Orchids (Orchidaceae spp.) are houseplants enjoyed for their elaborate, diverse blooms and waxy leaves. Though common, orchids have somewhat exacting care requirements that must be met if they are to thrive. Yellow, sickly leaves are a common orchid problem that can usually be fixed by changing the plant's growing conditions or checking for pests.
If an orchid produces yellow leaves that also show signs of rot, the plant may be getting too much water. Allow the plant to dry out for several weeks before watering it again. Take care to only water in the morning, as this allows the plant time to dry out before nighttime. To keep from overwatering, poke your finger in the soil. If the soil is moist below the first inch, do not water again until it's dry.
Most orchids suffer if grown in direct sunlight, as this can burn the plant. Orchids with yellowish green or yellowish orange leaves may simply be receiving too much direct sunlight. Orchid species such as Paphiopedilum, Phalaenopsis and Oncidium are especially sensitive to high levels of sunlight. If the leaves of the plant feel warm or hot to the touch, this can also indicate that the plant is getting too much light. Move the plant to a shadier environment.
Pests such as scale may cause yellowing leaves. Scale forms dense colonies on the undersides of leaves, sucking nutrients from the plant and causing dark yellow patches on leaves. Leaves may drop prematurely if a scale infestation is present. Mild infestations can be remedied simply by dipping a cotton swab in rubbing alcohol and swiping it across the insects. More severe infestations may call for chemical sprays or insecticidal soap.
An orchid that has insufficient nutrients may produce yellow leaves. Orchids should be fertilised regularly during the growing season to ensure healthy foliage and flowers. Apply a water-soluble orchid fertiliser monthly during the growing season, or apply a diluted fertiliser every week or two. Water thoroughly before applying fertiliser, as undiluted fertiliser can burn the plant's roots. Dry or granulated fertilisers must be dissolved first before they are applied to the soil.