ORCHID ROOTS AND REST PERIODS by Brian Milligan
What's most important to an orchid - its roots or its leaves? Actually, it all depends on the orchid. Some orchids never have leaves. One example is the Australian native terrestrial Dipodium roseum, commonly called the hyacinth orchid because its pink flowers resemble those of the hyacinth. It's often found flowering under trees in the mountains in summer. Dipodium roseum and many other saprophytic orchids rely on mycorrhizal fungi growing in association with tree roots to provide them with all the nutrients that they need. Other leafless orchids, such as the epiphytic Polyrrhiza lindenii and Chiloschista lunifera, are not saprophytic and need to manufacture their own sugars and other components necessary for plant growth. They do so in the same manner as leafed orchids, that is, by photosynthesis from water and carbon dioxide, using the green pigment chlorophyll as catalyst. The only difference is that these leafless orchids carry chlorophyll in their roots, whereas it's present in the leaves of most orchids.
Many orchids have leaves for only part of the year. Common examples in our collections are Bletilla striata, Pleione formosana and many of our native terrestrial orchids. The first two lose their leaves in autumn, leaving only their tubers to survive the icy winters of their natural habitats. In contrast, greenhoods (Pterostylis)and donkey orchids (Diuris) lose their leaves in late spring, leaving their tubers underground to survive our hot, dry summers.
Some epiphytic orchids survive stressful times in their native habitats, usually cool dry times, by entering what orchid growers call a rest period. They indicate their intention of entering this phase by gradually extending the protective layer of velamen that normally covers most of the root until the green, actively growing tip is completely encompassed. At this stage the root tips absorb water and nutrients much less efficiently and the plant drastically slows its growth. Flower spikes may continue to grow and buds to open, but the energy for these processes is derived from nutrients already stored in the pseudobulbs. Because orchids absorb little water or nutrients through their roots during the rest period, frequent watering and fertilising at this time is unnecessary and may be harmful.
Common examples of orchids that enter a rest period during winter are Laela anceps, Oncidium varicosum and its hybrids, epidendrums and cattleyas. But not all orchids have a rest period during winter; these plants continue to grow, albeit at a reduced rate. Cymbidiums are a prime example. They benefit from continued watering and fertilising during winter, although at a much less frequent rate than in summer.
So, how do you tell which orchids need a rest period during winter from those that don't? It's simply a matter of examining the root tips. If they are green and shiny, the orchid is still growing actively. But if the root tips are no longer shiny and are completely sealed with velamen, then the orchid has entered its rest period and it should be watered much less frequently, perhaps once every two or three weeks. Occasional misting of the foliage of these plants, especially in bright weather, will help to keep them from excessive dehydration.
As spring arrives, these 'resting' orchids will show signs of re-awakening by developing fresh root tips. Sometimes the old roots also develop new branches with shiny green tips. These are signs that you should gradually resume normal watering and fertilising. Because of the importance of active root tips in absorbing water and nutrients, it is vital to the well-being of your orchids that these tips should not be damaged. Ensure that your plants are secure in their pots and use snail bait to kill snails, slugs, slaters and earwigs.*
*Pet owners are warned that most snail baits are poisonous to animals as well as to humans.