One of the most important climatic factors in growing orchids, and most other plants, is undoubtedly light. Daylight in the natural habitats of most orchids is diffused due to tree shade or clouds and fog. The concept of orchid lighting is a riddle for many an orchid grower. A human eye senses light very subjectively and can easily adapt to dark or bright light conditions. As a result, it is not a reliable factor in determining light conditions for plants. Leaf or flower tissue damages are an incurable reaction to stress from sunburn. To a certain extent, a plant can defend itself against harmful exposure to daylight (leaves and roots will turn red) but when the light intensity exceeds a certain level the damage is unavoidable. Plants which used to grow in shady environments are particularly delicate and should not be directly exposed to daylight - they‘re not well adapted to quick changes.
1 lux – illumination of one burning candle 1000 lux – light quantity required in most shade tolerant plants – ferns etc. 5000 lux – minimal light quantity required by shade tolerant orchids (Phalaenopsis, Paphiopedilum) 10.000 lux – minimal light quantity required in Cattleyas, Dendrobiums, Miltonias and Oncidiums 15.000 lux – minimal light quantity required in Vandas, Ascocendas and similar types.
The function of roots
Roots in orchids perform the same function as roots in other plants. They attach the plant to the ground and absorb water and nutrients from the soil. Roots in some orchid types are capable of performing photosynthesis, a rarity among plants. There are quite a few orchid genera which do not grow leaves at all as their function is performed solely by roots (Dendrophylax, Microcoelia, Polyradicon etc.) These orchids are slightly more delicate to grow since they have to be grown on tree bark where the roots are exposed to daylight. It is probably not necessary to point out that this causes the roots to dry out more easily, which can be avoided by sprinkling this part of the plant several times throughout the day. Phalaenopsis is among the most widely known orchid types where roots are capable of performing photosynthesis. Of course Phalaenopsis orchids grow leaves, therefore their roots serve only as additional generators of sugar. If Phalaenopsis orchids are grown in transparent pots they will be capable of performing photosynthesis through its roots, but placing them in decorative pots will not really affect the growth and development of your plant either.
Orchid roots are more complex than roots in other plants. Throughout the process of evolution they have adapted to humble living conditions high in tree canopies. Despite the regular precipitation and abundance of decaying organic material, their living environment is quite poor since water and nutrients are washed away very quickly from the tree. The plant replaces this shortage in many ways (e.g. fleshy leaves, pesudobulbs). One of the ways to adapt to such living conditions is velamen or spongy part of the roots. The thick multi-layered part of the faded outer root cells retains water in the area around the central vascular tissue of the roots. Growers should keep in mind that only the green tips of the roots extract humidity and nutrients while the rest of the surface is not active in obtaining these vital substances. When watering, it is however important to water the complete root area together with the velamen. The water from velamen will not penetrate into veins and the plant itself, it will, however, protect the plant from dehydration and water stress. Besides protective function, velamen performs another important task. Due to its specific structure it acts as an agent in the exchange of gases. If your orchid is in water it will quickly rot away, which will be the consequence of a non-transpirational velamen. The green tips of the roots absorb approximately 1 – 2.5 ml of water per day through the process of osmosis. Nutrients flow into the plant through an active transfer which requres energy from the plant.
The root tips are very sensitive to physical damage and will stop growing at the slightest injury. It is wise to avoid, as much as possible, physical contact with them and to be extremely careful when transplanting them to new pots. For orchids which need rest, roots are a very good indicator of the orchid’s active engagement. When an orchid starts to prepare for a rest period, due to low temperature and short days, the root growth will slow down and the green tips will be covered in a grey velamen layer. In spring, when the temperature increases, the roots will start to grow again, followed by the growth of vegetative offshoots – new leaves. Some orchids develop root hair on the surface called rhizoids, which is grown to ensure better fixation of the plant in some genera and in others it is active in absorbing humidity and nutrients. Rhizoids are even more sensitive to physical damage and will die away if the plant is not watered in time. There is one more adaptation to humidity shortage in orchids, which is worth mentioning. Despite the fact that orchids are mainly tropical plants, their photosynthesis is processed differently compared to other plants of tropical ecosystems. This type of photosynthesis is technically called CAM. These plants (mostly succulent plants found in desert areas) are more economical in handling water, by applying other mechanisms during their daily rhythm, surviving dry spells far better. This type of plant typically 'work' even during the night, since the synthesis of sugar is in process even when they receive no light. Therefore it is nothing wrong with watering the orchids in the evening, though you should be careful not to pour water over its leaves and flowers.
