Q. Two plants in my living room have yellowing leaves. Can you suggest what’s wrong? -Diane Broyles, San Luis Obispo
A. Causes of yellowing leaves on houseplants can often be diagnosed with a little detective work. If a previously happy plant turns yellow, inspect it for spider mites. These tiny insects suck plant juices, causing yellowing and perhaps death. If you see light-colored stippling on leaves and webbing at leaf interstices, mites are probably present. The tiny suckers (usually found on the undersides of leaves) can be seen with a magnifying glass. Other insect pests are usually more visible.
Spider mites may be controlled by washing the leaves either with a forceful spray of water, with insecticidal soap, or a horticultural oil. Make sure the product you select is labeled for houseplants and for use indoors and follow the label instructions. If a plant is too large to move, try soaking a cotton ball and using it to wash each leaf and stem. Then, follow up with insecticidal soap or oil sprays.
Treatments may need to be repeated every 7 to 10 days until the mites are gone. With a serious infestation, it may make sense to dispose of the plant. Spider mites often attack dusty plants so mist or wash your plants regularly.
If mites are not present, look for other problems. Has the plant been receiving too little or too much sunlight? Plants have differing light requirements — you may need to move them seasonally to keep them happy. Lack of nutrients can cause yellowing; follow feeding recommendations for specific types of house plants.
Overwatering is another cause of yellow leaves. Assure your plant has good drainage and is not sitting in soggy soil. Finally, if the yellowing plant has not produced new leaves for a while, it may need repotting. If possible, tip the pot and pull out the plant to check if roots completely fill the space. If you see roots protruding from the drainage hole, the plant may need to be repotted. Local Master Gardeners can give you some repotting instructions, if needed.
Got a Gardening Question?Contact the University of California Cooperative Extension Master Gardeners: at 781-5939 from 1 to 5 p.m. on Monday and Thursday; at 473-7190 from 10 a.m. to noon in Arroyo Grande; and at 434-4105 from 9 a.m. to noon on Wednesday in Templeton. Visit the UCCE Master Gardeners Web site at groups.ucanr.org/slomg/ or e-mail firstname.lastname@example.org.