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Dendrobium is an enormous genus, and the plants sold under that single label come from climates as far apart as the seasonally dry Himalayan foothills and the wet lowland tropics. The result is that two orchids on the same shelf, with the same label, want opposite treatment for half of every year. Care advice that does not begin by asking which kind you own is worthless, because the instruction that makes one type flower is the instruction that kills the other.
The split that matters is deciduous against evergreen. The deciduous kinds build a tall jointed cane through the growing season, then shed every leaf in autumn and stand through winter as a bare, ribbed, silvery stick. That looks like a dying plant and it is not. Those bare canes need a hard cold dry rest, with no water at all for weeks, and only then do flower buds break directly out of the nodes along the cane in spring. The evergreen kinds keep their leaves year round, never take a dry rest, and are simply kept warm, bright and lightly watered through winter.
Everything else in this guide is written twice for that reason. Light, potting, feeding and pest control are broadly shared. Winter water is not, and getting it wrong is the single most common reason a dendrobium that looks perfectly healthy produces no flowers at all.
Telling which type you have
You do not need a label. The plant tells you in autumn and again when it flowers, and once you have watched a single full cycle the question never comes up again. Work through the table below with the plant in front of you, and give most weight to what happens to the leaves as the nights turn cold and to where the flowers come out of the cane.
| What to look at | Deciduous type | Evergreen type |
|---|---|---|
| Leaves in late autumn | Yellow and fall from the older canes, leaving bare sticks | Stay green on canes several years old |
| The cane itself | Slim, jointed like bamboo, becomes ribbed and silvery when leafless | Thicker and harder, often club shaped near the top, stays green |
| Where flowers appear | Buds break from nodes all along the length of a bare cane | A spike rises from near the top of a leafy cane |
| Flowering season | Spring, after a cold dry winter | Any time, often more than once a year |
| Winter cold it wants | Nights of 45F to 55F (7C to 13C), tolerates colder | Nights above 60F (15C), sulks below 55F (13C) |
| Winter water | None at all for six to ten weeks | Reduced but never stopped |
If a plant kept warm and watered all winter still drops its leaves, it is deciduous and it has been given the wrong winter. Leaf fall is the plant asking for the rest, not a symptom of anything you did to the roots.
What both types want during the growing season
From the point in spring when a new shoot appears at the base of a cane until that shoot finishes its growth in late summer, the two types are managed identically and both are hungry, thirsty and greedy for light. This is the only window in which the plant builds anything, because next year's flowers come out of canes made this year, and a thin cane carries few flowers no matter how well you treat it later.
Give the strongest light the plant will take without burning, water heavily while the mix is draining freely, and feed a balanced orchid fertilizer at roughly quarter strength at every second watering. New canes rise fast, and a well-fed one can add several inches a month at the peak. Support it early with a cane and a soft tie if it leans, because a tall growth that flops over kinks at the base and never straightens.
The new growth finishes when a small leaf appears at the top and the cane stops extending, usually in late summer. That moment is the handover, and it is when the two types part company. Cut the nitrogen back and switch to a low-nitrogen, high-potassium feed for the last few weeks so the cane hardens instead of pushing more soft leaf. From here, a deciduous plant is heading for its dry rest and an evergreen one simply carries on more slowly.
Never cut the old bare canes
The most damaging thing done to dendrobiums is tidying. A leafless cane looks finished, so people cut it off, and in doing so they remove exactly the wood that was about to flower. On the deciduous types, buds break from the dormant nodes along the length of canes that are one, two and sometimes three years old, and the bare silvery ones are usually the best flowering wood on the plant.
Old canes serve a second purpose even when they have finished flowering. They are the plant's water and starch reserve, which is why they are thick and firm rather than hollow. A plant stripped of its old canes has nothing to draw on when it pushes a new growth in spring, and the growth comes up thin. Leave them. A well-grown dendrobium looks like a crowded bundle of sticks, and that is the correct appearance.
The only cane worth removing is one that is genuinely dead: brown or black through, soft, hollow, or shriveled to a papery thread that snaps rather than bends. Cut those at the base with a sterile blade. A cane that is merely leafless, wrinkled or unattractive stays on the plant.
After flowering, cut off only the short flower stalks where they meet the cane. Leave the cane itself. Many nodes further up the same cane can still flower the following spring, and cutting the cane back removes them.
Pot, mix and the things that go wrong
Dendrobiums want a small pot and a coarse open mix, and both points are counterintuitive when the plant is tall. A three foot (90 cm) cane in a four inch (10 cm) pot looks absurdly top heavy and is exactly right. The roots are few and thick, they resent being surrounded by wet material, and a plant given a generous pot spends two years filling it instead of flowering. Stand the pot inside a heavier outer container for stability rather than potting up.
The mix should be coarse bark with grit or perlite through it, chunky enough that you can see daylight between the pieces. Ordinary potting mix holds far too much water around thick roots, breaks down within a season, and produces the sour black sludge that rots canes from the base. If the mix in a pot has gone dark and fine and no longer drains, that is a repotting reason on its own regardless of how crowded the plant looks.
Most failures with this genus come down to three mistakes. Watering a deciduous plant through winter, which produces plantlets along the cane instead of flowers. Cutting the old canes, which removes the flowering wood. And potting too large in too fine a mix, which rots the roots and stalls the plant for years. Avoid those three and the plant is undemanding.
What to write down
A dendrobium year is too long to hold in your head, and the cause of a spring failure lies six months back. Keep four notes per plant and the guesswork disappears. Record the date the first new growth appeared at the base, the date that growth stopped extending and made its top leaf, the date you stopped watering in autumn if the plant is deciduous, and the date the first bud broke on a cane.
Add one line about which cane flowered and how old it was. After two seasons that tells you whether your plant flowers on last year's canes, on two year old wood, or on both, and that single fact settles every future argument about what to cut and what to leave.
Note the lowest night temperature the plant actually sat through in winter, not the temperature you intended. Deciduous types that spend winter in a room that never drops below 65F (18C) make leaves and plantlets rather than flowers, and a written temperature is the fastest way to see that the rest was too warm rather than too short.