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This is part of a series intended to introduce the principles of basic orchid
culture. The series originates on a number of articles I have written over a number of years. I hope
that it will answer a number of questions often raised on these matters.
Introduction
Many people are growing a selection of orchids which come from diverse natural habitats. It is the aim of growers
to re-create the plants natural conditions in their glasshouses as far as is practically possible in order that
maximum growth and flowering can be obtained. It is of assistance to consider individually the factors which are
essential for plant growth to take place when environmental factors relevant to the particular plant are being
considered. A break-up of growth conditions to individual factors can also assist in providing a means of analysis
if a plant or group of plants is not achieving the growth expected. Obviously the specific requirements of all
plants cannot be met artificially, and some genera are more tolerant of conditions different from those experienced
naturally than others. Conditions in even a small glasshouse can, however, vary quite markedly from one part or
another, and it is often possible to select a position that more closely recreates the requirements of an individual
specimen, if care it taken. An understanding of the requirements of plant growth can assist in this management.
GROWTH AND DEVELOPMENT
Growth of a plant involves as increase in size, whereas development represents a change in the
type of growth.
During the spring we are all familiar with the new shoots appearing, and increasing in size over subsequent
months.. At a certain stage this vegetative growth will slow down, and development takes place which involves flowering
and fruiting. A plants requirements can vary during the stage of dormancy (during winter) growth and development
and certain environmental factors are involved in the control of the various stages. A basis understanding of this
can assist in obtaining of maximum results from the plant involved.
FACTORS ESSENTIAL FOR PLANT GROWTH AND DEVELOPMENT
- Adequate temperatures
- Adequate moisture
- Sufficient light
- Adequate air (oxygen and carbon dioxide present is suitable amounts)
- A supply of the required mineral salts.
These factors will be considered individually
A TEMPERATURE
This is commonly described as the master factor of plant growth, although moisture and light levels are so closely
inter-woven it is often difficult to separate them entirely. Certain plants require a relatively narrow range of
temperatures for optimum growth, and if this is not provided, difficulties can occur. Other plants can, however,
withstand substantial variations although aspects of growth may be affected. The seasonal variation in temperatures
will also affect growth, as there will generally be a period when temperatures are conducive to maximum growth.
Seasonal variation is also important in regulating plant development, eg warm spring conditions will bring about
vegetative growth, with often cooler autumn temperatures are 0necessary to initiate flowering, although other factors
are also involved.
Temperature variations throughout the day and night, and associated relationship with humidity (air movement)
also form an essential, although complex, characteristic of a particular environment.
B MOISTURE
This is a factor of major significance in setting plant growth. Moisture involves not only water freely present
in the growing media permeated by the roots, but also water vapour present in the air. As previously noted, this
factor is closely related to temperature, variations in temperature significantly affect the amount of water vapour
present in the atmosphere.
The supply of water in whatever form can be subject to seasonal variation, and this aspect often has an important
bearing on both plant growth and development. Flowering can be initiated by changes in water available – for example
the onset of the seasonal wet or dry season can initiate flower production.
C LIGHT
The effect of light on plant growth and development covers a number of areas
- The level of photosynthesis carried out
- Affects the direction and proportions of growth
- Heat effects
- Effects of alteration of light and dark periods on the life cycle of the plant
It will be well known that ‘white’ light is a blending of light of different colours or wave lengths. The different
portions of light spectrum have different effects on plant growth. The red/orange/yellow section is most concerned
with photosynthesis; the blue/ultra-violet section affecting the direct and proportion of growth, the red wave
lengths most significantly affecting temperatures.
Light of the shorter wave lengths ie. ultraviolet is more easily filtered out of daylight by clouds mist or
glass than light of the red/infa-red sector. In different situations therefore ‘daylight’ in fact can comprise
different proportions of the colours or wave lengths of light. In high mountains the light has a relatively high
ultraviolet content, as less of the UV portion of the spectrum has been filtered out by the dusty air. Plants in
the higher altitudes are generally more compact in growth. Plants grown in glasshouses will tend to be more ‘leggy’
because the glass keeps out a greater proportion of the UV light.
In a glasshouse the heating effect of the sun arises because the direct sunlight is able to penetrate the glass,
but the reflected heat of a different wave length being unable to pass through, resulting in the heat being retained
increasing the internal glasshouse temperature.
It is apparent from the above that the materials used in glasshouse construction can have a significant effect
on the conditions created inside the structure. Therefore, the colour and nature of the cladding materials used
must be carefully considered. It is worth noting at many of the plastic materials available have different light
transmission characteristics which can affect plant growth, and particular care must be exercised to ensure that
satisfactory results can be obtained from the materials chosen.
The intensity (or brightness) of light will have a considerable effect on the rate at which the plant can complete
the photosynthetic process. Generally the greater the light, the faster photosynthesis takes place, although if
in its natural habitat the plant is conditioned to weak light, brighter light will not increase the rate of growth.
It will be well known that plants manufacture their own food for immediate growth, or stored for future use.
Green leaves are able to utilise the suns energy, transforming this by means of the green chlorophyll (which is
mainly in the plants leaves), producing food. Photosynthesis combines carbon dioxide from the air with water to
from sugars, oxygen being given off as a by-product. This essential process can be affected by the amount and quality
of light reaching the plant, as is therefore a significant factor influencing the amount of growth and flower production
taking place.
