This article is aimed at new growers of Australian terrestrial orchids (Pterostylis, Diuris, Chiloglottis, Caladenia etc.) and other European (e.g. Satyrium) or South African (e.g. Pterygodium) terrestrial orchids. In general, these orchids have an annual cycle of dormancy, growth and flowering. They are adapted to conditions of hot dry summers and cool moister winters so they tend to be dormant below ground in summer and grow and flower from autumn to spring.
We'll start our discussion in spring, as that is when many people buy their first terrestrial orchids, in bloom.
While the plants are in spike and bloom, ensure you keep the mix moist. After flowering has finished, the leaves will naturally start to yellow and dry down so reduce watering to almost nil so that the tubers do not rot. In December and January, repot the tubers, water once and then not again until the leaves appear. Thereafter water regularly to keep the soil moist. After a time, flower stalks will appear and the next round begins.
Standard pots (i.e. those that are taller than wide) are best as it gives the plants more opportunity to send new tubers deeper to where they find the optimum moisture level. If you have a few pots of these orchids, it's best to standardise on one size/shape of pot so you can manage the watering more easily.
There are as many mixes for terrestrial orchids as there are growers, but many of us in Canberra have success with a fine seed-raising mix (preferably without added fertiliser although that is hard to find these days). The advantage to such as mix is that it is easy to sieve out any new tiny tubers at repotting time.
Repot in December and January when the tubers are dormant. Most growers repot annually because the colony-formers can produce a lot of new tubers in a year and the pots can become over-crowded very quickly. Having said that, there are some species (e.g. Caladenia sp.) that are resent repotting and should be disturbed as little as possible. If you get hooked on growing Australian terrestrials, you should join the Orchid Society to get tips/tricks/plants from our experienced growers.
Scrape the mulch off the top of the post and tip the mix into a tray. You can either put the mix through a fine sieve or sort through it by hand, collecting all the tubers.
A 150 mm diameter pot will hold 15 to 20 large tubers. The object is to have the surface of the pot covered by foliage so put in extra small tubers that will not flower but will produce leaves and cover the surface.
Don't line the bottom of the pots with shade cloth, or you may find tubers caught up in it the next time you repot. Fill your pot to within 65mm of the top, place a small pinch of blood and bone over the mix, remembering that like all Australian native plants they don't like much fertiliser, and then place 25mm of mix on top. Arrange your tubers on top of this mix, eye upwards. When you examine the tubers you will see that each tuber has an eye from which the new growth will appear. If the tuber is dirty and you cannot see the eye, the tubers can be rinsed in warm water. However, like most garden bulbs, the positioning of the eye doesn't seem to matter too much and the plant will grow to the surface, so don't stress too much.
After having placed the tubers, cover with another 25mm of mix and top the pot with a cover of clean and chopped casuarina or pine needles. This is to make watering easier as there is less soil disturbance, and also the plants sit nicely on top of the needles to create a good display.
Give the pot a good watering to settle the mix, and then place in the shade or dappled light. Be careful to give minimal water until the plant starts to show new growth, then water regularly until flowering finishes and the whole cycle begins again.
These orchids are quite hardy and can be grown outside in Canberra, with a little protection from frost in winter and rain in summer. Balconies are good, shade houses are good and even windowsills work. While the plants are dormant, keep the pots in a shady place out of the sun so the pot doesn't heat up and cook the tubers. While the plants are actively growing, they do best with bright indirect light, dappled sunlight or morning sun. If the plants are growing tall instead of making a flat rosette, it is a sign that they need more light. If the flowering stems bend towards the light, make sure to rotate the pot regularly to keep them growing straight.
Here are some examples of terrestrial orchids: