UNDERSTANDING LIGHT

  • This chapter is written to enable the amateur grower to understand a little about light and its effect on growing plants.       Knowledge of this subject, correctly used, can greatly increase both size and floriferousness of your plants. Light as we know it comes in two forms. 
    Natural, radiated by the sun thus allowing us to see during the hours of daylight and Artificial, which enables us to see in the dark, the prime source being mains electricity, which is most suited for use by the amateur grower.     In everyday situations we see light as illumination, whether natural or artificial.   It allows us to focus our eyes on an object and with our individual perception of colours, observe its definition in every detail.       Green plants on the other hand perceive light in a totally different form, as radiant energy, which they absorb using their leaves, known as photosynthesis, the method by which the plant converts soluble nutrients into carbohydrates for use by the plant.    The radiant energy from the sun differs from artificial light in several ways.  The two most significant being:-Light.  Light  transmitted by the sun has a total spectrum which is measured by wavelength from Ultraviolet through to Infrared whereas artificial light has a very limited spectrum which can be varied using different types of transmitters, e.g. light bulbs or fluorescent tubes.Heat.    In addition to light the sun also transmits heat the levels of which rise and fall with the light intensity.    When sunlight is filtered by cloud or other atmospheric conditions such as dust, rain, mist or fog, the intensity of light and heat are reduced accordingly and outdoors are always uniquely balanced.  With different types of artificial light the radiant heat varies and is localised, therefore if used, it needs to be supplemented with  heat from a separate source.    The various types of transmitters and their values will be debated a little later.  In addition, air circulation must be improved either by ventilation or a circulating fan to increase the availability of carbon dioxide if the plants are to benefit.  The use of  heat to promote plant growth presents the amateur grower with a number of problems.        Invariably, the heat will be increased  with little or no regard  paid to the light factor.    It is during the winter months when the enthusiastic amateur turns up the heat in an effort to promote new growth for an early start to the season not appreciating the correlation between heat, light and the third element carbon dioxide.       This later element, carbon dioxide, is a gas present in the air and is formed by burning or decomposition of organic substances including fossil fuels.   The percentage of carbon dioxide in the air is 0.03 per cent.   In the limited confines of a greenhouse or conservatory growing plants can quickly reduce this amount  to 0.01 per cent where there is little or no air movement.   When this occurs plant growth is inhibited and it now becomes the limiting factor.  This anomaly can be easily remedied simply by installing a circulating fan to move the air in and around the plants thus replenishing the carbon dioxide being used in the process of photosynthesis.    Increased ventilation will also assist especially if using fossil fuels as a heat supplement.   A lack or imbalance of any one or more of these elements can be a limiting factor for optimum plant growth therefore they require to be balanced correctly.The use of artificial light can be a great asset to the grower wishing to aspire to the 'Show bench' or the enthusiast striving for perfection.  It is one of the permissible forms of assisting, tampering with nature and if used correctly can prolong the normal growing season either by supplementing natural light or extending the hours of daylight in the winter, spring or autumn.Both light and heat radiated by the sun are always uniquely balanced and allow plant life to grow and survive in their natural surroundings, whether in the Artic or the Tropics it is the intensity and balance of these two factors coupled with a water supply which dictates which plants grow and survive in different parts of the world.      This is an extremely complex subject for the amateur grower to understand.       It will suffice to reason that plants growing in their natural habitat will thrive with little or no assistance.    However, for plants grown in an artificial environment such as a greenhouse or conservatory it is a different story.  As an enthusiastic showman I was perplexed by the difference in quality and size of my plants from year to year even though I adhered to a strict and identical routine. The quality and timing (flowering) of my show plants were never quite the same each year.     A study of my records showed that in a bright spring and summer my plants were early and had to be disbudded but in a dark damp spring and summer I missed some shows completely.   The plants were late flowering even though they were treated exactly the same as the previous year.     There had to be a common factor, one that differed each year and influenced plant growth.   That factor was light, intensity of light, which was the problem.     This prompted research into light and it's affect on plants. I never missed a show again.    In dull summers, I was now able to extend the normal daylight by several hours or just use artificial light to increase its intensity.   This later factor I also use during the winter and early spring to promote growth on stock plants and early cuttings.     If you wish to use artificial light for growing plants a little knowledge and understanding of its effect on and use by plants will help you achieve optimum results and a great deal of pleasure.    I shall try to be concise as possible in explaining the following points which need to be understood if success using artificial light is to be achieved.   In addition, a little knowledge of why and how green plants use light will prove beneficial.Photosynthesis.     As plant growers we need to understand a little about the process of beneficial.

