BACK TO BASICS - WATER AND ORCHIDS
Noel J. Grundon Atherton Qld
The most frequently asked-for articles by new growers are those on culture; these can be generalised as the “How to do something” article. As a new grower myself some 40 years ago, I enjoyed reading many such articles in the orchid journals of that time. When our editor asked if he could reprint an address I had given recently at an orchid gathering, he finally made up my mind for me to put down some of my experiences with growing orchids in the hope that they may fill part of the gap wanted by new growers. What follows are a series of articles, not in any structured order, on some of the basics that need to be considered in orchid culture. With a little chemistry, a little physics, and a little biology, I hope we can all arrive at a lot of common sense in culturing orchids.
A little chemistry and physics
Water is a colourless, odourless and tasteless liquid at normal temperature and pressure.
Water is the most common liquid on earth, and more substances dissolve in water than in any other liquid. This property is very important for biological systems because all biochemical reactions take place in solution.
Water is one of the few, perhaps the only, liquid that expands when it solidifies into ice at 0.4 oC. Therefore, ice is lighter than water and floats. If water did not have this property, all the fresh water would be as ice at the bottom of the oceans.
Water has a higher surface tension than most liquids. Think of this as the water molecules liking to stick close together. Hence, a drop of water is spherical in shape whereas a drop is alcohol is flat in shape because alcohol has a much lower surface tension than does water. In a thin column of water such as in a very small capillary tube, a tug on one end causes the whole column of water to move rather than the column to break. This property of surface tension becomes important in moving water around the plant.
Water has a high latent heat. This means that a lot of extra heat must be absorbed when ice melts and this heat must be given up again when water freezes. Similarly, when liquid water evaporates, it absorbs more heat that most other liquids. These properties can be used to cool or heat the environment of orchids.
Water - the essence of life
We can consider a plant cell as a water-filled sack containing substances in solution and solid structures such as the nucleus and mitochondria where certain chemical reactions can occur. Membranes surround this sack of water-solution and control the entry of water and other substances into and out of the cell. The whole lot if contained within solid walls that give structure to the plant but these walls have pores that allow adjacent cells to have contact with each other and exchange chemicals between them. So plants can be thought of as a lot of inter-connected water-filled sacks held together within permeable stiff walls.
It has been said that more orchids have been killed by an oversupply of water than by too little water. So it is important that we understand the role of water in the daily life of an orchid plant if we are to grow and flower that plant at its best. If we understand why we need to water an orchid plant, we can better understand how and when to water it. To understand why we must water plants, we need to understand the term relative humidity, and how plants use water in their life cycle to absorb nutrients and move nutrients from the roots to the shoots; that is we need to understand the processes of transpiration, nutrient uptake and nutrient translocation.
Water and relative humidity
Plants must loose water to the atmosphere in the process of transpiration to function correctly. If the air is saturated with water at 100% relative humidity, plants cannot transpire. So maintaining too high a relative humidity is not an advantage for the plant. Likewise, too low a relative humidity is not an advantage because the plant may loose water to the atmosphere more rapidly than it can absorb more water from the compost. Under these conditions, the plant can quickly become desiccated and die.
Because plants must loose water from their leaves or transpire if they are to function correctly, we need to consider the process of transpiration and its relationship to plant growth.
Water and transpiration
Inside the plant root and stem, there is a continuous ‘pipe’ of dead cells called the xylem. These cells act as a series of interconnected very small capillary tubes that the plant uses to transport water and dissolved substances from the root to the leaves. Think of the cells within the leaf as being bathed in a continuous film of water. On the underside of the orchid leaf there are small pores called stomata that open and shut in response to light, carbon dioxide concentration of the atmosphere, and the water relationships of the plant. When the stomata open, water evaporates from the film covering the cells within the leaf. Because water molecules like to stick together, the surface tension of water causes a potential to build up inside the leaf for water to move towards the place from which water is being lost. Think of this potential as a force pulling water towards the leaf through the small capillary tubes of the xylem in the leaf, stem and roots. So the evaporation of water from the surface of cells within the leaf the process called transpiration pulls water up the roots and stem and into the leaf. Thus, water moves from a location of high water potential [think of it as a high water concentration or high energy level] in the roots to an area of lower water potential in the leaves because the surface tension of water makes it want to come to the same potential. Therefore, to get water from the compost into the orchid plant, we make use of the high surface tension of water.
