BACK TO BASICS – WATER AND ORCHIDS – (Part Two)

BACK TO BASICS – WATER AND ORCHIDS – (Part Two) St Ives Orchid Fair

Water first or fertiliser first?
Now we come to the question of, “Do we water first then add the fertiliser solution; or do we add the fertiliser solution first?” Put another way, “Should we add fertiliser to the dry compost, or water the compost 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?
It is important that plants transpire or loose water for the reasons outlined in the previous section, but it is equally important that the plants do not transpire too much water or they will become dehydrated and growth will slow down. Hence, we need to supply water to orchids for reasons of water needed for transpiration and nutrient supply. So, why, when, and how do we water the orchid? And did we need to water it today? These sound like stupid questions – but pause and think for a moment! The answer could determine if you needed to apply water to the compost or to the floor of the orchid house instead!!

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
How often you water will be determined by many factors including the size of the pot, the size of the plant, the closeness or fineness of the compost, and the prevailing weather conditions. Most epiphytic orchids are plants that have many adaptations for surviving in very droughty environments. They have a number of adaptations that assist their survival, such as thickened leaves, thick waxy cuticles on the surface of their leaves, and stems that store water. However, the more terrestrial genera have thin leaves, thinner cuticles, and often have no water storage organs, and are not as well adapted to withstanding extended dry periods during their period of active growth.

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?
Each watering should provide sufficient water to wet all the compost in the pot. That is, watering should continue until water runs freely from the drainage holes of the pot. If you are watering by hand, it is easy to provide just the correct amount of water to each individual pot, but remember that the compost will absorb some of the water and it is a good idea to go back and rewater the pots after about 15 – 30 minutes to make sure all the compost is wet. If you have a timing device to water your plants, ensure that the water is not turned off until water is dripping freely from the drainage holes of the pots.

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.

Water logging
The compost is where the roots live. To live and function correctly, roots need a source of energy that is provided by converting sugars such as sucrose into carbon dioxide and water. The sugars are transported down from the shoot. However, oxygen is needed to convert the sugars into carbon dioxide and water, and this oxygen is absorbed directly by the root from the air in the compost.

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.

Over-watering
It is often said that more plants are killed from over-watering than from under-watering. However, the real killer is waterlogging, and the real problem is not over-watering but too fine a compost. You cannot over-water a compost which has been correctly designed with a graded size of components so that water will drain from the larger pores and allow entry of air within an hour of watering. Your compost must be sufficiently coarse that it will drain and allow entry of air even when it rains all day. `Over-watering’ is a symptom of a poorly designed compost. Change the compost! Do not blame the water!!

Water quality
Most orchid growers do not need to worry about water quality. By water quality, I refer to the amounts of dissolved salts, especially sodium chloride, that are in the water. Most town or city water supplies contain low or acceptable levels of dissolved salts. However, some growers, especially those using bore water may need to check the quality of their water if they are experiencing difficulty in growing their orchids. From my own experience, I know that water containing more than 100 parts per million (ppm or micrograms per litre) sodium can kill Cymbidium, Oncidium, Odontoglossum, Lycaste, and many Australian native orchids. Especially sensitive are the terrestrial genera, the epiphytic Dendrobiums from inland north Queensland (e.g. Dendrobium semifuscum, D. fleckeri, D. adae, D. agrostophyllum), all Bulbophyllums, and epiphytic members of the Sarcanthineae. The lithophytic species such as D. kingianum, D. speciosum, Sarcochilus ceciliae, S. hartmannii, and S. roseus, and the epiphytic species from coastal habitats, such as D. discolor, appear to be more tolerant of poor water quality.

Conclusions
Watering orchids is more than just grabbing the hose and splashing a bit about. So the next time you pick up the hose, ask yourself a few questions first.

  • Does the compost or the bush house 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 water log 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.

Back To Part One Here

“This article first appeared in Orchids Australia.
Reprinted with permission of Orchids Australia and the author, Noel Grundon.”

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