Gadenin日本語Get the appWhy this genus needs it more often than the others
A cattleya sits in coarse bark for three years and comes to no harm. A paphiopedilum cannot, for two connected reasons. First, it grows in a finer, more moisture-retentive mix, and fine material decomposes faster; within about eighteen months it has turned to a dense, sour, airless mush that suffocates roots. Second, because the mix is kept constantly damp rather than being dried out between waterings, that breakdown happens faster still.
The plant also has no pseudobulbs to live off while it recovers, so root damage translates immediately into a smaller new fan and a lost flowering. Repotting on time is therefore not maintenance you can put off; it is the single most important routine job with this plant.
Plan on every twelve to eighteen months as a default, and shorten that if the mix looks dark, feels soft when you squeeze a piece, or smells sour. Do not wait for the plant to complain, because by the time leaves show it, a season has already been lost.
| Signal | What it means | Action |
|---|---|---|
| Bark crumbles between finger and thumb | The mix has broken down and is holding no air | Repot now, whatever the season |
| Pot stays wet far longer than it used to | Fine particles have filled the air spaces | Repot into fresh mix, same or one size larger |
| Sour or musty smell from the pot | Anaerobic decay around the roots | Repot immediately and inspect every root |
| New fan smaller than the previous one | Root loss, usually from old mix | Tip the plant out and look before doing anything else |
| Twelve to eighteen months since the last repot | Routine renewal is due | Repot after the flower finishes |
When in the year to do it
The best moment is just after flowering, when the plant is starting its next fan and will put out new roots quickly. That is the window to aim for whenever you have the choice, because roots produced immediately after a repot establish in the fresh mix and the plant loses nothing.
Unlike orchids with a dormant season, a paphiopedilum has no period when it must not be disturbed, so a plant in genuinely bad mix can be repotted at any time of year. Rotten mix is a bigger threat than an inconvenient date.
The one time to hold off is while a spike is extending or a bud is forming. Disturbing the roots then commonly causes the bud to yellow and drop. Wait for the flower to finish, then repot straight away.
The mix and the pot
Use a fine to medium grade orchid bark as the base, roughly quarter-inch (6 mm) pieces, with perlite for air and a proportion of something water-holding such as chopped sphagnum moss or coarse peat-free compost. The result should feel damp for days after watering but never turn to mud, and water should drain freely rather than sit on top. A common working blend is around four parts fine bark, one part perlite and one part chopped moss.
Many growers add crushed oyster shell, dolomite lime or limestone chips at about a tablespoon per pot, because a good number of slipper orchids grow on limestone in the wild and appreciate the calcium and the slightly less acid mix. It is not essential, but it helps.
Pot size is where people go wrong. Choose the smallest pot that holds the root ball with about an inch (2.5 cm) of space around it, which is roughly one season of growth. An oversized pot holds a mass of unrooted, permanently wet mix that rots the plant. Plastic pots suit a plant that must stay damp better than clay, which dries too fast. Drainage holes must be generous.
Never bury the crown. The base of each fan, where the leaves meet, must sit at the surface of the mix, not below it. A crown buried even half an inch (1 cm) deep sits wet permanently and rots, and this is one of the commonest ways a repot kills an otherwise healthy plant.
Doing it
Work with sterilized tools and clean hands; slipper orchid virus and bacterial rot both travel on blades. Sterilize the blade between every plant, by flaming it or with a fresh disposable razor. Have the mix and the pot ready before you start, so the roots are out of the pot for as little time as possible.
- Soften and slide the plant out Water the plant lightly a few hours beforehand so the roots are pliable rather than brittle. Squeeze a plastic pot all round, or run a clean knife between the mix and the pot wall, and ease the plant out by the base rather than pulling on the leaves.
- Clear the old mix away Tease out as much of the decomposed material as you reasonably can with your fingers, working from the outside in. Soaking the root ball in a bowl of tepid water for ten minutes makes stubborn old mix release without tearing roots.
- Inspect and trim the roots Good roots are firm, pale tan and hairy to the touch. Dead ones are brown, hollow and slide apart between your fingers leaving a wiry core. Cut every dead root back to firm tissue with a sterilized blade, and remove any fan whose leaves have gone yellow and dry.
- Set the plant in the new pot Hold the plant so the crowns sit level with the eventual mix surface and the oldest fan is against one side of the pot, leaving room on the side where new growths are heading. Trickle the mix in around the roots, tapping the pot to settle it rather than pressing hard.
- Firm it just enough to stand The plant must not rock, because a wobbling plant snaps its new root tips. Press the surface gently with your thumbs, add a stake and a loose tie if it is still unstable, and check that the crowns are at the surface and not buried.
- Water and settle it back in Water thoroughly at the pot edge to settle the mix, drain fully, and return the plant to its usual position out of direct sun. Keep it evenly damp as normal from that point; do not withhold water afterwards, because this plant has no reserves to draw on.
Afterwards, and what to log
Do not feed for two to three weeks after a repot, to let cut roots seal, then resume the usual quarter-strength routine. Expect the plant to look untouched for a month or so; a paphiopedilum does not sulk visibly the way some orchids do, and the sign that it has taken is a new fan or new root tips appearing.
Write down the date, the pot size, and what the roots looked like when they came out. That last detail is the useful one. If the roots were poor, the note tells you next time to shorten the interval and open the mix up; if they filled the pot with firm tan roots, you know your current mix and watering are right and you can leave the recipe alone.