Journal Number 102
February 2007
CONSERVATION BY CULTIVATION
The Terrestrial Orchid
From ANOS Geelong Group June 2002
What is a terrestrial orchid? It grows in the ground, usually has a growing period and a dormant period, usually has a tuber although sometimes a thickened root. The plant usually replaces itself each year, has one flower spike with from
1 to 50 flowers, in most instances does not have a root as we know it and is reliant on a mycorrhizal fungus.
Let us look at the structure of the terrestrial orchid. It has a leaf or leaves, flower stem, tuber, tuber droppers, and a very reduced root structure. Therefore a terrestrial orchid is dependent on mycorrhizal fungi to grow successfully and needs a growing medium in which the fungi can grow.
The orchid may have up to four different fungi and four or more different bacteria involved in feeding it. In some orchids there is a three way relationship e.g. Dipodium, orchid-fungus-gum tree.
As mentioned earlier the mycorrhizal fungus lives in certain underground parts of the plant. When the plant is dormant the fungus seems to live in the old tissue. Thus when repotting use half of the old mix including plant parts to maintain the fungus in the pot.
What do we grow terrestrial orchids in?
A mix that
- holds moisture but is not wet
- dries out and encloses the tuber when the plants are dormant
- is open enough to include sufficient air
- has food for the fungi
- will support the flower spike.
The most common potting mix is
- 2 parts gravel
- 1 part leaf mould - still with texture
- 1 part wood shavings or Eucla Mix.
(Check that the particle size is not too large or contains chemicals that may harm
the orchid)
- 1 part good soil; check that it does not have fertiliser or other things added
- a little blood and bone.
Variations of the mix components are made to meet the specific requirements of more difficult to cultivate orchids.
The pots need extra drainage holes in the bottom and a piece of shade cloth or fly wire in the bottom to stop the mix falling out. 12 to 15 cm diameter pots are most commonly used.
The mix should be damp when used. The tubers are placed about 3cm below the top of the mix, with the top centimetre of the pot free of mix to assist with watering. Up to 20 tubers may be placed in a pot. When only a few tubers are placed in a pot they should be placed near each other. This seems to assist fungal activity and hence give stronger orchid growth.
A layer of mulch is needed on the top of the pot to stop the water disturbing the medium (and hence the fungi) and to keep the leaves off the ground to reduce rotting. Chopped casuarina needles or larger gravel are commonly used.
At the completion of potting the orchids are watered.
How do we grow terrestrial orchids?
Most of the available terrestrial orchids grow in a cycle of
- Rain in late autumn
- Rain through winter and early spring
- Little rain through a dry summer and early autumn.
This is different to most epiphytes. Thus if we are trying to grow them with epiphytes it is very difficult as they get too much water in winter and much too much water in summer resulting in tuber rot.
Most of the readily available orchids grow under trees which provide shade and heat protection. Thus we need to protect them from the heat, sun and from excessive rain using a solid roof that is partly transparent. We need to give them plenty of ventilation using shade cloth walls. And we need to lift the pots off the ground to protect our back, make them easier to view, give better air circulation and keep them away from snails and slugs.
Watering, or incorrect watering is the cause of most plant deaths. Most commonly grown terrestrial orchids need a good soaking at the beginning of February. When watering, check that the mix has wetted, as water can run around the side of the pot instead of through the mix.
Keep the pot just damp until leaves emerge and then water every one to two weeks, depending on the weather. A warm wind can dry a pot in a day. By checking the bottom of the plant name tag you can see how moist the pot is. For newer growers it can be helpful to make up a couple of extra pots of mix. You can then dig down into the media and check its moisture level.
In late spring the leaves will start to go yellow. This is the time to reduce watering and let the pot almost dry out.
Keep the pot almost dry over summer. Just damp enough to stop the tubers dehydrating.
Summer and autumn flowering orchids are usually watered earlier and require a different watering cycle.
Pests And Diseases
Pest and disease control is similar to that required for epiphytic orchids with snails, slugs, caterpillars, spider mites and aphids being the most common pests. Rust and virus are controlled by removing infected plants.
How Do You Increase Your Orchid Numbers?
Collect seed pods from your plants and store the seeds in paper envelopes in the refrigerator. In autumn sprinkle the seed around the parent plant and gently water the pot. The pot would not be repotted the first summer after sowing seed. Some species can be induced to produce an extra while others increase their numbers naturally.
An Excellent Resource
The reference book Cultivation of Australian Native Orchids by Richards, Wootton and Datodi ($A10 from www.orchidaceousbooks.com.au - Ed) is excellent. I find it to be an essential reference book when growing terrestrial orchids. Page 52 gives the basic potting mix with the following pages giving variations to the mix needed for specific orchids and the times to commence watering and the particular needs of each species.
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