Disclaimer: The growing guides are provided only as a starting basis to cultivation. Local conditions in your area may require modification to these suggestions. Bankstown Orchid Society Inc. will not be responsible for the results of your cultivation practices.
Native terrestrial orchids comprise many varying Genus and species; for the most part they tend to be deciduous, sending out new leaves, flowering then dying back to a subterranean tuber in a yearly cycle of growth. The more commonly grown genuses in cultivation are Acianthus, Caladenia, Diuris, Pterostylis and Thelymitra's. This guide will be based on these orchids; other Native Terrestrial Genus's not mentioned may still be cultivated under these guidelines with further research.
This intriguing group of orchids has some of the most amazing blooms of any orchid; from the curious Pterostylis (Greenhoods), the delicate Caladenias (Spider Orchids), the brilliantly coloured Diuris (Donkey Orchids) and the spectacular Thelymitras (Sun Orchids) these have some the most amazing colour combinations of any orchid.
General Culture: The first thing to understand about these orchids is that they have a cycle of active growth followed by a period of dormancy, very similar to many other exotic and native epiphytic orchids. Where this differs is that the major period of growth and flowering tends to be in the cooler months of the year, while the dormant period is in the warmer months. As a general rule this is the opposite to most other species of orchid.
They can be happily housed in shade house conditions (50%), with (ideally) a section having a solid alsynite style roof for the period of dormancy. They can cope with light frosts in winter and temps as high as 40oC in the summer.
Watering: This by far is the most important rule for growing these orchids. When in active growth the plants should be watered regularly, to the point were the potting mix should remain moist and not wet . As the plants finish flowering and start to die back (This is when the new tubers are maturing) watering needs to be decreased to the point where the mix should be almost completely dry. If the mix remains wet at this time the tubers will surely rot. Late December early January start to water lightly occasionally and as the new growths appear and accelerate increase the water accordingly. I will also during the active growing period allow them to get a light watering from rain as I find rain water far more beneficial than tap water.
Light and Humidity: Light varies depending on the genus, Pterostylis and Acianthus prefer shadier conditions than the Diuris, Caladenia and the aptly named 'Sun Orchids' Thelymitra's (so called because the flowers open as the sun rises and close by night). A good indicator is if the leaves or rosette (for Pterostylis) are long and rangy then more light should be given. All these species resent stagnant humid conditions preferring an open position with plenty of air movement, as plants can get attacked by fungal infections.
Fertilizing and Pests: A small percentage of organic style fertilizer like blood and bone can be added to the potting mix, apart from this steer clear of fertilizers as this can damage or even kill these plants. The only exception to this rule is that Diuris and Pterostylis do benefit from very light applications of foliar feeding.
As for pests the usual suspects slugs and snails can do immense damage in a very short time, baits for these are a necessary evil. Keeping plants suspended on mesh benches is a good deterrent. Thrips and Red Spider can at times also cause problems.
Potting: As far as potting mix is concerned A good standard mix is 45% potting mix, 50% coarse sand and 5% organic matter (Leaf Litter etc). Also as mentioned before the addition of a small amount of Blood and Bone is beneficial to the mix. Repotting should be carried out on a yearly basis in December or January for best results, pass mix through a sieve to collect the dormant tubers and save a small amount of the old mix and add this to the new pot. If potting is left for a year or two species like Pterostylis that are colony formers can out grow a pot very quickly.
Pot size, 100mm pot is a good starting point, to this size place 5 to 7 tubers, for a 150mm pot place 10 tubers, 200mm pot 20 tubers etc. (for optimum results 200mm pots should be the maximum size used until more experience is gained)
Finally when potting is complete place a small layer of chopped sheoak needles on the top of the pot, this will act as mulch allowing the rosettes or leaves to be stay above the damp mix thus preventing fungal problems.
These orchids are very different from the norm and very rewarding none the less, people who grow these orchids tend to be heavily addicted by there unique charm. So give them a go you may catch the bug.