Pocket Botanist
Check these first
- Inspect the undersides of the leaves for signs of fungus.
- Check the soil moisture level with your finger.
- Observe the air movement around the plant.
Likely causes
Fungal Leaf Spot
Most likelyHow to tell: You will see small brown or black spots with yellow halos on the leaves. These spots often spread during periods of high humidity or frequent rain.
What to do: Remove and destroy all infected leaves immediately to stop the spread. Improve airflow around the plant by thinning out dense branches and avoid overhead watering.
Overwatering
PossibleHow to tell: The spots may appear soft or water soaked rather than dry and papery. The surrounding leaf tissue might also turn yellow.
What to do: Check the soil moisture before watering again. Allow the top 2 inches of soil to dry out between waterings to keep the roots healthy.
Find out what's wrong with your giant 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
Worry if the spots are spreading rapidly across the entire plant. A few spots on older leaves are usually just a sign of normal aging.
Stopping it coming back
Water at the base of the plant instead of over the leaves to keep foliage dry. Space plants properly to ensure good air circulation.
Giant knotweed: spots? Check yours with a photo
Every plant is a little different. Take a photo of your giant 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
- Should I use a fungicide?
- Usually, you do not need one. Removing infected leaves and improving airflow is almost always enough to manage the issue.
- Will the spots go away?
- No, the damaged tissue will not heal. The plant will grow new, healthy leaves once the fungal issue is under control.
Guides to these causes
More problems with Giant knotweed
Back to the Giant 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.