Watering a Dendrobium: The Winter Where You Give None at All

Two watering years in one genus

In summer, every dendrobium is watered the same way: heavily, whenever the coarse mix has dried, which in warm weather can mean every two or three days. The roots are thick, the mix drains almost instantly, and a growing plant in strong light drinks far more than people expect from an orchid. Nobody gets into trouble in summer.

Winter is where the genus divides. A deciduous dendrobium that has dropped its leaves is asking to be kept completely dry and cold for the rest of the winter, and completely dry means no water at all, not a light drink every few weeks. An evergreen dendrobium never takes that rest and is watered through winter at a reduced rate, roughly every ten to fourteen days, in a warm bright room.

Reading which type you have is therefore a watering decision, not a botanical curiosity. Leaves falling in autumn is the signal. When the leaves go yellow and drop from a jointed, ribbed cane as the nights cool, the watering can goes away until buds appear. When the leaves stay green through a cold spell, they are meant to, and the plant carries on being watered.

The dry rest: how and how long

The rest begins once the new cane has finished extending and made its small top leaf, and the nights have started dropping. Reduce watering gradually over two or three weeks rather than stopping in one step, then stop altogether. Keep the plant in the brightest, coolest frost-free place you have, with nights of 45F to 55F (7C to 13C), and simply do not water it for six to ten weeks.

1. Taper as the cane finishes and the leaves yellow Stretch the interval from every few days to weekly, then to fortnightly, over two or three weeks. An abrupt stop while the cane is still filling can leave it thin, so let the plant finish first.
2. Stop completely once the leaves have fallen From this point give no water and no feed. Bare canes lose almost nothing, and the plant is living on the reserves stored in them. This is the part that feels wrong and is the part that works.
3. Keep it cold as well as dry Dry alone is not the signal. Nights of 45F to 55F (7C to 13C) with the plant in full light are what turn dormant nodes into flower buds. A dry plant in a warm room simply shrivels without setting anything.
4. Watch the canes for wrinkling and hold your nerve Light lengthwise wrinkling on the canes over the rest period is normal and expected. Deep shriveling that makes a cane feel soft and empty means the rest has gone on too long or the room is too warm, and that is the point to give one light watering.
5. Resume when buds break, not before Small swellings appear at the nodes along the bare canes in late winter or early spring. Once you can see them, start watering lightly, then normally as the buds extend. Watering before they show is what costs you the flowers.

Six to ten weeks is the usual range, but the honest end point is the buds rather than a count of weeks. A plant that has taken enough cold and enough dryness will show swelling nodes on its own, and nothing you do earlier will hurry it.

Why watering during the rest makes plantlets instead of flowers

Each node along a dendrobium cane holds a dormant bud that can become either a flower or a small plantlet, a complete miniature plant with its own leaves and roots. Which one it becomes is decided by the conditions the plant experiences over winter. Cold and dry sends the node down the flowering route. Warm and wet, especially with any nitrogen in the water, sends it down the vegetative route instead.

So a plant watered through winter does not fail silently. It produces a crop of little leafy plantlets up and down the canes, and the owner concludes the orchid has decided to reproduce rather than bloom. That is exactly what happened, and it was the watering can that decided it. The same is true of feeding: any fertilizer given during the rest period pushes nodes towards plantlets.

The plantlets themselves are not a disaster and can be grown on into new plants once they have made roots an inch or two (3 to 5 cm) long. But a cane that has spent its stored reserves on plantlets will not then flower from those nodes, so the season is gone. If you find them forming, the correction is not to remove them but to fix next winter.

Judging when to water in the growing season

From the moment buds break in spring until the new cane finishes in late summer, water generously and let the pot drain hard between waterings. The interval depends on heat, pot size and how root bound the plant is rather than on any calendar, so judge it by weight and by pushing a finger into the mix. Cool and damp means wait. Dry and light means water now.

Water in the morning, pour over the whole surface until it runs freely from the base, wait a minute and do it again. Coarse bark repels water on first contact and a single quick pass leaves the center of a root bound pot dry. Never leave the pot standing in a saucer of water; these are epiphytic roots and they need air within minutes of being wet.

The canes are the long-term gauge. Plump, firm canes mean the plant is getting enough. Canes that wrinkle during the growing season mean either too long an interval or dead roots that cannot take water up, and wrinkling on a pot that is always wet is a root problem, not thirst. Tip the plant out and look rather than watering more.

Water quality, feeding and salts

Rainwater is ideal and tap water is fine in most areas. Water passed through a domestic ion-exchange softener is the one to avoid, because it replaces calcium with sodium and the sodium accumulates in the bark around the roots. Where the tap water is very hard, expect a white crust on the pot rim and flush more often.

Feeding rides on the watering. Use a balanced orchid feed at about a quarter of the label strength at every second watering while a new cane is growing, then switch to a low-nitrogen, high-potassium formula from late summer to harden the cane. Stop feeding entirely once the plant enters its rest, and for evergreen types feed only very lightly through winter.

Because the feed is frequent, flush the pot with plain water every fourth or fifth watering, running two or three pot volumes through until it pours out of the base. Fertilizer salts otherwise build up in the mix and burn the fine root tips, showing as blackened root ends and brown dry leaf tips. Keep a rough note of how often you watered in each season and what the weather was doing, and the following year the interval stops being guesswork.