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"TIPS AND TRUTHS REGARDING LIGHT AND THE GROWING OF ORCHIDS" Author:
Dénis van Rensburg (Tel:
012-803 3336) Pretoria A.
THE PRINCIPLE OF
LIMITING FACTORS Whenever
you have less than perfect growing conditions your plants will be under stress
and respond with less than maximum growth.
We are involved here with what is called in biological circles as the
"Principle of Limiting Factors".
This idea may be explained by saying that whenever any process such as
growth is controlled by a variety of factors, any one of them can be limiting at
a given time. At a certain time it
may be the amount of light, at another, the temperature etc. Whenever the balance between the factors is not optimal the
plant is stressed. If the stress is
too great, it may die, or alternatively produce smaller growths, fail to flower,
produce fewer or poorer flowers, lose leaves, form blind sheaths or simply rot. As
a rule we create stress by doing things backwards.
We usually have the bottoms of our orchid plants too wet and the tops too
dry! Remember;
in nature the plant is surrounded by high humidity and the roots dry out
quickly. B.
EFFECT OF LIGHT ON THE GROWING OF ORCHIDS Green
plants possess the amazing ability to grow and manufacture their own food
through the process of photosynthesis. Photosynthesis
can be defined as the process through which plants manufacture food using the
energy of sunlight and this process is often expressed by the simple chemical
equation depicted in Figure 1 Figure 1:
+ PHOTOSYNTHESIS CO2
+ H2O + Chlorophyll
RESPIRATION
Green
plants have the unique ability to utilize the gaseous carbon dioxide (CO2)
of the atmosphere plus water (H2O) in the presence of an energy
source (sunlight) and the chlorophyll in the leaves to produce glucose (a kind
of sugar that is a basic food for the plant) and give off oxygen into the
atmosphere as a byproduct. It is
important to note that plants do not "eat" like animals or humans,
they manufacture their own food. (Hence
we cannot "feed" out plants!) Plants
manufacture their food beginning with the glucose produced during photosynthesis
and adding the mineral salts in fertilizers (or the naturally occurring mineral
salts in the environment) to complete the manufacturing of food, fabricate plant
parts and to perform all metabolic processes.
A critical distinction between food and mineral nutrients required by
both plants and animals is that food provides energy minerals do not. The
leaf is the sugar or energy producing factory for the plant and like any factory
there are many needs or inputs that are required, in the correct form and at the
correct time, in order for the factory to put out an acceptable product
(limiting factors). Carbon dioxide
and water are major nutrients in our plant factory and enter the plant through
"holes" or stomata on the underside of the leaves amongst other
structures like roots (Orchids have about
3500 stomata per cm2). If the stomata are open the factory is working.
The stomata open and close as required by environmental conditions within
and external to the plant. The loss
of water vapor via stomata is called transpiration.
Although the above is true for most plants many orchids are CAM plants
and absorb and store their carbon dioxide at night, closing the stomata during
the day to prevent water loss, and their “factories” therefore work when the
stomata are closed. When
plants are placed in a CO2 free atmosphere the stomata tend to stay
open, even in the dark. The
converse is also true. If the plant
is placed in a CO2 rich atmosphere the stomata may close even in the
presence of light. (Exceptions
exist especially in forest floor tropical plants.) Angela
van Rooyen shared some useful information:
"One is always tempted to give your orchids a spray of lovely cool
water or open the sprinklers when very hot "to cool them off".
IT has taken us many years to learn that we are actually causing more
harm than helping plants. If leaves
are wet the stomata on the undersides of the leaves close and photosynthesis
will stop. This can cause the leaf
to overheat as it cannot cool itself by the normal process of evaporation
(transpiration) and thus growth is slowed down."