Different orchids require different substrate in which they will thrive. Orchids which grow in the ground in a moderate climate will grow in regular soil (Bletilla), while terrestrial orchids from tropical areas (some species in the genus Cymbidium and Paphiopedilum) require very porous yet rich substrate. Young plants and seedlings need finer substrate than adult plants. Clearly, there is no universally applicable substrate which would suit all orchids. If you think of yourself as an advanced orchid grower, you will easily be able to make your substrate from basic ingredients. Mixing your own substrate is easier than it may seem at first sight, with the only problem being that some materials are not readilly available everywhere. Below is a description of ingredients contained in a good substrate, which can not be said for most commercially sold substrates found in gardening centres.
Peat moss: also called Sphagnum is a classic compound in most substrates for orchids. Among other areas, peat moss also grows in Slovenia, but can today only be found in rare peat bogs. It is prohibited to pick up peat moss, as this would further endanger the plant and its disappearing habitat. Orchid society members and excursion-goers usually buy peat moss at commercial exhibitions in Europe, which Slovenian Orchid Society members visit at least once a year. Sphagnum, the best quality, can be found in New Zealand and Chile. Its characteristics are good water absorption and aeration. Phragmipediums and some other orchids are best grown in a substratum composed exclusively of sphagnum but, for most orchids, peat moss is mixed with tree bark which does not retain water very well. Peat moss can also be used in planting orchids on tree bark where the roots are wrapped in moss and then attached to large or small bark pieces. Other mosses which grow in the woods are definitely not adequate for orchid planting.
Shredded bark: the basic ingredient contained in most substrates is required for orchid growth. The most common bark found in these substrates is pine, while Americans will also use the bark of Pseudotsuga menziesii, known as Douglas fir. When buying tree bark, be sure to get the most expensive type to avoid wood particles in the bark. Shredded bark is sold in various fractions, from finely shredded (up to 25 mm in size), mid-sized (up to 40 mm), and roughly shredded (over 40 mm) particles. In most case, growers will use bark with fine shredded particles. For larger orchids (some species in Cattleyas, Angraecums etc.) and those which demand good aeration of roots (Vanda and its hybrids), growers also use the mid-sized and roughly shredded particle bark which can be torn in smaller pieces if necessary. It is essential that bark is soaked in water for a while and then washed at least once to get rid off tannins and fine dust particles, which are not good for tender orchid roots.
Peat and coconut fibre: supplements which improve water characteristics in substrates such as peat and fibrous husk, unlike bark, are able to retain some water. Growers can buy lumped peat sold for this purpose in markets abroad, which is much better than the ground or shredded peat available in Slovenia. Coconut fibre can retain less water than peat and is rarely found on the shelves of Slovenian stores.
Charcoal: charcoal is added due to its absorption capabilities. It can extract a number of waste substances from the substrate and, to a certain extent, also the nutrients contained in it, which is not desirable. Charcoal does not absorb much water and, similarly to shredded bark, it helps to airate the substrate.
Styro foam: some substrats for professional growers contain fine particles of styro foam which improve airation. Since these particles are completely inert (they do not decompose), styro foam is long-lasting and does not give out any nutrients as other ingredients in substrate do. Large pieces of styro foam can be placed at the bottom of the pot, which will provide excellent non-decomposing drainage material (small stones can also be used for this purpose).
Oher ingredients: some orchid growers add perlite, vermiculite, bits of decayed compost and similar material. These ingredients are used as special supplements in fertilising more demanding orchid types (Disa), but in most cases they will not be necessary.
Mixing different substrate types: a mixture of fine or mid-sized bark particles (depending on the orchid size) and sphagnum in a 4:1 ratio is recommended for most orchids. For Cymbidiums, Paphiopedilums, Zygopetalums and Ludisia, we recommend you add an additional part of rough peat or coconut fibre. If you have garden charcoal, use it as you feel, while styro foam can replace one part of the bark. As mentioned earlier, large pieces of styro foam can be used for the purpose of drainage which is very important. Drainage material can also be provided by using large pieces of bark or small stones. Masdevalias and Miltonias require a 2:1 ratio of shredded bark and sphagnum. Vandas, Ascocentrums, Arachnis, and similar natural and artificially bred types of orchids require only roughly shredded bark.