Orchids: How to grow Disas

Disa uniflora develops a large storage tuber, whereas species such as D.tripetaloides, D. cardinalis and D.aurata develop thick fleshy roots which take on the function of storage tubers. The substrate that the plants are found in is either a Table Mountain Sandstone derived sand with organic silt trapped in it; or an organic fibrous humus well impregnated with sandy silt. Both substrates have extremely good drainage properties.

Disa uniflora, the related species D. cardinalis, D. tripetaloides, D. aurata and D. caulescens and their hybrids are frequently seen in collections as they have considerable appeal to collectors and are also comparatively easy to cultivate. Plants of these species can also easily be purchased from specialist nurseries or through orchid societies. Unfortunately, most of the smaller-flowered disas are quite difficult to grow in pots, and are also seldom obtainable.

Plants in cultivation are best grown under roof with 50% shading. Growing the plants under a roof will protect the flowers from rain damage too. A cool environment with excellent ventilation is a must.

Growing media

A very well draining medium that retains moisture successfully without remaining soggy, is ideal. A soggy anaerobic condition is detrimental at all times. Various potting components and combinations can be used. Peat moss, fine milled bark, perlite, vermiculite, silica sand, river sand, fern fibre, and coconut fibre have all been used with varying degrees of success. Availability will tend to influence choice of material. Whatever the choice, the requirements are:

  • 80% coarse inert material, such as silica sand or river sand
  • 10% fine relatively inert material for assisting capillary action, e.g. fine river sand or perlite or vermiculite
  • 10% absorbent material to aid water retention and prevent excessive dehydration of the potting medium, such as peat moss or fine milled bark, vermiculite, fern fibre or coconut fibre.

The pH value of the medium should err on the acid side. Anything between, 5.0 and 6.5 is acceptable. Peat moss, fern fibre and milled bark all are acidic (varying between 3.5 - 4.5). This can be used advantageously to keep the pH values of the medium acidic.

Repotting

Disas respond well to repotting. Repotting of mature plants is generally done shortly after the flowering season is over and the new shoots have appeared. The pot is tipped over and the old mix gently shaken loose. Dead roots and any other decaying matter such as the remainder of the previous year's tuber are removed. (The dead material is identified by being dark brown in colour and soft and structureless in texture.) The plant is then immediately dropped into a basin containing a Cabtab solution. Should this not be available, Dithane may be used as well. The whole plant can be submersed in the solution. All grit and other matter is gently rinsed from the plant. This treatment has the advantage of preventing and treating fungal infections. If the plant has produced several shoots which have matured, these can often be separated easily during the rinsing action.

The cleaned plant can now be potted into a clean pot with new medium. Gently hold the plant in the new pot and pour the new mix between the roots. Fill the pot to the top, making sure that the plant is potted well up to the first rosette of leaves and all white areas of the stem are covered. Disas prefer to be set deeply in the soil. As the plant is watered, the level of the potting mix will drop. Some granules of a slow release fertilizer such as Osmocote or Horticote can now be pushed into the grit. (Maximum of 10 granules per pot). Drench the plant well so that water drains freely from the pot.

Watering

Although water quality of utmost importance with all plants, disas in particular do not tolerate impure water. Care must be taken to ensure that the water to be used is suitable. Even moderate salt levels will not be tolerated. pH values should range between 5.0 and 6.8 maximum.

As disas grow on stream banks and seepage areas watering is critical. By using a flooding method of watering, the leaves of the plants are kept drier and burn marks on the leaves and in the crown can be avoided. If grown on a bench, flood bench to half the height of pot, approximately 5cm height. Allow to drain - ensure that bench drains fully within 24 hours. Drained water can be re-used for other gardening practices. If no bench is available, improvise with a suitable large, shallow container which drains slowly.

Overhead watering is used in winter, spring and autumn. These are the seasons where fungal activity can cause the most damage. Keeping the plants on the drier side will result in more robust growth and increase resistance to crown rot and damping off. Overhead watering twice a week, drenching each pot and ensuring ample water flow through the pot, will provide an adequate water supply. This method of watering is more suitable for the cooler seasons as the weaker sun will not cause burn marks on the leaves. Salt build up or any other toxins in the media will be leached out.

Feeding

A reliable method of ensuring that the plants have appropriate nutrients throughout the year, is by applying a temperature controlled, slow release fertilizer. (Kirstenbosch NBG has been using Horticote 7.) Horticote releases over a period of one year. As it is temperature controlled, it releases less nutrients in winter when the plant's metabolic processes are at a minimum and ambient temperatures are low. As temperatures and plant growth activity increases and there is a greater need for nutrients, more nutrients are released. The convenience of a one year duration is that the granules can be incorporated in the potting mix, thereby ensuring easy even distribution. Alternatively, a maximum of 10 granules can be placed on the medium in each pot. Push the granules into the medium to prevent the granules from splashing out of the pot during watering.

Spring marks the beginning of the growth season. As temperatures increase, vegetative growth also increases. The greater the light intensity, the greater the amount of nutrients the plant is able to utilise. Spring is also the season when the main rosette, from which the flower spike will arise, starts showing active growth. During this active growth period available nutrients are well utilised. One inorganic fertilizer that can be used is Multifeed 10. It may be watered in with a watering can, normal strength as indicated, or applied using a mix nozzle. Be sure not to apply excessive amounts of nitrogen, as this cause the plant not to form its normal storage tuber. If no tuber is present, the plant will die after flowering, since there will be no sustenance from which the new shoot can arise. An application of Trelmix trace elements once a month can be applied as foliar feed in the cool of the day.

Fertilizers that have been successfully utilised are Nutrisol, Nitrisol, and Seagro. Slow release granules such as Osmocote and Horticote are also suitable. Generally, the tendency is to steer away from organic fertilizers, partly because the nutrient concentrations aren't always exact, but mostly because they can be a source of bacterial and fungal pathogens. However, good results have been achieved with organic fertilizers and they should not be entirely dismissed. Although not a fertilizer, Kelpak stimulates growth and root development. Kelpak is made from kelp and contains natural auxins and cytokinins, two of the most important growth hormones that stimulate shoot and root growth. It can also be applied at the recommended dosage by watering can or mix nozzle.

Pests and diseases

Main pests affecting disas are aphids, red spider and thrips. All spraying formulations should be applied in the wettable powder form. Red spider and thrips are usually an indication that the environment is too hot and dry for the disas. Cooling the environment will decrease their activity tremendously. Humidity can also be increased, provided that temperatures are kept low. Warm humid conditions are completely inappropriate for disas and will cause the plants to rot. Crown rot and damping off can be well treated with formulations containing the active ingredients Captab and/or Metalaxyl, but again, only in wettable powder form.


Author: Hildegard Crous
October 2000


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