My mint growing by the window in my sunroom (facing west) started having leaves turning black (looked burnt and dry like charcoal) on the tip. I tried watering more but it didn’t seem to change. Actually, something similar is happening to the sage growing next to it. Please advise as to the cause and solution.
Both herbs are hardy perennials, happy to grow outdoors. They may not be as well-suited to growing indoors.
Consider the following factors, one or more of which could be responsible for the black leaves:
Light: Adequate sun exposure is critical in maintaining herb health. It sounds like the mint and sage are getting enough sun, especially in the past month or so (it is late April).
Heat: However, if it’s too hot in the sunroom, the plants could be suffering from the heat. Herbs generally like temperatures of around 18-21 degrees C (65-70 degrees F) during the day. Also, the leaves should not touch the window, as they would be affected by cold winter temperatures or hot summer temperatures.
Water: Watering – either too little or too much – could be an issue. Mint likes a moist soil, and excessively dry soil could contribute to the leaves turning black. On the other hand, overwatering can also cause the leaves to rot and turn black.
As for the sage, this plant does not like soil to remain wet – after watering, let the top of the soil dry out before watering again.
Soil: Both herbs need to be in a well-draining soil, or they may be more susceptible to rot or diseases/pests. To increase the draining capacity of the soil, you could add sand or perlite to the soil mix. Consider repotting the plants if you’ve had them in the same soil for a few years. Remember too that clay pots permit better soil drainage than plastic or glass pots.
If the surface of the soil has areas that are white in colour, this is salt crystals, which the herbs do not like. Salts can contribute to turning leaf tips brown and dry/crisp. Fertilizer should help with this issue. As well, flushing the soil 4-5 times with water, permitting it to fully drain between flushes, will help get rid of the salts. I sometimes also remove the top few centimetres of soil, to get rid of the crusty salts, and replace this with fresh potting soil.
Fertilizer: Now that it’s spring, and the plants are actively growing, fertilize once every 1-2 weeks with organic fish emulsion or liquid fertilizer (e.g., 12-12-12). This should keep them healthy and less susceptible to diseases/pests.
Diseases/Pests: From your description, it does not sound like your plants are affected by insect pests. However, fungus could be a problem. Fungal blight affects weakened mint plants, causing stems or entire leaves to turn black and fall off. SF Gate’s Indoor Mint Leaves With Black/Brown Spots discusses additional diseases and pests that can strike mint. Note that fungal leaf spots can affect sage. If either plant is affected by fungus, discard the plant and soil (don’t compost it, as fungal spores may live on!) and to start afresh with new herbs.
Finally, keep the plants several inches apart, in case whatever affects one spreads to the other!
See also Missouri Botanical Garden’s Herb problems indoors.