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Oncidiums keep a cycle rather than a calendar, and different plants run it at different times of year. The stages always come in the same order, though: a new growth appears at the base of the last bulb, expands into leaves, fattens into a pseudobulb, hardens off, sends a spike from the base of that finished bulb, flowers, and then rests briefly before starting again. One full lap normally takes nine to twelve months.
Work out which stage your plant is in and everything else follows. A plant in expansion wants the most water, the most feed and the most light. A plant with a hardened bulb and no spike yet wants steady conditions and less nitrogen. A plant in bud wants to be left alone. A plant that has just finished flowering is the one to repot.
Ambient season still matters, because it controls light and warmth. Wherever you are, the bright warm half of the year is when the plant should be building bulbs and the dim cool half is when it slows down. If your plant's cycle happens to put bulb-building in the darkest part of the year, supplement with a grow light or accept that the bulb will be smaller.
| Stage | How long | Water | Feed | Main job |
|---|---|---|---|---|
| New growth appears | 2 to 4 weeks | Normal, dry between | Quarter strength, balanced | Repot now if it is due |
| Leaves expanding | 6 to 10 weeks | Most of the year's water | Every watering, weak | Maximum light without scorch |
| Bulb filling out | 4 to 8 weeks | Full soaks, still dry between | Weak, lower nitrogen | Do not disturb the pot |
| Bulb hardened | 2 to 6 weeks | Slightly less often | Reduce | Watch the sheath for a spike |
| Spike extending | 6 to 10 weeks | Normal | Light feeding | Stake early, stop rotating the pot |
| In flower | 4 to 8 weeks | Normal, never bone dry | Optional, weak | Cool, bright, stable, no moving |
| After flowering | 2 to 6 weeks | Reduce a little | Resume as growth starts | Cut the spike out at the base |
The bright warm half of the year
This is when the plant earns its flowers. Water thoroughly and often, letting the mix approach dry between soaks, and expect that to mean every four or five days in warm weather rather than the weekly rhythm of winter. Feed at quarter strength at most waterings and flush the pot with plain water every fourth or fifth time to keep salts from building up in the bark.
Give it the brightest position it will take without scorching, adjusting over two weeks rather than in one move. If you can put it outdoors once nights hold above 55F (13C), do it, in morning sun and afternoon shade. A summer outside builds visibly fatter pseudobulbs than any windowsill, and fat bulbs are the whole game.
Keep an eye out for pests weekly during this stretch, since warmth speeds up scale and mite populations dramatically. Check the leaf undersides and under the sheaths, not the tops of the leaves. This is also the season to leave the plant alone otherwise: no repotting, no dividing, no moving a growth that is halfway through expanding.
The cool dim half of the year
As light drops and the room cools, growth slows and water use falls sharply. Stretch the intervals, and check the pot weight before every watering rather than sticking to a rhythm that worked in summer. A mix that stays wet through a cool dim spell is the most common cause of winter root loss in this genus, and the loss does not show up until spring.
Move the plant closer to the glass and take down any summer shading, because indoor winter light is far weaker than it looks and will not burn oncidium foliage in any climate where the plant is indoors. Keep the leaves off cold panes overnight and out of the direct blast of heating vents, which cook the leaf tips and invite spider mites.
Ease off the feed while growth is slow, and stop it altogether if the plant is doing nothing at all. Many oncidiums flower during this half of the year, so if a spike is up, hold the plant somewhere cool and bright, stake it early, and stop turning the pot. Cut a finished spike off at the base as soon as the last flower falls.
What to write down for next year
This plant runs slowly enough that memory is useless and a few notes are worth more than any general rule. The cycle takes most of a year, so a mistake made while a bulb was filling only shows up as a missing spike months later. Notes are how you connect the two.
Record the date each new growth appeared and the date its bulb stopped enlarging, so you know how long your plant's cycle actually runs and when to expect a spike. Record where the plant stood in each season and whether the leaves ran yellow-green or dark green there, because that is your honest measure of light. Record the date of each repot, what the roots looked like, and how many pseudobulbs each division kept.
Then record the outcomes against those notes: the date the spike appeared and which bulb it came from, the date the first flower opened, how long the display lasted, and the size of the finished bulb compared with the year before. Add the date any pest or rot first appeared. Over two cycles that record will tell you exactly which change made the difference, and it will settle the usual argument about whether a shy plant needs more light or more time.
The single most useful entry is a note of how fat the newest pseudobulb was when it hardened, alongside where the plant spent that season. Compare two years of that and the light question answers itself.