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Japanese knotweed · Reynoutria japonica

Why does my Japanese knotweed have dark spots on its leaves?

Japanese knotweed is a tough plant, but it can develop fungal leaf spots during long periods of high humidity or heavy rain. Check the undersides of the leaves for signs of fungal growth first.

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spots on a plant
Photo: Scot Nelson, CC0

Check these first

  1. Inspect the undersides of the leaves for fuzzy growth.
  2. Check if the weather has been unusually wet or humid lately.
  3. Look for signs of poor airflow around the base of the plant.

Likely causes

  1. Fungal Leaf Spot

    Most likely

    How to tell: You will see dark, irregular spots that may have a yellow halo around them. These often appear after a stretch of wet, humid weather.

    What to do: Remove and bag any infected leaves to prevent spores from spreading. Improve air circulation around the plant by thinning out dense stems.

  2. Overhead Watering

    Possible

    How to tell: Spots appear shortly after you have watered the plant using a sprinkler or hose that wets the foliage. The leaves stay damp for too long, encouraging fungal growth.

    What to do: Switch to watering at the base of the plant only. Keep the foliage dry to prevent spores from germinating.

Leaf spots on other plants

Find out what's wrong with your japanese knotweed

The list above covers the usual causes. Pocket Botanist looks at a photo of your actual plant and tells you the likely cause, how serious it is, and what to try first. Identification and a health check are free.

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When to worry

It is usually just a cosmetic issue that does not kill the plant. Worry only if the spots cover more than half of the foliage.

Stopping it coming back

Keep the area around the plant clear of debris and ensure it has plenty of space for air to move through the stems.

Japanese knotweed: spots? Check yours with a photo

Every plant is a little different. Take a photo of your japanese knotweed and Pocket Botanist tells you which cause is most likely, how serious it is, and what to try first. A few checks a day are free. Save the plant and you get care reminders timed for it.

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Questions people ask

Will these spots kill my plant?
No, Japanese knotweed is extremely resilient and will likely survive even with significant spotting.
Should I use a fungicide?
Usually, it is not necessary. Simply removing the affected leaves and fixing your watering habits is enough.

Guides to these causes

More problems with Japanese knotweed

Back to the Japanese knotweed guide

This article was generated with AI on from the plant’s care profile and is general information, not horticultural advice. Read about how we build these guides.