Another function of transpiration is to cool the plant. The leaves could become very hot if it were not for the loss of water by evaporation through the stomata. This is where the high latent heat of water becomes important to plants. As water evaporates, it absorbs more heat per unit volume than most other liquids and this heat is taken from the leaves as the water evaporates during transpiration. As well as using the property of the high latent heat of water to cool our plants, we can make use of this property of water to also warm our plants during cool nights. By placing bottles of water amongst the plants during the day, the water is warmed. This heat can be returned to the atmosphere, as the air becomes colder during the night. It may be only a few degrees that we are talking about, but that may be all that is needed to keep the plants growing better.
Water and nutrient uptake
Water and nutrient translocation
Hence, the supply of water and fertilisers is linked. It has been said that water is the vehicle of good fertilising practice. Although little research has been undertaken on the specific fertiliser needs of orchids, we can assume that because orchids are plants, they will behave like all other plants. Therefore, we can make the following assumptions:
Orchids require the same nutrient elements for healthy growth that other plants need.
Orchids can only absorb these nutrient elements when the nutrients are dissolved in water.
The roots of most orchids cannot function when they are starved for oxygen in waterlogged compost.
One of the aims of watering an orchid is to provide the water needed for growth, and to provide the vehicle for dissolving the nutrient elements the plant uses in its cycle of life. Providing sufficient water and nutrient elements for healthy growth and flowering is part of the art of good culture.
A plant held continuously in an atmosphere of 100% relative humidity would not loose water to the atmosphere. Thus, it will be unable to transpire and so would slowly die from lack of nutrients because its transpiration stream would be virtually closed down permanently. Therefore low relative humidity is not altogether a bad thing, provided the plant can obtain sufficient supplies of water from the compost to replace the water lost in the transpiration stream.
"This article first appeared in Orchids Australia.
BACK TO BASICS - WATER AND ORCHIDS
Water first or fertiliser first?
Much good advice says “never fertilise a dry compost; water it first, and add the fertiliser solution second.”
I don’t think this makes sense! Consider an orchid root. Most orchids have roots that are adapted to a dry environment. Only the central cylinder is alive; the outer layers are made up of dead cells. These dead cells act like a sponge; they absorb water and any dissolved nutrients as soon as the water solution passes over them. And like any sponge, when they are full of water, no more water can be absorbed. So when you water first, you fill up the sponge with water. Then when you add the fertiliser solution, none of the fertiliser nutrients can get into the sponge until the plant has absorbed water from the sponge layer. To me, it makes more sense to fill up the spongy outer layer with fertiliser solution first.
When should I water?
For example, did you water the orchid because the compost was dry and you did not want the orchid to dehydrate? Correct - go ahead and water the compost, even if the orchid is in its resting season. If the orchid needs to absorb water to prevent dehydration, it will need to have water applied to its roots. Even orchid roots, which do not have growing root tips, can still absorb water. Most of the water need by an orchid plant must be absorbed through the roots; very little water can be absorbed through the leaves and stems that have special waxy coatings that prevent loss and absorption of water.
Did you water the orchid and compost because the temperature was too high and you wanted to cool the house down? Wrong - water the house, walls, and floor, not the orchid! Mist or very lightly water the orchid plant to cool it if necessary, but do not water the compost. There is no need to water the compost if it is already moist enough to provide the orchid plant with all its water needs. To water the compost at this time might cause poorer growth from over-watering.
Did you water the orchid plant and compost to increase the humidity of the atmosphere and so prevent excessive transpiration or water loss? Correct - but only if the compost is dry and the orchid plant needs water to prevent dehydration. However, it would be wrong to water the compost when the compost is already moist enough and the orchid needs no water; in this case apply water the floor and walls of the house instead.
