Why are my orchid leaves turning yellow?

Read the roots before you do anything

What makes orchid yellowing different from most houseplants is that Phalaenopsis are epiphytes. In the wild they cling to tree bark, never to soil, so their thick green roots are built to grab moisture in a downpour and then dry out fully within hours. That biology is your best diagnostic tool, because almost every cause of yellow leaves shows up in the roots first, and most orchids are sold in clear pots so you can see them.

Slide the inner pot out and look. Healthy roots are firm and plump, silvery-grey when dry and turning bright green within a minute of getting wet. Rotting roots are brown or black, soft, and so hollow they collapse to a thread when pinched. Dehydrated roots look grey, flat and wrinkled even after watering. The root condition, more than the leaf, tells you which fix you need.

Overwatering and root rot: the top cause

Overwatering is the most common reason a Phalaenopsis yellows. Sitting in soggy bark starves the roots of oxygen, they rot, and the plant can no longer pull up water, so it sacrifices its lower leaves first. The leaves turn soft and yellow rather than crisp, often several at once, and the crown stays damp. This usually traces back to one of three habits: watering on a fixed schedule regardless of the bark, a pot with no drainage, or decorative bark that has broken down into a dense, water-holding mush after a year or two. The Missouri Botanical Garden notes that overwatering symptoms often mimic underwatering because rotted roots cannot move water either way, which is exactly why checking the roots beats guessing from the leaf.

Other causes

These rank below overwatering but can overlap with it.

• Natural aging. A single oldest bottom leaf slowly yellows while the rest of the plant looks healthy and the roots are firm. A Phalaenopsis typically carries five or six leaves and drops the lowest one once or twice a year as it pushes new growth from the crown. This is normal and needs no action.
• Too much direct sun. A south or west window in summer can scorch the foliage; look for yellow or bleached patches, sometimes with dry tan sunburn spots, on the side facing the glass. Leaves should be a mid olive-green, not dark green (too little light) and not yellow-green (too much).
• Underwatering. Months of going too dry shrivels the roots; they look grey and flat, the leaves go limp and leathery, and the bark is bone dry well past a normal watering gap.
• Cold draught or shock. A sudden chill below about 55F (13C) near a single-glazed window or an air-conditioning vent can yellow leaves; the change follows a cold snap or a recent move.

A worked example

Say two lower leaves on your orchid have gone soft and yellow over a week, while the top leaves still look fine. You water every Sunday no matter what. You unpot it and the bark is still wet from six days ago, and several roots are brown and squashy. That combination, soft yellow lower leaves plus wet bark plus mushy roots, points to overwatering, not aging or sun. Aging would be one leaf, not two at once with rotted roots. The fix is to repot into fresh dry bark, cut the dead roots, and water by root colour from now on rather than by the calendar.

How to fix it

1. Inspect the roots. Slip the plant from its clear pot and check for firm, green roots; brown, soft, hollow roots confirm rot.
2. Trim and repot. Cut away mushy roots with a blade wiped in rubbing alcohol and repot into fresh, dry orchid bark in a pot with drainage holes.
3. Water correctly. Water only when the roots turn silvery and the bark is nearly dry, usually every 7 to 10 days, then let it drain fully.
4. Empty the saucer. Never let the pot stand in collected water, which keeps the roots wet and rotting.
5. Fix the light. Move it to bright indirect light, such as an east window or a few feet back from a brighter one, out of direct midday sun.
6. Shelter from draughts. Keep it away from cold windows, doors and air vents, ideally between 64 and 80F (18 to 27C).
7. Leave aging leaves alone. Let a naturally yellowing lower leaf detach on its own instead of cutting it.