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BEGINNER'S SERIES Anerican Orchid Society

Orchid Culture — Part 10 — Repotting
Text and photos by STEPHEN R. BATCHELOR

Most orchid growers face the chore of repotting (or reslabbing) their orchids
reluctantly. However unappealing this task, it is a very necessary one. As previously
discussed in this series, organic media unfortunately decompose into humus, a material,
by itself, unfit for orchid roots. In a decomposed medium roots rarely have
adequate oxygen to survive and function (see illustration). Therefore repotting
must occur before significant media decomposition takes place, in order to avoid
major root loss. Once again, healthy roots are a must for healthy orchids!

Even as a slab, tree fern eventually decomposes. If remounting
had been done before the decayof this slab, the nearly total
loss of the root system of this epidendrum would have been avoided.

Fast-growing sympodial orchids (those which grow horizontally by rhizomes)
may bring on the need for repotting before media decomposition (see illustration,
page 1318 of the November 1981 BULLETIN). A plant which has "crawled out of its
pot" and is rambling along as it pleases in mid-air will require repotting and/or dividing
before things get out of hand!

WHEN TO REPOT

Repotting is an unavoidable disruption to the potted orchid. This disruption can be

lessened with expert technique, yet inevitably some injury is going to result in the
process of replacing old medium with new. Nevertheless, minor root damage in repotting
is of little consequence when compared to the alternative of major root decay
if a change of medium is delayed. With new root growth orchids overcome any
damage which may occur with repotting. For this reason it is very important to repot
at a time when the orchid is just beginning, or is capable of beginning, new root
growth.
By way of explanation, consider deciduous calanthes. Like a number of orchids,
they have a distinct period of inactivity. This rest period typically follows the
maturation of new growth, the loss of leaves, and, finally, flowering. Annual repotting
is recommended, the procedure being to remove and divide the plants after
flowering, and to maintain the pseudobulbs, bare-root, until new growths begin.
Only then is repotting to occur (Hawkes, 1965). Repotting after new growth has initiated
comes just prior to the period of greatest root growth (see illustration, page
949 of the August 1981 BULLETIN). Placed into a new medium at this stage, a calanthe
can fairly rapidly grow into the mix, establishing itself for the water and nutrient
uptake needed to produce the growth and flowering to follow

The cluster of root tips at the base of the Cattleya hybrid indicates
a good time to repot. Photo: Greg Allikas

Deciding whether an orchid could produce new roots at the time of repotting is
just as important for orchids without well-defined rest periods. Most complex Cattleya
hybrids are capable of forming new roots nearly anytime. Even so, repotting a
plant with an actively expanding pseudobulb just beginning or about to produce its
own cluster of roots is ideal timing (see illustration). Such a practice often means
quick establishment in the new medium. To wait until root development on the
new growth is nearly completed is less than ideal, not only because another such
flush of new, penetrating root growth is less likely, but also because those newer
roots will be exposed, like the rest, to mechanical damage in repotting.
Proper timing of repotting also considers the flowering period of the orchid involved.
Since a change of medium is to some degree a shock to a plant, repotting at
a time when an orchid is in bud or flower is risky. If repotting is done during the
period of bud development, bud drop could result. At the very least, some lessening
in the ultimate size and longevity of the flower(s) is apt to occur.
Sometimes timing repotting to coincide with a period of active root development
is not possible, due to a dangerously decomposed medium. In this case, it is
far better to repot before all roots are lost, and to encourage new root formation
into fresh medium where there is a good chance of survival. Whether it be a result
of higher oxygen levels, or a lesser chance of disease attack, orchids with few viable
roots (and those with a good number) are generally more inclined to form new
roots in a drier medium than one which is wet. After restricting water to bring this
about, the trick then is to avoid severe desiccation during this tenuous, rootdeprived
period. This can be done by providing conditions which discourage high
transpiration (water loss) rates (such as high humidity, lower light intensity, etc.)
until new roots are formed and on their way towards establishment. Very validly,
many growers will not pot up a rootless orchid until new roots are initiated, keeping
the plant in a bag, under a bench, or on a flat of moist medium to achieve these
low-transpiration conditions.

REPOTTING PROCEDURE

It is fairly obvious when a sympodial orchid is about to outgrow its container
and will soon need repotting. (see FIGURE 1 of the repotting sequence illustrations).
For orchids which grow vertically (monopodial types), this is not likely to be
the reason for repotting. Determining whether repotting is necessary because of
media decomposition requires a bit more investigating. Naturally, as media break
down and become less porous, they take up less space in the pot. A lowering of the
level of a medium is a good indication of decomposition (see illustration). An examination
of any roots along the surface will reveal whether they have declined, as
is the case in the accompanying illustration. As previously mentioned in this series,
jiggling the plant in its pot will also indicate root condition. Any significant shifting
of an established plant suggests substantial root loss. Examining the resistance of
the medium with a probing rod or finger is another method of assessing decomposition.
If there is little resistance, one can assume major decomposition of the mix
has taken place.

Though this Phalaenopsis hybrid has not outgrown its pot, it is nevertheless
inneed of repotting because of media decay. Decomposition is indicated
by the shrinking of the media and the decline of the surface roots.