Repotting should be considered when the potting mix is excessively decomposed as to cause compromised root health OR the plant is growing over the edges of the pot. Some plants respond poorly to repotting, but they usually die without roots.
Slipper orchids can generally be gently repotted any time of year, even when in spike or in bloom although flower longevity may be shortened for that particular bloom. If you're careful, the same can apply to Moth orchids. Corsage orchids should be repotted when new roots are just appearing at the base of the youngest growths. Any plant that seems to be of poor health due to root loss should be repotted. It is our feeling that we'd rather risk a single poor blooming and know the entire health of a plant than risk it not seeing another bloom.
Unpotting orchids allows you to examine root health as well as inspect the entire plant for disease or creatures that may be hiding. We use Chilean or New Zealand sphagnum moss, three grades of fir bark alone and in various mixes, and a home blended soil-less mix for our plants depending on each plant's requirements. Fine fir bark is used straight from the bag without any grading other than sifting out the fines with a colander for plants in larger pots or those that like to dry out a little more. Bark marketed as "medium" grade is sized through a section of one half inch grid "egg crate" light diffuser mentioned above in the section on humidity. Bark that falls through the grid is separated from the rest- we creatively call it small fir bark. Charcoal grade #4 is used with fine fir bark, and grade #2 is used with small and medium fir bark. Sponge rock, a very large grade of perlite, is the final component of our mixes. Typically, we use a 4 to 6 parts bark or moss: 1 part sponge rock: 1 part charcoal mix, but proportions are adjusted to each plant's needs.
As general guidelines: For slipper orchids in two to three inch pots, we usually use fine fir bark mix; slippers in three to six inch pots get small fir bark mix; and those in pots greater than six inches are potted in medium fir bark mix or a mixture of small and medium fir bark mixes. We use plastic pots for all of our slipper orchids, and they are repotted at least every twelve months; some Phragmipedium that like it wetter at the roots get repotted even more frequently. Moth orchids are potted in New Zealand sphagnum moss in plastic pots up to four or five inches. Moth orchids in larger pots typically get planted in small or medium fir bark mix depending on the pot size and root system. We are starting to pot moth orchids in bark mixes more frequently because potting in New Zealand sphagnum is more tedious. We are extremely pleased with the results from these plants in moss (some plants were jumping from four inch pots to being stuffed into six inch pots after only eight to ten month's growth!), but it simply takes too long to repot many plants in it. It seems they grow well in bark using the same basic guidelines for slipper orchids above. Specimen Phalaenopsis in six to ten inch pots get potted with medium grade fir bark with soil-less mix tapped in on top. Our soil-less mix is composed of two parts sphagnum peat moss, two parts vermiculite, two parts perlite, and one part coarse sand (pool filter sand not play sand!). Compact and miniature corsage orchids can use the same mixes for pot sizes listed above, but be sure to let the plants dry out more than slipper and moth orchids. Whenever fine bark is used for corsage orchids, be sure to remove the fine particles for maximum drainage and quick drying, and it may be desirable to add a little extra sponge rock to the mix or to put a layer of larger sized bark at the bottom of the pot for improved drainage. Clay pots can also be used instead of plastic to encourage faster drying.
Once you decide on the appropriate pots and media, you can get started with repotting by gathering a few items: a stack of opened newspaper to work on; sharp, fine-tip garden or bonsai shears; a container of saturated trisodium phosphate solution; disposable gloves; pots; labels; potting mix; trash container; pencil...
The trisodium phosphate, or TSP, is used to disinfect cutting tools between plants. TSP is available from us or in the paint department of some hardware stores. Read the label carefully to be sure you get TSP and not a TSP substitute or TSP-PF (phosphate free). Add the TSP powder bit by bit to luke-warm water in a container with a lid and stir- repeating until the powder no longer dissolves. Allow the undissolved TSP to remain on the bottom of the container. The solution can be stored and reused until it becomes discolored. TSP is very corrosive; be sure to label the container clearly and keep your TSP solution out of the reach of children and pets. Cutting tools should be allowed to soak in the TSP solution between each plant. Alternatively, disposable razor blades can be used one per plant or cutting tools can be sterilized with a small torch. A simple candle is nowhere near hot enough to properly disinfect. Bleach works well as a ten percent solution in water, but a contact time of at least twenty minutes is required for proper disinfection. We use a ten percent bleach solution to disinfect pots and labels before reuse by soaking at least twenty-four hours after a thorough cleaning with soap and warm water.
We recommend wearing a new pair of disposable gloves for each plant repotted. This is another step to prevent the spread of disease from one plant to another, and they also provide you protection against splinters from the fir bark and nasty fungal infections from sphagnum moss.
