Plants in containers with drainage holes can be thoroughly watered, as the excess drains out. If there are no drainage holes, check soil two inches beneath the surface by sticking your finger into the soil to detect whether the soil is still damp before adding water. Plants without drain holes will often have yellowing lower leaves and will gradually decline. Too much water encourages root rot. Move plants to better drained pots if you suspect inadequate drainage.
Insect pests can move in unexpectedly.
Check the undersides of leaves regularly, especially on those plants brought in from outdoors. Wash the foliage regularly with a mild soapy solution, taking care to rinse all soap off completely. A fine water spray or wiping the leaves with alcohol-soaked cotton will take care of most insects. Rinse off all the alcohol. Use any chemical insecticide with great care.