This statement might not be fully correct for CAM orchids but never the
less useful for information for all non CAM plants. (On
really hot summer days, it is best to water in the early morning or late
afternoon as cold water on hot plants is such a shock to the plant that it may
stop growing for a few hours.) Light,
apart from the production of food, has other effects on the plant, namely, the
production of heat. This occurs within the tissues of the plant, as well as
within the greenhouse which is really an infra-red trap. Each plant, according to its origins in nature, on low
savannah or high cloud forest, grows best within a particular temperature range.
Exceeding this range can slow down the rate of metabolism and growth or,
in other words, produce a stress that prevents maximum results.
So, while increasing the light for photosynthesis, be careful to control
heat buildup. If
the temperature of the air near your orchid is measured, say in full sunlight,
and then compared to the actual leaf temperature of the orchid, the latter will
be found to be at least 3°C warmer than the former, if there is no air
movement. The
heat also causes an increased water loss from the leaves so that they become
collapsed, soft and pliable, and the bulbs become wrinkled and ridged.
One immediately proceeds, as an average response, to over water the
plants to make up for this obvious dehydration, only to kill the roots and
worsen the total picture. Decrease
the heat by 1.
Increasing air movement 2.
Diffuse the light 3.
Increase humidity to prevent excess water loss; don’t over water the
roots Air
movement is beneficial for three reasons: 1.
To cool the leaves 2.
The thin film of air next to the leaves can very rapidly become depleted
of carbon dioxide and constantly replacing that air with a cool and refreshing
breeze will help maintain a higher rate of photosynthesis 3.
Those fungi and bacteria that cause dreaded rots, adore warm stagnant
air. Remember
Orchids grow slowly and they die the same way, so fortunately, you usually have
an opportunity to change whatever is not optimal and alter their deterioration. Carbon
dioxide and water are the two major nutrients of plants and for the plant
factory to convert these nutrients, sunlight (or the energy that is contained in
sunlight) is absolutely necessary. But
sunlight contains many different wavelengths or types of energy.
Just like a radio or TV set that only "works" when it is tuned
to a certain wavelength (or energy packet), so do plants only react to certain
wavelengths and use that specific energy to manufacture food through
photosynthesis. All the other light
(wavelengths) are totally useless to the plant and may even be harmful. Because
oxygen is produced when plants photosynthesize it is fairly easy to determine
which wavelengths present in light are indeed preferred by the plant.
All you need to do is to expose a plant to each different wavelength and
measure the oxygen produced. If no
oxygen is produced that specific wavelength has no effect on the chemistry of
the plant. Photosynthesis seems to
be very high in the "blue" and "red" wavelength ranges and
much less effective in the "green" region, and absent in all the other
wavelengths regions. When
light falls on our orchids it is not the intensity (number of energy packets)
that matters but rather whether the correct wavelengths (correct energy
packet) for absorption are present. If
the light is not absorbed by the plant it cannot be used to manufacture food In
their natural habitats, plants are subjected to a day and night temperature
differential and it is important to imitate this as far as possible.
Photosynthesis is affected by temperature as well as light intensity. As long as it doesn't become too hot, the rate of
photosynthesis and hence growth increases with rising temperature, but only as
long as there is sufficient light (and carbon dioxide).
Not only is there no point in increasing the temperature of the
greenhouse beyond 25°C if there isn't sufficient light it is positively
harmful, as the rate of photosynthesis will fall below the rate of
transpiration, even at 25°C. Note: Ultraviolet radiation has much more energy per packet (or
quantum) and the energy is indeed enough to initiate chemical reactions in
paint, plastics (e.g. plant containers etc).
This is why plastics exposed to sunlight need some form of UV inhibition
(filters) otherwise they will deteriorate rapidly. It is this ultraviolet radiation that also causes most damage
to our plants. A.
EFFECT OF GREEN LIGHT AND NEAR UV LIGHT Many
studies have been done on the effects of near UV and green light on plant
growth. The conclusions reached are
that the near UV and green wavelengths are capable of suppressing
the growth of plants which otherwise receive adequate levels of those
wavelengths necessary for photosynthesis and normal development.