Lay the newspaper out on your work surface and gently unpot the plant by inverting the pot while supporting the plant. Slipper orchids usually come out of the pot fairly easily, but moth and corsage orchid roots can get a tight grip on their pot. Plastic pots can be squeezed and worked to loosen roots, and clay pots can be carefully broken with a hammer to free plants. Sometimes, watering the plant will help loosen its grip. Once out of the pot, inspect the plant from top to bottom. Remove any squishy roots and dead lower leaves. Look for insects. A strong stream of room temperature water from a faucet can easily wash away pests and eggs. Remove as much of the old mix as possible without damaging the roots.
Decide on an appropriate pot size based on the root system of the plant not the size of the leaves. Slipper orchids should be potted into pots that will hold the roots comfortably with only a small amount of extra space. Moth orchids with vigorous root systems can be potted into pots with an extra inch or so of clearance all around. Corsage orchids should be potted to allow for about two year's growth. This will depend on how close the pseudobulbs are on the plant. Some will add new pseudobulbs very close to the old ones, whereas others will space them out further.
Potting orchids in excessively large pots can lead to root rot because of the extra water-holding mix present. Sympodial orchids such as slippers and Cattleya types are positioned in the pot so old growth is against the pot's edge and new growth will spread out into the new mix. This is not as important with Slipper orchids as new growths are generally tight up against the old. Exceptions of Paphiopedilum are Paph. armeniacum and Paph. micranthum and some of their hybrids and Phragmipedium besseae and some hybrids. These tend to be stoloniferous, new growths are on leads away from the old growths. This can be problematic if new growths get trapped in the potting mix and rot. We have tried a couple different methods of potting Paphiopedilum armeniacum. Baskets are sometimes recommended. We made four inch square baskets from half inch poultry wire, lined it with coconut husk, and then potted the plants in fine bark. We also tried clear plastic pots. We like the clear plastic pots better because you can see new leads as they approach and then grow along the pot below the media's surface. We then gently push aside the bark and direct the growth upward. The baskets allow new growths to emerge, but the media will dry faster than in plastic pots and require special attention and additional monitoring. Monopodial orchids like Moth orchids are usually centered in their pots since they grow upwards.
Once the plant's roots are positioned in the pot, begin to fill potting mix in around it. Add a little mix then tap or jiggle the pot while supporting the plant to get the mix evenly distributed around the roots. Repeat until the mix is just covering the very bottom of the base of the plant.
We used to soak bark mixes overnight with a couple drops of dish soap as a wetting agent before use, but gradually got away from this as our collection grew. It's faster for us to pot with dry mix. We now use dry fir bark mixes and water extremely well three or four times once the plants are in their pots.
New Zealand and Chilean sphagnum moss are soaked just before use and mixed with charcoal and sponge rock as needed. Potting with sphagnum moss isn't as easy since it can't be simply shaken into place. It needs to be inserted bit by bit being careful not to pack it too tightly. We usually pack it so water will not pool on its surface when watered; there should be good drainage. Again, water the plants several times immediately after potting to thoroughly wet the media. Use chlorine free water on plants in sphagnum moss as the chlorine will greatly hasten the decay of the moss.
Some slipper orchids grow in detritus on limestone rocks. Since we use very pure water for watering, these plants can become deficient in calcium and magnesium. For slipper orchids that grow in these calcicolous areas, we usually lightly top dress the mix with crushed oyster shell and Epsom salts after potting. Dolomitic lime can also be used. The top dress is evaluated regularly and reapplied based on each plant's growth and health. The practice of such supplementation should be carefully considered! If your water already has calcium and magnesium or if you add these elements in your fertilizer, topdressing may not be necessary! Again, identification tags provided with our plants typically indicate the supplementation preference of each species or hybrid with Ca and PCa as follows:
Ca- all parents calcicolous
PCa- for hybrids, some parents calcicolous; for species, those that do not always grow in such environments
If the species or hybrid does not appreciate calcium and/or magnesium supplementation, no Ca/PCa code will appear on the tag.
After potting, place plants in a lower light area and try to keep them humid without inhibiting air flow to encourage root healing and growth. We were amazed at root growth our plants put on when we moved from Washington, DC to Chicago, IL. The orchids sat in cardboard boxes in a comfortably warm and humid room for over a week because we were too tired and busy to set up the shelving units and unpack them. When we opened the boxes, roots, especially on the Moth orchids, were everywhere. The plants apparently got bored and couldn't really add green growth since there was no light, so they started growing roots. This is an extreme case to be sure. Try a spot with bright but very diffused light for about a week.