Skip to content
Pocket Botanist
Get the app

Knotweed · Persicaria

Why are there spots on my knotweed leaves?

Seeing spots on your knotweed usually points to a fungal infection triggered by high humidity or poor airflow. Check the undersides of the leaves first to see if the spots are fuzzy or have a distinct border.

Not sure it's this? Check a photo of your plant for free

spots on a plant
Photo: Scot Nelson, CC0

Check these first

  1. Inspect the leaves for fuzzy growth or spores.
  2. Check the soil moisture level with your finger.
  3. Look at the spacing between your plants for airflow.

Likely causes

  1. Fungal Leaf Spot

    Most likely

    How to tell: You will see brown or black circular spots often surrounded by a yellow halo. These spots tend to spread during rainy or humid weather.

    What to do: Remove and destroy all affected leaves immediately to stop the spread. Improve air circulation around the plant by thinning out dense foliage and avoid overhead watering.

  2. Overwatering

    Possible

    How to tell: The spots may look water soaked or mushy rather than dry and crisp. The soil stays wet for more than 7 days.

    What to do: Allow the top 2 inches of soil to dry out before watering again. Ensure your pot has drainage holes so excess water can escape.

Leaf spots on other plants

Find out what's wrong with your 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.

Download on the App StoreGet it on Google Play

When to worry

Worry if the spots are spreading rapidly across the entire plant. A few spots on older leaves are normal as the season progresses.

Stopping it coming back

Water at the base of the plant to keep foliage dry and provide plenty of space between plants.

Knotweed: spots? Check yours with a photo

Every plant is a little different. Take a photo of your 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.

Download on the App StoreGet it on Google Play

Get the app · Or check it from a photo now, free

Questions people ask

Should I use a fungicide?
Usually, you do not need one. Simply removing the infected leaves and fixing your watering habits is enough to stop the issue.
Will the spots go away?
No, the damaged tissue will not heal. You should remove the spotted leaves to keep the plant looking tidy.

Guides to these causes

More problems with Knotweed

Back to the 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.