost most of its lower foliage two years running is not worth persevering with. Dig it out, take the surrounding soil with it, and do not replant susceptible plants in the same spot for a season.

Because every root fragment of this plant regrows, removing an infested clump is a two-season job. Dig the area over, sieve out every piece of root you can find, and go back in spring to pull anything that reappears before it makes leaves.

Aphids and mildew, both minor

Black aphids colonise the upper flower stems and buds in summer, sometimes in dense sleeves. They distort buds if numbers get very high, but on an established plant they rarely do real harm and predators usually deal with them within a couple of weeks. Intervene only if buds are visibly deforming.

1. Check the growing tips weekly in early summer Aphids build up on the softest tissue, which is the top few inches of the flower stems and the buds themselves. A ten-second look each week while you are deadheading catches a colony while it is still small enough to deal with by hand.
2. Knock them off with water first A firm jet of water aimed at the stem tips removes most of a colony and does no damage to the plant. Repeat after two or three days. This is usually enough and it leaves the predators, ladybird larvae and hoverfly larvae, in place to finish the job.
3. Squash the rest between finger and thumb For the remainder, run finger and thumb up the stem. Faster than any treatment on a plant of this size and it does not affect anything else in the border.
4. Leave low numbers alone A scatter of aphids is what attracts and feeds the predators that keep the whole border in balance. Clearing every last one in early summer usually means a worse outbreak later, once the predators have gone elsewhere.

What looks like a problem and is not

Crisp brown leaf margins in midsummer are drought, not disease. This plant wants soil that stays moist through summer, and in a hot dry spell in full sun it will scorch. The cure is a deep soak and a mulch, and repositioning to somewhere with afternoon shade if it happens every year. No spray will help.

A plant that does nothing in its first year, or produces a clump of leaves and no flowers, is behaving normally. Japanese anemone characteristically sulks for a season or two after planting while it builds roots, then performs. Judging it before its third summer is unfair and leads to perfectly good plants being dug up.

Late emergence in spring worries people every year. This plant comes up noticeably later than most border perennials, and bare soil in mid spring where you know a clump is planted is not a sign of death. Wait until other late risers are up before you go digging around to check.

And the shoots coming up two feet (60 cm) from where you planted it are not a disease either. That is the plant working exactly as designed, and it belongs in the containment routine rather than the pest list.