Conversely the selective removal of near UV and green wavelengths
from white light gave enhanced growth. From
these scientific experiments it is evident that green shade cloth should as a
matter of routine not be used as a covering in shade houses.
Due to the
total reflectance/transparency of white shade cloth types a higher than
normal shade percentage should be used when the shade cloth is white and not
the usual black. B.
EFFECT OF DAY-LENGTH ON ORCHID GROWTH Most
of us are used to days being longer and warmer in summer.
This difference between summer and winter becomes more pronounced the
further one gets from the Equator. At
the Equator the length of day is always the same, although the amount of solar
energy delivered in a day does vary, being greatest at the spring and autumn
equinoxes. The
power of the sun never reaches more than about 90% of equatorial levels in South
Africa, but because our days are longer the solar energy delivered in a day
exceeds equatorial levels from mid-October to mid-December.
However on a midwinter's day the solar energy delivered is only about 30%
of that at the equator. We
must remember that our plants need a certain amount of solar energy to be able
to grow. Many plants native to
South Africa, including some of our orchids, give up the unequal struggle, shed
their leaves (which have little to do) and hibernate.
Many of the orchids we grow were originally imported from the tropics and
have not learnt this trick. If they
are kept too warm while there is not enough light to be able to photosynthesize
adequately, the result is soft, spindly growth.
Some plants, for example some New Guinea dendrobiums, which are used to
growing and flowering for 12 months of the year in their native habitat, can do
quite badly during our dark winter days. Many
of the plants we grow, especially the popular cooler growing ones, come from the
limits of the tropics, 20-30° from the Equator.
Here the day length varies between 10.5 and 13.5 hours and the daily
solar energy gains by 45% over the year. There
are marked seasons and many of the plants are synchronized with this, growing at
one time and perhaps flowering at another.
This seasonality is important. Most
cymbidiums need long bright days to initiate flower spikes, which then flower in
the shorter days. Angraecum sesquipedale is another plant that flowers during short
days, coinciding with the drier season in its home in Madagascar. C.
MEASURING THE BRIGHTNESS OF LIGHT Having
looked mainly at the duration of light, there is also a question of how bright
should the light be? The human eye can adapt to a huge range in brightness which
has many advantages, but unfortunately it means we are very poor at judging how
bright light is. It is therefore
necessary to resort to instruments and technical terms. The
amount of light illuminating a surface (and thus available for photosynthesis if
the surface is a plant leaf) is measured in lux.
The total amount of light leaving a source of light, e.g. a lamp, is
measured in lumens. One lux is one
lumen per square meter. Since we
are interested in plants photosynthesizing properly, we are mainly interested in
lux, but when buying artificial lighting, lamps may be rated in lumens. Lumens are the best measure of how much light a lamp
produces. One lux is not much
light, so thousands of lux (kilo-lux, abbreviated klx) may be used.
For those hankering after candle-power, do not, candles are unsuitable
for plant cultivation (1 klx = 92.9 foot candles i.e. the illumination produced
by 92.9 candles with their flames all at the same point, a foot (304.8mm) away
from the surface to be illuminated). D.
HOW MUCH LIGHT DO PLANTS NEED? Plants
vary in the amount of light they need and they are adapted to different
ecological niches. A plant adapted
to low light levels, such as may be found on the forest floor where there is a
reliable source of water and nutrients will absorb so much light, if exposed to
full sunlight, that the leaves will be scorched by the energy absorbed.
Conversely, a plant adapted to the high light levels in the forest canopy
will not be able to capture enough energy (light) to thrive and flower, if kept
in conditions that are too shaded. Table
1 gives a summary of the light levels required by our main orchid families. Table
1: THE
AMERICAN ORCHID SOCIETY HAS SUGGESTED THE FOLLOWING LIGHT LEVELS: NB:
Full sun on a clear day = 10 000 foot candles = 100 klx
PS.
