Why are my orchid leaves turning yellow?

A moth orchid (Phalaenopsis) grows new leaves from the center and slowly lets go of the old ones at the bottom. University of Maryland Extension calls yellowing and drying of the oldest leaves expected. The American Orchid Society describes how it looks: a clear, even yellow that fades gradually, often in spring. On a Cattleya, the same thing happens to the leaves on the old pseudobulbs at the back of the plant.

What makes it normal is the pattern:

• Which leaf: the oldest, lowest one.
• How many: one at a time.
• Color and feel: an even yellow, with the rest of the plant firm and green.

Timing matters too. The New York Botanical Garden says bottom leaves yellowing in spring is part of the normal cycle, while yellowing later in the year is more likely a sign the plant is short on resources. Even in spring, NYBG suggests a quick look at the roots. And a Maryland Extension answer adds that losing old leaves earlier than usual can mean stress from water, feeding or light.

Orchid roots need air as much as water. When the potting mix stays soggy, or old bark breaks down into dense, soil-like crumbs, the roots smother and rot. The AOS, NYBG and Maryland Extension all trace yellow leaves back to this. NYBG adds that the damage is often worst deep inside moss or bark, where it stays wet the longest.

The tell-tale signs:

• More than one leaf turning yellow, often newer ones too.
• Leaves that feel soft or limp.
• Mix that stays wet for days, or looks dark and broken down.
• Roots that are short, brown and mushy instead of firm (described by the AOS).

A quick root check: slide the plant out of its pot. Firm roots are alive. Brown or black roots that are mushy or slimy are dead. Our root guide shows what healthy and rotten roots look like, and our root rot guide covers rescue in more depth. To keep it from happening again, see how to water an orchid and repotting.

Yellow-green all over. Maryland Extension says leaves that turn yellow-green to nearly yellow, often with pink or reddish edges, are getting too much light. The AOS explains that too much light breaks down the leaf's green pigment, and Iowa State Extension agrees on the yellow-green color.

Sunburn. Leaves can overheat after a sudden change, such as a move to a sunnier window or outdoors in spring. The damage sits on the most exposed, sun-facing part of the leaf. Sources describe the first stage a little differently: a black spot (the AOS), white patches that turn brown (Iowa State), or reddish-purple freckles (the AOS heat stress page). In time the spot dries to a tan scar.

What to do:

• Move the plant out of direct sun right away. Iowa State says burn damage can get worse quickly.
• Raise the light gradually after that.
• Leave mildly burned leaves on. The AOS says the scar stays, but the rest of the leaf keeps working. Badly damaged leaves can come off.

Watch the scar: the AOS notes it can get infected.

Damage from cold above freezing is called chill injury. The AOS describes pitting, sunken spots, discolored patches and water-soaked tissue that later turns brown. Its warnings:

• Symptoms can show up days after the cold.
• They can look like underwatering or root rot, so think back to any recent cold nights.
• Moth orchids are prone to losing leaves after a chill.

The NYBG says nights below 16 °C (60 °F) can make a moth orchid's lower leaves yellow, and the AOS gives the same 16 °C (60 °F) as the night floor moth orchids prefer. Chicago Botanic Garden warns that leaves touching a very cold windowpane can be harmed.

Cold water counts too. The AOS names ice cubes as one cause of collapsed leaf tissue and recommends room-temperature water. (Ice cubes are a popular myth: see are ice cubes a good way to water orchids?)

Move the plant away from cold glass and drafts, and remove leaves only if they are badly damaged.

Too little or too much fertilizer. An orchid that hasn't been fed in months can slowly yellow from its older leaves forward while its roots look fine. The AOS and a Maryland Extension answer both describe this, and Maryland suggests a diluted liquid orchid fertilizer. Too much can also be a problem: Iowa State says overfeeding can yellow leaves too, with brown root and leaf tips and a salty crust on the pot.

Low light. Most sources link too little light to leaves that turn dark green: Maryland Extension, Chicago Botanic Garden and the AOS all say so. One Maryland Extension answer adds that a spot that's too dim may make old leaves drop sooner. That's a single source, so treat low light as a less likely reason for yellowing.

A Dendrobium's winter rest. Some Dendrobiums, like Dendrobium nobile and its relatives, drop their leaves every winter and bloom in spring from the bare canes. The AOS and the RHS both call this natural. The RHS says to keep the canes unless they are completely shriveled and pale brown. Hard-cane Dendrobiums (the Phalaenopsis-type, or "antelope", kinds) keep their leaves. The AOS says if their bottom leaves yellow and drop, leaving a palm-tree look, they're likely too cold: below about 13 to 16 °C (55 to 60 °F). For the rest that sets up blooms, see getting it to bloom again.

Crown rot. If the newest leaf in the center turns yellow or brown and rots at the base, water has likely been sitting in the crown, where the leaves meet. The AOS says bacteria rot the growing tip, and a moth orchid that loses it can't grow new leaves from there. To prevent it, water in the morning so the plant is dry by night (Maryland Extension) and blot any water that pools between the leaves (the AOS).

Scale insects. Small yellowish speckles on top of a leaf can mean insects feeding underneath. The AOS and NYBG both say to check the undersides. The AOS names white Boisduval scale as a common one and says to act quickly.

Both approaches come from good sources:

• Wait (AOS). The American Orchid Society says to leave the leaf until the plant has drawn its nutrients back and the leaf separates on its own. Then it comes off with a gentle tug.
• Remove it at the base (Maryland). University of Maryland Extension says to remove old yellowed leaves at the base of the plant. The NYBG also says dead leaves can be trimmed.

A middle path that fits both: wait until the leaf is fully yellow, and remove it once it pulls away easily or has dried. For a leaf badly damaged by sun or cold, the AOS says removing it is fine.