When you decide to increase the humidity of the orchid house to reduce the rate of transpiration or water loss from the orchid plant, keep in mind that increasing the humidity of the atmosphere will benefit the orchid only if the humidity is high when the orchid opens its stomata or pores; while the stomata are closed, the orchid plant looses very little water by transpiration. But when do orchids open their stomata? The thin-leafed genera such as varicosum Oncidium, Lycaste, Odontoglossum, and Paphiopedilum apparently open their stomata during daylight hours. Hence, for these genera, it is important not to let the humidity of the air get too low at all times. However, some the genera open their stomata only during the evening or night. These genera can undertake what is called the `CAM-photosynthetic pathway' and characteristically have very thick leaves. Hence, the humidity must be high during the evening and at night for genera such as Cattleya, Vanda, Dendrobium, and certain species of the Cymbidium genus. Because these genera do not open their stomata during the day, it really does not matter to them what the daytime humidity is like.
Frequency of watering
With epiphytic genera such as Dendrobium, Oncidium and members of the Vanda and Cattleya alliances, the compost can be allowed to become dry before they need to be watered, but the more terrestrial genera such as Paphiopedilum need watering just before the compost becomes dry.
If the compost is close or fine (i.e. with ample peat moss), you will have to water less frequently than if you have a coarser compost (i.e. with large bark, charcoal or gravel). If the plants are large for the pot size, they will use up the water more quickly than will small plants in the same sized pots, and may need more frequent watering. If the temperature is hot, or if there are drying winds, watering will need to be done more frequently.
We often read in books that such and such genera need a resting period if they are to flower at their best. During this resting phase, total withholding of water is often recommended. However, at other times, a light misting is sometimes recommended. Plants that require this resting phase often grow in monsoonal climates that are characterised by warm wet summers and cooler dry winters. Even when very little rain falls for a 6-month period, scattered showers still occur, and the relative humidity is high, especially during the early morning. Thus, even these resting plants may benefit from a light misting of their leaves and the floor to maintain a high humidity and to prevent too much desiccation of the plant during the resting phase.
How much water to apply?
By supplying excess water at each watering, you make sure that the compost is completely wet-up and contains sufficient water to last the plant until the next watering. By providing enough water to cause it to drip freely from the drainage holes of the pot, you ensure that unused fertiliser does not build up in the compost and cause damage to roots or leaves.
Good compost is composed of a solid phase, a solution phase, and a gaseous phase. The solid phase is the components of the mix, such as bark, gravel, peat moss, and the like. The solution phase is the water and dissolved fertiliser nutrients. The gaseous phase is the air from which the roots will absorb oxygen. Before watering, the compost will be mainly the solid phase and the gaseous phase. During watering, water enters the pores between the solid phase components. After watering, water drains from the larger pores, allowing air to enter the compost. Within an hour after watering, all three phases will exist within good compost.
However, if the compost is too fine, there may be no pores large enough to drain free of water and allow entry of air. The roots in such compost will be starved for oxygen, and will be unable to function fully. Such compost is said to be waterlogged. If you suspect that your compost is not draining enough to allow entry of air, open it up by using less peat moss or fine sand, and more coarse bark or small gravel. Remember, it is easier to water more frequently with a slightly coarser compost than to try to remove water from a waterlogged compost.
Does the compost or the bushhouse need the water?
Am I trying to decrease the air temperature, increase the humidity of the atmosphere, or do I need to water the compost to prevent the orchid dehydrating?
Is this the right time of the day or night to be applying the water?
Is the compost moist enough already, or should I leave the watering for tomorrow, or, worse still, should I have done it yesterday?
How much water should I apply?
Will I waterlog the compost if I water now?
By the time you have answered these questions, perhaps you will feel like putting the hose away, having a `cuppa', and meditating on the problems of watering orchids correctly. And perhaps your orchid might grow better as a result.
It is difficult to kill an orchid, but it is more difficult to grow it well. Part of the answer to an orchid that is growing well is to firstly water it correctly.
"This article first appeared in Orchids Australia.