Where to Keep a Dendrobium: Bright Light, Summer Outside, Cold Winter Nights

These are high-light orchids

Dendrobiums grow perched on branches and rock faces in open, exposed positions, not deep under a forest canopy, and they want far more light than the moth orchids most people compare them to. Indoors, that means the brightest window in the house, right against the glass, with direct sun in the early or late part of the day. A dendrobium set back on a table a few feet from a window will make thin canes and never bloom.

The leaves score the light for you. A well-lit plant has stiff foliage in a light yellow-green, sometimes flushed slightly bronze or red on the sun side, and canes that stand up on their own. A plant in shade has soft dark green leaves and floppy canes that need staking. Dark green is not a sign of health here. It is the most common reason a plant that looks fine refuses to flower.

Too much light is possible and shows differently. Bleached cream or white patches with a dry papery feel, appearing over one or two days, are sunburn and do not heal. Burn happens when a plant grown indoors is put into direct sun without acclimatizing, almost never to a plant that has been out all season.

Summer outdoors: where the canes are built

Once nights hold reliably above 55F (13C), both types benefit enormously from a summer outside. Outdoor light in open shade is several times brighter than the best indoor windowsill, and moving air keeps the canes dry and the mix sweet. A plant that summers outdoors builds fat canes with plenty of nodes, and a plant that summers on a windowsill builds thin ones that flower sparsely if at all.

Choose a spot with morning sun and shade through the hottest part of the afternoon, or the dappled shade of a tree, or shade cloth cutting roughly a third of the light. Harden the plant off over two to three weeks: full shade first, then two hours of morning sun, then its summer position. Leaves grown indoors scorch in a single unshaded afternoon.

Stand the pot on feet or a slab rather than directly on soil, so slugs cannot enter through the drainage holes. Dendrobiums are tall and light, and a small pot under a three foot (90 cm) cane blows over in the first summer storm, snapping canes at the base. Drop the pot into a heavier outer container, or tie the plant to a stake driven into the ground.

Do not pot up for stability. A larger pot costs you flowers. Weight the outside of the pot instead, or stand it inside a heavy ceramic cover pot with the drainage kept clear.

Autumn: the point where the two types separate

For deciduous dendrobiums, the chilly nights of autumn are the whole point of the year. Leave the plant outside as the evenings turn cold, with nights in the 45F to 55F (7C to 13C) range against warmer days, until frost is genuinely forecast. That cold, combined with the dry rest described in the watering guide, is what turns dormant nodes into flower buds. Bringing the plant in at the first cool evening feels like kindness and costs the whole season.

For evergreen dendrobiums the instruction is the opposite. They come in before the nights drop below about 60F (15C), because a prolonged chill makes them drop leaves, stall and sometimes rot at the base. If the two are standing side by side outdoors, they need separating in early autumn, and this is the one moment in the year where knowing which is which is urgent.

Once indoors, the deciduous plant wants the coldest bright frost-free place you have: an unheated spare room, a porch, a stairwell window, a cool conservatory. The evergreen plant wants the warmest bright place, above 60F (15C) at night, away from cold glass. They should not share a windowsill in winter.

Air, humidity and drafts

Moving air matters more to dendrobiums than humidity does. These are plants used to breezes over an exposed branch, and still stagnant air is what allows rot to start in a wet leaf axil and mold to settle on flowers. A gently moving room, an open window on mild days, or a small fan on the other side of the room all help. What they do not want is a cold draft blowing directly across them from a door or a vent.

Humidity of 50 to 70 percent suits them, which is more than a heated winter room provides. Standing the pot on a tray of damp gravel, with the pot base clear of the water, lifts the air around the plant enough to matter. Do not mist a plant in flower, and do not mist a deciduous plant during its dry rest, because the point of the rest is dryness at the roots and on the canes alike.

Keep every dendrobium away from radiators, heating vents and wood burners. Dry rising heat makes buds yellow at the stalk and drop before opening. Ripening fruit in the same room does the same thing, because the ethylene it gives off makes orchid buds abort. Once a plant has buds, settle it in its final position and leave it alone.

Fixing a plant kept warm indoors for years

A deciduous dendrobium that has lived on a warm windowsill and been watered every week is usually tall, dark green, floppy, hung with plantlets, and completely flowerless. It is not diseased and it does not need feeding harder. It needs one full outdoor summer followed by a real cold dry winter, and it will not respond to anything else.

Expect the correction to take a full cycle. The canes made in shade cannot be improved, but the new ones will come up thicker and better lit, and it is those, plus the older bare ones, that will flower after the following winter's rest. Give it two seasons before concluding anything is wrong with the plant itself.

Write down where the plant stood each season, the date it went out and came in, and the lowest night temperature it actually experienced. When a plant fails to flower, that record separates the three possible causes at once: too little summer light, too little autumn cold, or a rest that was broken by watering. Without it you are guessing between three fixes that look identical from the outside.