Calathea leaves yellowing due to spider mites.
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The DIY Spider-Mite Repellent House Digest's Garden Expert Swears By
Spider mites suck the moisture out of plants, causing yellowing leaves. Fortunately, you can naturally repel them using peppermint soap, neem oil, and rosemary essential oil.
Mix a tablespoon of peppermint soap with a teaspoon each of neem and rosemary oil. Add a liter of water before pouring the mix into a spray bottle and misting the affected plants.
When treating houseplants, keep in mind that the smell of this mixture is going to be pungent — neem oil alone has a pretty unpleasant smell — so it’s best to apply this outdoors.
If not, place your affected houseplants in the tub, spray them, and turn on the exhaust fan. Let the solution sit for a few hours before you wash it off and put the plants back.
For outdoor plants, make sure you spray the solution in the evening as it contains neem and rosemary oils that can burn the delicate tissues of your plant’s leaves in full sun.
While this solution will help trap and suffocate spider mites, it might not work immediately. You might need to repeat the spraying process several more times for complete relief.