  • Photosynthesis

    As plant growers we need to understand a little about the process of photosynthesis which we can manipulate to increase the yield from our plants whether it be flowers, fruit or seed production by ensuring optimum conditions for healthy growth.    This process, derived from the radiant energy of natural or artificial light, transforms the carbon dioxide absorbed from the surrounding air by the stomata cells on the underside of green leaves into the carbon containing organic materials for use by the plant.     This is one of the unique features of green plants whereby they are able to utilise light as a source of energy.  These carbon compounds are essential to the plant in the processing and conversion of nutrients suspended in water and drawn up through the root system from the soil by a process called vascular action, a system of pumping water from one cell to another.     This transports the raw materials from the soil to the leaves for processing into the complex chemical compounds which together make up plant tissue.     Simultaneously, the photosynthetic products of the leaves, sugars, fats, proteins and a host of other substances are then transported to the roots, leaves and other parts of the plant for growth and development. The light energy used in the photosynthetic transformation of carbon dioxide and other material is absorbed by Chlorophyll, the green pigment of the leaves, then passed on to the leaf's enzyme system to act as a catalyst in the production of plant food and other material.      Chlorophyll only absorbs light energy from the red and blue wave lengths of the colour spectrum.    Where artificial light is to be used to stimulate growth the emphasis must be on these colours if plants are to benefit nutritionally.      If additional suitable light is used then the limiting factor shifts to heat, then to carbon dioxide, back to light and so on.    Therefore a balance must always be maintained. To summarise, if the use of additional artificial light is contemplated then additional heat and carbon dioxide must be provided and vice versa.   It is worth noting that too much heat without extra light will produce unwanted soft lush leggy growth.    It is better to provide an excess of light rather than an  heat but better still try to keep them balanced.       Consider and understand each element in turn:- First, the provision of artificial light.     This can be provided in many ways but the two types for the amateur to consider are:-  A]     Incandescent Light.      This is produced by ordinary 'Tungsten filament' light bulbs as used in the home.    These are rather inefficient by virtue of their design.     Tungsten filament, in addition to light, radiates heat which dictates their positioning above plants.   The minimum I should recommend is 80 cm.   This effectively reduces the light available for use by the plant.    If positioned any closer, there is always the danger of burning the apex of the plant.   In addition, the light transmitted by tungsten bulbs is in the red end of the spectrum which tends to over encourage elongation of the internodal stem, whereby the plant takes on a lush leggy appearance not particularly suited for quality fuchsias.  If the use of tungsten filament bulbs is the only option, then the specification to use is as follows:- Use only pearl bulbs in holders manufactured from neoprene or other similar material.   Never use the metal type holders or second hand fittings that have been used previously. B]   Fluorescent tubes.      Installation is more expensive than tungsten but its efficiency more than justifies the cost.    Each standard six foot  tube emits four times the light for a given wattage and a life span of between 5,000 to 7,000 hours, far in excess of that provided by tungsten.     In addition, the tube provides ultraviolet/ blue light which is extremely  beneficial to plants.     Another bonus is the low radiant heat which allows for positioning closer to the plants with minimal danger of scorching the apex.   The colour output of the fluorescent tube is very important.     Use only 'warm white' or ideally, the slightly more expensive, Grolux or Triton tubes which are widely used for illuminating tropical fish tanks.      These special tubes transmit a higher degree of blue light  beneficial in maintaining balanced growth.   If these prove to be too expensive then warm white tubes are a close second.   I have used these to great effect for many years on my own plants.       The installation procedure outlined for tungsten lights also apply.   If you are not competent, use the expertise of a qualified electrician.     The extra benefit of using fluorescent lights is they can be positioned much closer to the plants, 30cm as opposed to 80cm for tungsten.      In addition, a reflector using tin foil can be draped over the lighting unit to a depth of 10cm below the level of the tubes. This will prevent light escaping.    If the foil is crumpled slightly it will scatter light back down onto the plants.     A simple way of adding a reflector to increase the light values.      Double fluorescent lights are ideal for covering a staging area 1m wide and when butted up can cover the whole length of the staging.      A simple arrangement of pulleys can allow for the raising and lowering of the lights as required.      The use of artificial light for growing adds another dimension to  growing plants in the confines of a greenhouse.      I have used this system for many years to bring show plants into flower by extending the daylight by up to four hours when the timing has been upset by a poor summer and low light levels. The lights need to be switched on at least half an hour before dusk to stop the plants going to sleep.    Additionally, artificial light can be used for a 'night break'  when the length of the night exceeds nine hours.  A timing device can switch the lights on automatically after four hours then after one hour switch them off again allowing the plants to go back to sleep for a further four hours.      