Yellow leaves can also mean the plant is overwatered. A great way to decipher watering issues is to carefully remove the plant from its container and look at the roots.
“With some exceptions, healthy plants generally have white roots,” Vogel said. “If the roots are rusty orange to light brown, it is underwatered, and if the roots are dark brown to black, it is overwatered.”
Gently place the plant back in the container and adjust the watering schedule accordingly. If overwatering is the issue, withhold water until the soil is quite dry and gradually begin a new watering schedule. After two weeks, apply a water-soluble, houseplant-specific fertilizer. If no improvement is seen in the next two weeks, repot in fresh soil.
Yellowing may also occur if a plant needs fertilizing. Depending on what nutrient is needed, yellowing may begin on the outer edges of leaves, or from the vein outward, or the leaf may even curl. Become familiar with the fertilization needs of each species. Occasionally, a plant needs fertilization when it becomes root-bound and may simply need to be repotted into a larger container.
• Spots, leggedness. Leaf spots on foliage can be caused by too much sun. When spots appear consistently, move the plant to a place it receives less sun.
“Leggy, or stretching plants occur when they are reaching for more light,” Vogel said. “Move them to a brighter location, or turn them occasionally to keep them growing evenly.”