Gadenin日本語Get the appThe roots need air, so the pot is full of bark, not compost
A phalaenopsis is an epiphyte. In the wild its roots are clamped to the bark of a tree, exposed to the air, wetted by rain and then dried by the breeze within a few hours. Those roots are built for that cycle: the spongy velamen coat absorbs a burst of water quickly, and the living tissue inside takes oxygen directly from the air around it. A moth orchid root that cannot breathe suffocates and rots, even in perfectly clean water.
Ordinary potting mix is designed to do the opposite. It holds water against roots for days, closes up around them and excludes air, and it collapses further every time you water it. Pot a moth orchid in all-purpose potting mix and it will look fine for two months and then lose most of its root system to rot, and by the time the leaves go limp there is usually nothing left underground to save. This is not a matter of degree or of watering carefully. Compost is simply the wrong material for this plant.
What goes in the pot instead is a coarse, chunky, open medium: graded orchid bark, on its own or with perlite, charcoal or coarse pumice mixed in. When you pour water in it runs straight through, and when you look into the pot you can see gaps between the pieces. Those gaps are the point. They are where the air is.
| Medium | How it behaves | Suitable for |
|---|---|---|
| Medium grade bark, 0.4 to 0.6 in (10 to 15 mm) | Open, dries in about a week, lasts 2 years | The standard choice for a mature plant |
| Fine grade bark, 0.2 to 0.4 in (5 to 10 mm) | Holds more water, breaks down faster | Seedlings, keikis, very dry rooms |
| Bark with 20 percent perlite or pumice | Slightly more moisture, still very open | Warm rooms where bark alone dries too fast |
| Sphagnum moss | Holds a lot of water, easy to overwater | Reviving a plant that has lost most roots |
| Ordinary potting mix | Waterlogs, excludes air, collapses | Nothing. Never use it for this plant |
| Old bark that has gone soft and dark | Behaves like compost, holds water, sour | Nothing. This is the signal to repot |
When to repot
Repot every eighteen to twenty-four months as a matter of routine, whether or not the plant looks like it needs it. The reason is the bark itself. Bark decomposes, and as it does the chunks soften, darken, crumble and pack down until the pot behaves exactly like a pot of compost: waterlogged, airless and sour. Squeeze a piece between finger and thumb. If it crushes wet and soft instead of snapping dry, the medium is finished.
The other triggers are visible from outside. Roots pushing out of the drainage holes and over the rim in large numbers, water that sits on the surface instead of running straight through, a plant that will not stay upright in its pot, or a persistent sour smell when you lift it, all mean repot now. So does a plant that suddenly seems to need water far more often than it used to, which usually means it has lost roots.
Time it for the weeks straight after flowering, when the plant is starting fresh leaf and root growth and will recover quickly. New roots emerging from the stem are the ideal cue: they will grow straight into the new bark and establish the plant within weeks. Do not repot while a spike is in bud unless the pot situation is an emergency, because disturbance at that point makes the buds yellow and drop.
If a plant was sold packed in a tight ball of sphagnum inside a plastic pot, repot it into bark at the first sensible opportunity rather than waiting two years. That moss core stays wet long after the surface dries, and it is where most shop-bought moth orchids quietly lose their roots.
Repotting step by step
Do the whole job on a covered table with everything within reach, because the plant should be out of its pot for as little time as possible. You need the new pot, dry bark, a sterile blade or snips, and a bowl of tepid water if the bark needs soaking.
- Water the plant a day ahead Roots that are fully hydrated are flexible and green, and they bend rather than snapping. Bone dry roots are brittle and you will break a lot of them getting the plant out.
- Ease the plant out and clear the old medium Squeeze the sides of the plastic pot to loosen it and lift the plant rather than pulling it by the leaves. Tease out the old bark with your fingers, working from the outside in. Take your time around roots that have grown through the drainage holes.
- Cut away every dead root Dead roots are brown or gray, hollow, flat and papery, and squash to nothing between your fingers. Live roots are firm and plump, white, silver or green. Cut the dead ones back to the base with a sterile blade. Do not cut a root just because it is long.
- Choose the smallest pot the live roots will fit into Most mature moth orchids are happy in a pot 4 to 6 in (10 to 15 cm) across. Go up a size only if the live root mass genuinely will not fit. An oversized pot holds a mass of wet bark that no root reaches, and that is where rot starts. A clear plastic pot with plenty of holes lets you keep reading the roots.
- Settle the plant so the crown sits at the rim Hold the plant with the base of the lowest leaf level with the top of the pot, and trickle bark in around the roots, tapping the pot on the table to work chunks into the gaps. Never bury the crown, and never press the bark down hard with your thumbs; you want it firm enough to hold the plant, no more.
- Leave the aerial roots outside the pot Roots that have been growing in open air for months have adapted to it and often rot if forced down into the medium. Tuck in the ones that go in easily and let the rest stay where they are.
The first few weeks after repotting
Do not water immediately. A freshly repotted plant has cut and damaged root ends, and water sitting against them lets rot in. Wait three to five days, then water thoroughly for the first time. If the bark was soaked before use, wait a full week.
Do not feed for a month. There are no working root tips taking anything up yet, and the fertilizer simply builds up as salt in the fresh bark. Start again at half strength once you see a new root tip or a new leaf moving.
Keep the plant a little shadier than usual and out of any draft for two or three weeks, and keep the humidity up. With fewer functioning roots than it had, the plant cannot easily replace water lost through the leaves, and a slightly limp leaf in the first fortnight after repotting is common. Resist the urge to solve it by watering more; that is precisely how a repotted plant is killed. Support it with humidity and shade instead, and let the roots grow.
If the plant will not stand up in its new pot, stake it rather than packing the bark tighter. A rocking plant snaps its new root tips as fast as it makes them, and a stake for the first couple of months solves that completely.
What to write down
Write the repotting date on a label pushed into the pot, along with the medium and the grade of bark. Two years later you will not remember, and the date is the single most useful piece of information you can have about the pot, because it tells you when the bark is due to fail.
Note roughly how many live roots the plant had, even as a rough count of ten, twenty or forty, and how many you cut away. Compared with the same note at the next repot, this is the clearest measure there is of whether your watering is right. A plant with more live roots than last time is being grown well, whatever the leaves look like.
Also record the pot size, and the date you first watered and first fed after the repot. If a plant sulks for months afterwards, that record usually shows why: too big a pot, watered too soon, or fed before it had roots to feed.