Potted Peppermint and Sage Have Yellow Leaves and Black Spots
Answered by: Conrad Richter
Question from: Jane Kelley
Posted on: October 23, 2005

My potted peppermint plants are getting yellow leaves, some with black spots on them. My sage plant is doing this also. Do they need some fertilizer over the winter? We live in Minnesota.

Yellow leaves are a common symptom of malnutrition in plants. There are other common causes for yellow leaves also, such as overwatering and underwatering. Plants in pots, especially at the end of the summer season and into the fall season tend to be both rootbound and undernourished.

Whether or not your plants are in the garden or in pots, during this fall season, lower temperatures and light levels can also cause the leaves to yellow also, as the plants begin to withdraw resources back to the roots in preparation for the winter. If your pots are outside this is a likely cause. The black spots are consistent with an autumn-induced withdrawal of resources as opportunistic diseases often show up on weakened leaves. But these seasonally induced diseases are rarely fatal to the plant; the vigourous new leaves in spring usually are free of disease.

If you are wintering your plants indoors you should repot the plants in larger pots if plants are rootbound. Supplement the light with artificial light supplied by a grow light set up on the window where the plants are growing: this way the plants get both natural light during the day plus artificial light in the early morning or evening. The lights should be on a timer so plants get a total of 16 hours of light. Remove the weak and diseased leaves.

If the plants are not rootbound then increase the light and apply a weak fertilizer solution (quarter to half strength) to give them a boost.

Watch for pest problems, especially on the undersides of the leaves. Look for spider mites and mealy bugs.

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