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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.
Further material
To supplement the above discussion, further relevant material on this
site can be found as follows:-
Fertilisers
Plant habitats
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tomnz@ihug.co.nz