Light can also influence the direction and proportions of plant growth, Parts of a normal plant will grow towards
the major light source – ie. the stem will grow upwards. Other parts, such as the roots, will always grow away
from the light. Under weak light conditions, the leaves will generally be long and flaccid, but with bright conditions
smaller parts will be produced, generally of a more robust structure. Where excessive light with regard to a plants
natural requirements will often cause the leaves and stems to take on a red pigmentation in order to protect the
underlying sensitive photosynthetic cells.
It is worth noting that generally there will be greater heat where strong light is involved, as these two factors
are closely associated. Thus glasshouse is shaded in the summer, not only to reduce the light levels, but also,
and often more importantly, to reduce the heat.
Growth and flowering of most plants is greatly affected by changing the relative periods of daylight and darkness.
The seasonal change of day and night periods affects flower bud initiation when associated with seasonal temperature
variations. When artificial light is used t o increase growth of small plants care must be taken that mature plants
are not affected, as the normal glowering sequence can be interfered with. This fact is however sometimes utilised
by commercial growers to control the flowering times of plants, even to the extent of producing flowers outside
of their natural flowering season.
D AIR
There must be adequate supplies of fresh air to enable photosynthesis and plant respiration to be completed.
Related to air supply is humidity, which can be critical fro many plants. Fresh and moving air can also be essential
with regard to ensuring plant health by helping create conditions under which diseases have difficult in becoming
established. Many bacterial and fungal conditions will become readily established under still stale air, especially
if humidity is high and temperatures are not at optimum levels for the plant involved.
E MINERAL SALTS
The adequacy of supply of mineral elements is also an essential factor affecting plant growth. Fertilisers are
involved in ensuring this aspect is met satisfactorily, and readers are referred to the Fertiliser article on this
site for greater detail. It is, however, worth noting that if has been shown that 16 elements are required in significant
amounts for growth to take place. Nine elements are required in significant amounts, Carbon, hydrogen and oxygen
are generally readily available from air and water. The six elements – nitrogen, phosphorus, potash, sulphur, calcium
and magnesium, are, however, often present in less that adequate amounts, and for optimum growth many have to be
specially supplied. There are quantities, which are usually available naturally, or made available when the more
significant fertilisers are being supplied, These elements are iron, manganese, boron, zinc, copper, molybdenum
and chlorine. The absence or shortage of any one element will affect plant growth, and actual influence being discussed
in greater detail later.
It is appropriate to note that while the factors essential for plant growth and development have been considered
individually, there is often a close and complex relationship between them in the natural environment. If one factor
is being altered artificially it is important to ensure that detrimental effects are not being created elsewhere.
WHAT CONTROLS PLANT GROWTH
We have considered the factors essential for plant growth and development to take place. From a practical pint
of view it is appropriate to consider what controls the size and production obtained from any plant.
You may think that the areas of your culture covered by the above factors that you complete best will control
your growing success. In fact this is not the case. It is the areas which you proved that least acceptable conditions
for a plant that will influence the results obtained. The position can be summarised as follows:-
- It is the factor of growth present in the minimum amount in relation to the plants requirements of each element
that controls the growth achieved.
- Growth rises and falls depending on whether this growth limiting factor is increased or decreased.
This can be illustrated by an example that well portrays the principles involved. If everything else is provided
in the amount and form required, but water is in short supply, then it is that lack of this water that will restrict
the growth. It matters little at all the other elements are present as required. If no moisture is available.,
the plant will die. If only a very small amount is available some growth will occur, but the plants full potential
will not be obtained. This potential will only be achieved when the plants water requirements are met, and all
other factors are present to the levels required.
The process which controls plant growth can be illustrated in the above diagram. As previously noted there are
five individual essential factors which must be present for plant growth and development to fake place. It will
be apparent that a plants requirements flor each factor will vary in total. What we are concerned about is the
amount of each factor that is available measured against the plants total requirements for that factor. It is therefore
often convenient to consider the supply of each as a percentage of the amount necessary for unrestricted growth.
In the diagram, of the five factors, only two - the supply of light (100% of the plants needs) and air (100%)
– fully meet the plants requirements. Three factors are in short supply. While temperature (50%) and minerals (75%)
are not present in the optimum quantities, three are not at this stage affecting plants. The plant will only growth
th the extent allowed by the shortage of water (25% of total needs) as at that level of growth there are adequate
levels of temperature and minerals. If the water supply is increased or decreased , which will allow variations
in growth to occur, although only while this factor remains the one least available in relation to the plants total
requirements.
To extend the example, if the water supply is increased to the amount required for optimum growth (100%) , then
the plant will increase growth to the level then allowed by the next restricting factor, in this case the temperature
conditions that exist at 50% of the plants requirements. Full potential growth and production levels will not be
achieved until ALL five factors are available at 100% of requirement.
It is relevant to note that the above principles apply also to EACH of the essential mineral elements. We are
not just talking of the supply of a total amount of mineral salt, but to the supply of each individual element
in relation to the plants requirements for each element.
CONCLUSION
It will be apparent from the discussion that, if optimum results from growing
your plants are to be obtained, all aspects of your culture must be maintained at satisfactorily levels. It is
no use concentrating on one or two areas are neglecting others as you would them be disappointed with the final
results obtained. The provision of optimum conditions is a challenge as complex processes are involved, but if
individual elements are considered and every effort made to meet the plants specific requirements, there is more
chance that the results so eagerly sought will be obtained.
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