100 klx = full sun
80 klx = 20% shade cloth
60 klx = 40% shade cloth etc. Interestingly,
recent articles on the optimal growing of Phalaenopsis
orchids suggest that 10 to 15 klx is the optimal light intensity with photo
inhibition occurring above 20 klx. (Today,
Phalaenopsis orchids are the most
widely grown orchid. Statistical
data from the Netherlands show that from 1983 to 1994 the number of potted Phalaenopsis
sold through the auction at Aalsmeer steadily increased from 50,000 to 3,150,000
plants. In 1983 Phalaenopsis
accounted for less than 5% of the market while in 1994 they accounted for over
66% of the market. Large scale
potted Phalaenopsis production is
taking place in China, Germany, Japan, the Netherlands, Taiwan and the United
States) If
a plant is receiving too little light, it may compensate by producing more light
absorbing pigment – the leaves go darker green.
It may also grow thin and leggy, trying to find light.
It will also probably put flowering on hold until conditions are more
suitable. Conversely, if a plant is
receiving too much light it may try to adapt by reducing the amount of pigment
– the leaves go paler, or it may produce a light-absorbing non-photosynthetic
pigment, a natural sun-block, and the leaves go red.
If the amount of light is really excessive, the resulting energy input
destroys tissue and the leaves are scorched. A.
EFFECT OF LIGHT ON FLOWER COLOUR It
is generally accepted that the colour of flowers is rarely influenced (or
changed) by geographical and/or seasonal influences.
Flower colour can change markedly with light intensity and the presence
or absence of sucrose (sugar). Flowers
that open in low light are less intensely coloured than flowers that open in
greater light intensities. As a
general rule, low light intensity reduces the petal pigmentation up to a certain
extent. Good fertilization and the
presence of sucrose will also stimulate the production of anthocyanin which is
the main source of purple, red and pink colours in plants. (A lack of anthocyanin pigmentation in a plant gives rise to
albino flowers. Remember; although
an albino plant (flower) may lack anthocyanins it may still contain chlorophyll
and carotenoids (yellow pigments). Low
light intensities are known to reduce flower colour; as a rule a 25% decrease in
light intensity gave rise to a 30% reduction in concentration of anthocyanin and
a 40% reduction in colour intensity.
A good example of anthocyanin production under the influence of light is
found in red pigmented apples where there are often green areas that have been
shaded by leaves etc. To
get more intense coloured flowers try to give all bright or dark-coloured
flowering plants brighter light conditions.
However, to get better pink, yellow, orange or green flowers a little
more shade is needed. B.
OTHER EFFECTS OF LIGHT ON PLANT GROWTH ·
More flowers are produced by plants growing in sunny positions than those
in shade ·
Flower stem length and leaf stalk length are both longer in shady habitat
than in sunny habitats (leaf size does not appear to vary much between these two
conditions) ·
Seed and fruit development is also increased by light. C.
LIGHT AS A CRITICAL STRESS FACTOR Light
is most often a critical factor and one that may easily cause stress.
If individual leaves are not saturated by light, each does not make all
the food that is possible through photosynthesis.
This means fewer reserves for cellular respiration and maintenance
processes in the plant, and it may mean a lack of sufficient additional reserves
for growth and flower production. When
reserves are plentiful, multiple growths may break, leaves and bulbs may turn a
healthy shade of red, and sugary nectar droplets will be formed on the sheaths
and flowers, even on the leaves of some orchids.
Since new growths depend upon the vigor and reserves of previous growths
with good root systems, continued lack of stress from insufficient light is
critical. D.
NOTES ON FERTILIZERS AND WATERING How
often do I water? Plants
with thinner softer foliage will generally require more water than those with
harder, more succulent leaves. Plants
with pseudo bulbs (such as dendrobiums and cattleyas) generally like to dry out
more between watering than those without (like Phalaenopsis). What
fertilizer should I use? If
anything, flowering plants need extra food.
