Gadenin日本語Get the appSoak thoroughly, then let the mix approach dry
The rhythm for this genus is simple and unforgiving at both ends: water heavily, then wait until the mix is nearly dry before watering again. In the wild these plants sit on branches where a downpour drenches the roots and a breeze dries them within hours. Roots kept permanently damp in a pot suffocate and rot; roots never soaked properly shrivel from the tips back.
Water at the sink, not with a small cup. Run tepid water through the pot for a good fifteen to thirty seconds so it floods every gap in the bark, let it drain fully, and then run it through once more. A pot watered a splash at a time develops dry pockets in the middle where roots die while the surface looks damp. Empty any saucer within a few minutes; a pot standing in water is the single fastest way to kill an oncidium.
Timing depends entirely on your pot, mix and conditions, not on a calendar. In a four inch (10 cm) pot of medium bark in a warm bright room, that usually works out at every five to seven days in the growing season and every ten to fourteen days in cool, dim winter weather. Treat those figures as a starting point and let the checks below correct them.
Three ways to check before you water
Do not water on a schedule. Check first, every time, using whichever of these you find easiest. All three are asking the same question: is there still water deep in the pot?
- Lift the pot Learn the weight of the pot immediately after a soak and again when it is dry. The difference is obvious once you have felt it twice. This is the most reliable check and takes a second, which matters because it is the one you will actually keep doing.
- Push a wooden skewer into the mix Slide a bamboo skewer down the side of the pot to the bottom, leave it a minute, and pull it out. Dark and cool means wait; dry and pale means water. Keep the same skewer in the pot and simply pull it out to read it.
- Look at the roots through the pot In a clear pot, wet oncidium roots are green or greenish white and plump. As they dry they turn silvery. Silver roots and no condensation on the inside of the pot mean it is time. Green roots and beaded condensation mean wait.
If you are unsure, wait a day. An oncidium recovers from being a day late far more easily than from being a day early, because the damage from overwatering is done to the roots and is not visible until long after the fact.
What the pseudobulbs are telling you, and when
The pseudobulbs, the flattened swollen stems at the base of each fan of leaves, are the plant's water tank. A bulb on the current growth should fatten steadily through the season until it is firm and plump, and that plumping is the precondition for flowering. A bulb that stays thin or turns wrinkled and ridged is reporting a shortfall of water reaching the leaves.
The catch is the delay. Bulbs shrivel slowly, over weeks, so a wrinkled bulb tells you about a problem that began well before you noticed it. It is not a signal to pour on more water today. It is a signal to find out why water is not getting into the plant, which is usually one of three things: the mix has broken down into airless mush and the roots have rotted, the mix is so old and hydrophobic that water runs straight through without wetting anything, or watering has genuinely been too sparse.
Sort out which by unpotting. Firm white or tan roots with green growing tips mean the roots are fine and you were simply underwatering, so water more often. Brown, hollow, slimy roots that pull apart between your fingers mean the mix has failed and the plant needs repotting into fresh bark, with the dead roots cut away. Adding more water to a rotted root system finishes the plant off.
| What you see | What it means | What to do |
|---|---|---|
| Bulbs plump, roots green and firm | Watering rhythm is right | Carry on |
| Bulbs wrinkled, roots brown and soft | Roots rotted, mix broken down | Repot into fresh bark, trim dead roots |
| Bulbs wrinkled, roots white and firm | Genuinely too dry | Water more often, soak longer |
| Water runs straight through in seconds | Mix has gone hydrophobic | Soak the whole pot for 15 minutes, plan to repot |
| New leaves pleated and accordion-like | Water shortage while that leaf formed | Fix the roots; the leaf stays pleated |
| Yellow lower leaf on an old bulb | Normal ageing | Leave it, remove when fully dry |
Season, water quality and feeding
Water most heavily while a new growth is expanding and its bulb is filling out, which is the plant's real working season. Once the bulb has hardened off and stopped enlarging, stretch the intervals noticeably. Through cool, low-light weather the plant uses very little and a mix that stays wet at that time of year is the classic route to winter root rot.
Oncidiums are more sensitive than most orchids to salts and to fluoride, and the symptom is blackened, dead leaf tips that creep back down the leaf. If your tap water is hard or heavily treated, use rainwater, distilled water or reverse osmosis water, and in any case flush the pot with plain water every fourth or fifth watering to wash accumulated fertilizer salts out of the bark.
Feed at quarter strength with a balanced orchid fertilizer at most waterings during active growth, then flush plain. Weak and frequent beats strong and occasional in an open bark mix, where a strong dose sits on the roots as a crust rather than being buffered by soil. Stop feeding when growth stops, and never feed a bone-dry pot; wet the mix first, then feed.