Repotting and Dividing an Oncidium: Fresh Bark and Clumps of Three Bulbs

When to do it, and when not to

Repot for one of three reasons: the mix has broken down into dark crumbly material that stays wet, the plant has grown over the pot edge and the newest bulbs have nowhere to sit, or the roots have rotted and the plant must be rescued. Bark decays in about two years in a warm home, and that decay, rather than the size of the plant, is usually what makes the decision.

The moment to do it is just as new roots begin to push from the base of a new growth, which normally falls right after flowering finishes. Roots caught at that stage grip the fresh mix within weeks and the plant barely checks. Roots that are already long and established get broken instead, and the plant spends months rebuilding.

Do not repot a plant in spike or in flower unless the roots are actively rotting. You will lose the flowers and often the current growth as well. Do not repot in the middle of a growth's expansion either. If the mix is failing at an awkward moment, water carefully and sparingly and wait for the right window rather than tearing the roots apart mid-build.

Pot and mix

Go up one size only. Oncidiums flower best in a pot that looks slightly too small, and a large pot holds a volume of wet mix the fine roots cannot use, which sours. For most plants that means moving from a four inch (10 cm) pot to a five or six inch (13 to 15 cm) one, and choosing a pot roughly deep enough to give two years of sideways room in front of the newest growth.

Use a coarse, free-draining orchid mix, not potting mix. Medium-grade fir bark with perlite and a little charcoal is the standard and suits the wet-then-dry rhythm these plants want. Fine bark or sphagnum holds water far too long for an oncidium in a normal pot and is the usual hidden cause of a plant that never seems to have healthy roots.

Clear plastic pots let you watch the roots and read moisture at a glance, which is genuinely useful with this genus. Clay dries faster and suits a heavy hand with the watering can. Whatever you choose, it needs generous drainage holes, and slotted orchid pots are better still. Soak new bark for a few hours before use, because dry bark repels water and leaves the fresh mix impossible to wet evenly.

The repotting itself, step by step

Work over newspaper with clean, sharp snips sterilized in flame or alcohol between plants. Have the new pot, soaked mix and a stake ready before you start, because bare oncidium roots dry quickly.

1. Water the day before and slide it out A moist root ball comes out in one piece and damp roots are far less brittle. Squeeze a plastic pot to release it, or run a knife around the inside of a rigid one. Never pull the plant out by its leaves.
2. Strip all the old mix away Tease and shake every bit of decayed bark from between the roots. Soaking the root ball in tepid water for ten minutes makes this easy. Leaving old spent mix in the middle is the most common reason a repot fails to help.
3. Cut out the dead roots Firm, white or tan roots stay. Brown, hollow, slimy or papery roots come off at the base. Do not be shy here: dead roots hold water and rot the live ones next to them. A healthy plant may still lose half its roots at this point.
4. Position the oldest bulbs against the rim Set the plant so the oldest bulb sits against one side of the pot and the newest growth faces the open space, giving it two years of room to walk forwards. The base of the bulbs sits level with the mix surface, never buried.
5. Fill and settle without ramming Trickle bark in around the roots, tapping the pot on the bench so it works into the gaps. Press lightly with your fingers only. Packed mix suffocates the roots. The plant should be firm enough not to rock, so stake it if it wobbles.
6. Wait, then resume watering Keep it in shade and hold off watering for three to five days so cut roots callus over, misting the foliage lightly if the room is dry. Then water normally. Start feeding again only once new root tips appear.

Dividing: three bulbs minimum, never singles

Repotting is the natural moment to divide, and it is easy to overdo. A division needs at least three pseudobulbs to have the reserves to build a strong new growth. Split into three-bulb or four-bulb clumps and each piece will usually flower within a year. Split into singles and each one spends two or three years slowly rebuilding, and many simply sit and shrivel.

Find the natural gap first. The rhizome, the horizontal woody stem linking the bulbs, has thinner points between clusters and the plant often shows you where it wants to break. Cut cleanly through the rhizome with sterilized snips, keeping roots attached to both sides, and dust the cut surfaces with cinnamon or leave them to dry an hour before potting.

Old leafless bulbs behind the division, sometimes called back bulbs, are still worth keeping if they are firm. Three of them potted together in a small pot of bark, kept shaded and barely moist, will often push a new growth from a dormant eye within a season. Soft or hollow back bulbs are just rot waiting to spread and belong in the bin.

Write down the date of the repot, what the roots looked like, and how many bulbs each division kept. When one piece blooms the next year and another does not, that note tells you whether the difference was the split size or the light.