Plants will have to be fertilized with a product appropriate to the
medium in which they are grown. In
general plants in bark-based mix will need a fertilizer high in nitrogen
(usually in a 3:1:1 ratio) while a balanced fertilizer will do for all others
(usually a 1:1:1 ratio). If in
doubt, feed with the same balanced fertilizer you would use for your other
container plants. Orchids
will do far better with too little fertilizer than with too much.
The old adage, "feed weakly weekly" is most appropriate:
Feed every week with a dilute solution. Solid
fertilizers vs. solutions Most
orchid growers seem to favor fertilizers in liquid form i.e. in solution, while
some growers advocate solid fertilizers (e.g. Shake and Grow or Multifeed).
I have always regarded both the above issues as a chancy business which
requires great attention to detail to be successful. If the substrate containing such fertilizers dries out to a
significant extent, i.e. less water, then the concentration of the fertilizer in
the remaining water will increase to such an extent that reverse-osmosis is
possible, the root tip adjacent to this high concentration of salts loses water,
blackens and dies (Water flows from the roots into the grow medium contrary to
the usual direction. All
plants treated with high P fertilizer have been found to have fewer flowers.
Continuous application of adequate N appears to be more important than
low N and increased P for optimal flowering. E.
GROWING MEDIUM Bark
pieces are still among the most widely used of media for growing orchids.
Obviously large pieces of bark, due to its hydrophobic nature and small
surface area, do not initially absorb much water and nutrients, especially when
allowed to dry out before the next irrigation.
This causes fertilizer runoff and slow plant growth.
The modern bark and polystyrene mixtures, as well as the rockwool ones
are almost completely sterile. They
provide no nutrients for the plant and so these have to be provided by the
grower. Plants
in a bark-bases mix will need a fertilizer high in nitrogen (usually in a 3:1:1
ratio) a balanced fertilizer (1:1:1) will do for all others. What
is the best potting material? The
best would be whatever your vendor or source recommends and stocks.
Orchids in general will grow satisfactorily in many different potting
mixes if watering and fertilizing are adjusted appropriately.
Over a period of one year bark
gave better results than rockwool, but soon deteriorated.
Rockwool gave better results over longer periods, however, root
development and maintenance was poor. If
the basic requirements for moisture, root aeration and support are accommodated,
the most readily available media in your particular area are probably those that
have proven to work best. Watering
frequency is generally inversely proportional to the porosity of the medium
used. In other words, the faster the mix drains, the more often you
will have to water. F.
SUMMARY OF GOOD CULTURAL PRACTISES 1.
Provide a light regime appropriate to the plant type 2.
Maintain good air movement within the growing area 3.
Do not crowd the plants 4.
Cull plants that become easily re-infected despite good cultural practice 5.
Keep the growing area free of dead leaves and spent flowers 6.
Remove and burn diseased foliage and flowers 7.
Use sterile cutting tools 8.
Treat cut surfaces with a fungicidal powder/paste G.
STOP THE SPREADING OF DISEASE Disease
spread under the following conditions: ·
Poor growing conditions ·
Too little light ·
Inadequate air movement ·
High humidity ·
Cold or warmth (plants unsuited for those conditions) Bacterial
and fungal diseases will spread rapidly, especially if the conditions are not
corrected. Plants infected with virus may or may not exhibit symptoms.
All infected plants are reservoirs of infection but viral diseases can
only spread by direct contact, by dripping onto another plant or in certain
cases, by sucking insects. PLANT
NUTRIENTS, FUNCTIONS, DEFICIENCY &
EXCESS SYMPTOMS
PLANT
NUTRIENTS, FUNCTIONS, DEFICIENCY & EXCESS SYMPTOMS
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Please Note: The opinions & recommendations made in articles that appear on this website, are those of the individual authors, and not those of the Orchid Society of Northern Transvaal (ONT). The ONT neither adopts nor endorses such opinions & recommendations and disclaims all responsibility for them. This includes products advertised on this website. |
C6H12O6 + O
(Glucose) + (Oxygen)