This deals with the culture of the better known Disa species and hybrids which grow in winter rainfall areas around Cape Town, South Africa. The main species of interest are Disa uniflora, D. racemosa, D. tripetaloides, D. aurata, D. cardinalis, D. venosa and D. caulescens. These are all terrestrial orchids which are associated with perennial water. Many other Disas are known, though few are commonly cultivated and many grow in drier conditions and have very different cultural requirements (e.g. D. sagittalis).

Media

Disas need excellent drainage, a neutral to acidic pH and a constantly moist, aerated medium. There are different ways of achieving such conditions. Long fibered New Zealand sphagnum moss is excellent, as is Supersphag, consisting of NZ sphagnum fines. Both benefit by mixing with some coarse Perlite or shredded Styrofoam to keep the medium buoyant. Coarse sand (not beach sand!), sometimes mixed with peat or fern fiber works well for some growers. I currently use New Zealand sphagnum (with or without the Perlite or Styrofoam), and this works well in both hydroculture and conventional culture. Algal growth on the surface can be reduced with a layer of small pebbles. Experiment with media to find the one which suits your plants and growing conditions the best. If a plant does not thrive, it may be time to try a slightly different medium. The best medium may also depend on the water supply being used. For example, if the water has a high pH, a more acidic medium may be called for.

Pots

For mature plants, 7 to 10 cm square plastic pots are fine; for seedlings, smaller pots are ok. Pots should be reasonably deep to conserve moisture and allow room for the roots and tuber. For specimen plants, bigger pots up to 15 cm square can be used. Netting pots with NZ sphagnum as a medium can be used to encourage vegetative multiplication of valuable clones.

Water quality and watering

Water quality is absolutely critical in Disa culture. The water should have a low concentration of dissolved mineral salts and should be low in choride and fluoride. Some tap water is ok, but most is not. Rainwater or reverse osmosis water is recommended. However, even rainwater can pick up unwelcome salts off a roof, especially the first rain coming off the roof, and if the tiles are made of cement. A slightly acidic pH (4.5 to 6.0) is preferred; unpolluted rainwater has a perfect pH of around 5.6. A dissolved solids tester, reading in ppm (parts per million) or microsiemens/cm is essential for any serious Disa grower. My rainwater  generally reads no more than 10 ppm  (~20 microsiemens/cm) and works well. I have a supply of simple pocket-sized testers for sale at $30 including shipping within USA.

Disas are different from most other orchids - never allow the medium to dry out. Just in case you weren't really paying attention right then...........NEVER! Adjust your watering frequency to meet this requirement, which may mean daily watering in extreme cases. But also try to avoid a waterlogged condition. Hydroponic systems, either continuous flow or ebb and flow, give excellent growth. Regular watering from above also works well. As with most orchids, watering is best done early in the day so that the leaves can dry out. Disas succumb rapidly when mineral salts accumulate in the medium; thorough watering from above will help to flush away unwelcome residues. It is important to cut back severely on watering during winter, otherwise problems with fungal rot can be expected.

Fertilizer

Use very dilute (about 1/4 to 1/10th strength), balanced fertilizer containing all trace elements every week or two during the spring to fall growth phase. Cut back on fertilizer during winter. Moderate to high nitrogen is recommended; mixtures like 10:5:5, 30:10:10, 20:20:20 and similar have all been used by successful growers. Here again, a dissolved solids tester is useful. Ideally, fertilizer solutions should be kept below about 200 ppm (300 microsiemens/cm), although some growers use higher concentrations without adverse results. Provided all essential elements are supplied to the disas, I don't believe that the choice of fertilizer is very critical. Put another way, if your other growing conditions are good, any complete fertilizer is likely to work well.

Temperature and humidity

Disas like plenty of air movement and relatively cool temperatures, especially in their root zones. They are stressed when the temperature goes above 80°F (27°C); also the humidity should ideally be kept above 50%, especially in hot weather. Shade netting in summer helps keep the temperature down and humidity up. A sharp frost will damage the leaves, and a hard, extended freeze is likely to kill the plants, but they will tolerate winter temperatures just slightly above freezing.

Lighting

During the growing season bright, filtered sunlight is important for Disas. My plants stay outdoors, under shade netting from spring to fall. During the colder months they come indoors at night but spend the days outdoors with no protection from the elements, except when temperatures dip below freezing. (A greenhouse works a lot better!)

Repotting

Mature Disas need annual repotting since the old plant, tuber and roots die and rot. Repotting is usually done in the fall, after flowering, or whenever there are indications of root problems. Healthy roots are pale and very brittle; dead roots are brown and soggy. These should be removed, and the plant should have a thorough washing before repotting. I also use repotting as an opportunity to give the plants a soaking in a fungicide solution. I disinfect pots and pumice medium before reuse.

Pests and diseases

If you are not prepared to use fungicides and insecticides on a regular basis, I would advise you against taking up Disa growing. Disas are susceptible to a virulent fungal rot, in which the whole plant and root system decomposes into a soggy black pulp. Correct culture and a fresh potting medium reduces the chances of this, but routine fungicide application at least from fall through spring is recommended. Among the fungicides which have been used are Aliette WP, Subdue, Cleary's 3336 WP, Captan WP and Rootshield, a biological fungicide. Bacterial soft rot has been treated with Physan 20. Insect pests and their treatment include aphids (Malathion, Sevin, Orthene), thrips (Cygon, Lebaycid, Orthene), gallmidge fly larvae (Lebaycid) and earwigs, which eat the flowers at night. Thrips are probably the worst of these pests - they do their dirty work largely unseen deep down in the growing tip, cause unsightly disfigurement of leaves and flowers, and can even kill off your plants.  Regular removal of dead and dying leaves reduces the risk of disease. As a rule, pesticides in wettable powder form are preferred to liquid formulations which may contain damaging solvents.

Growth cycle

Winter. Plants show little or no green growth during the coldest months, but mature plants can have significant underground activity in the way of developing tubers and shoots.

Spring. With the advent of warmer weather and longer days, leaf growth begins to accelerate and spikes may start developing.

Summer. As the weather warms up, strong growth continues and flowers should open. The hybrids tend to flower early, along with species such as D. tripetaloides. On the West Coast of North America, the main flowering season is between May and August. Flowers may last for six weeks.

Fall. The plants are genetically programmed to die back after flowering. With good management, other plantlets are already present, and a new tuber has been produced from which a strong new plant will sprout. Even unflowered plants may die back as the weather gets cold, but provided a new tuber has been produced, a healthy new shoot can be expected to grow in the spring.

Propagation

Mature Disa plants often produce several new plantlets annually, and these provide a simple way of increasing a collection. These new plantlets can grow from new tubers, as keikis alongside the main growth, or from long underground runners (stolons). Disas can be grown from seed to flowering in 2 to 3 years. Seed can be sown on boiled, damp moss, on peat, or on a sterile nutrient agar medium, which is quicker. One quarter strength Murashige and Skoog formulation, or similar mixes, with added banana pulp or powder, work well for sowing seed and replating.

General

Despite trying to follow other people’s proven growing methods, I have had my share of setbacks. It pays to experiment to find the best growing medium, watering routine, fertilizer, lighting etc for your specific conditions. Unfortunately, you will probably lose a few plants along the way!