Try to raise the ambient temperature to 15/16deg.C in the autumn when using artificial lights, not forgetting to maintain a good air circulation, to hasten flowering. One further point that may be of interest.   The use of artificial light for rooting cuttings.     After conducting a series of tests using various types of light and a control sample, I came to the conclusion that if the cuttings were taken five or six days earlier I could have saved a lot of electricity.       It worked well, the only advantage would be rooting cuttings in an environment where daylight is restricted such as indoors, otherwise just take the cuttings a little earlier.     This was a learning exercise, a useful experience.  Carbon Dioxide.      The importance of this element can not be over stressed.   Install a good circulating fan and position it, not near the roof, but as close to the staging as possible to blow air through the plants to replenish the used carbon dioxide.    The available carbon dioxide can be quickly used up by the plant if there is no air movement.     As explained earlier, this element is essential for the process of photosynthesis more especially where extra heat and light have been provided. To recap, you will require a six foot double fluorescent light unit with Grolux or warm white tubes,  a circulating fan, tin foil and a time switch.    Whilst on the subject of light, it may not be appreciated that light has a far more subtle effect on plants other than being an energy source for food production.   Fuchsias are light sensitive and if left in one position the growing tip will tend to bend towards the light source.    If not corrected by turning the plant at regular intervals the stems will ripen spoiling the shape, especially if growing standard whips.   Once the stem ripens it is difficult to correct and is a fault.     This sensitivity is known as phototropism which can be used to advantage especially when growing half baskets by placing the back to a south facing structure.        If your plants tend to grow 'leggy' then pay attention to correcting the  factors which influence this phenomenon.    Increase light and ventilation and reduce heat and shading.   Increasing light and reducing heat will result better and more compact growth.   

  • Greenhouse shading comes under this heading, which, if wrongly used can be disastrous especially for the show person striving for perfection.   A greenhouse or conservatory is basically a foreign environment for plants and needs to be customised.     The obvious are such things as staging, ventilation, electricity and most essential, a water supply.     What is not so obvious is how to control sunlight entering the greenhouse.  This needs to be monitored and controlled constantly throughout the year with the changing seasons.   During winter and early spring the greenhouse glass needs to be cleaned regularly to allow maximum penetration of light, but as the season progresses the sun radiates more light and heat and consideration must be given to applying some form of  shading to prevent the plants being scorched. Initially I followed the teaching of the Gardening Gurus who advocated, and still do, shading the whole greenhouse using a propriety Lime wash specially prepared and tinted green.    The green tint has since been removed.   There are now excellent products available that are both easy to apply and remove.      Under no circumstances use an emulsion paint.   I made this mistake and was obliged to use wire wool and many weary hours to remove it.      After only two seasons I realised this type of total shading was not suitable for the type of plants I wished to grow especially fuchsias and other light sensitive plants.      On a typical hot sunny day during summer shading is paramount to prevent the plants being scorched and drying out.   The problem arises when the sun disappears behind cloud.     The light levels are reduced but the temperature in the greenhouse remains constant.     This induces unwanted elongation of the internodal stem and the leaf size to increase, effectively weakening the plant.     This is further compounded when the weather is dull and humid.   To achieve balanced growth, plants are able to adjust their metabolism to their environment being able to sense heat and light factors.    If an imbalance is detected they will adjust their growth pattern accordingly.     In low light and high temperatures lush elongated growth is produced.    On the other hand high light and low temperatures produces small dark green leaves and short jointed internodal lengths.   As explained earlier, growing plants in a greenhouse is always a compromise.    In an attempt to remedy the situation, I experimented with various ways of shading and cooling the greenhouse but only one method suited the exact needs of my plants.      This was using vertical stripes of white shading [ pictured below] applied with a 60cm to 80cm soft bristle brush leaving a similar width of  glass clear, a zebra pattern for the want of a better description,    This was applied all over the greenhouse with the exception of the north face which was left completely clear.  With glass to ground greenhouses shade all the glass below the staging and above the stage for approximately 50cm.  This is to protect the pots nearest to the glass from the direct  sunlight  which would otherwise overheat the compost damaging the root system.   An anomaly often wrongly attributed to over watering.     The benefits of using this method were amazing.    In comparison to the two previous years the plants were more compact, leaf, flower size and overall balance were greatly improved.       On a sunny day, as the earth rotates on its axis each plant is bathed in pure light then shade continuously throughout the day.  Conversely, on a dull cloudy day each plant receives maximum light as well as shade.   A method well worth considering.       If the shading is applied to the outside  of the glass, the weather will reduce its intensity gradually during the summer allowing more light to filter through in the later cooler months of the year.      Keep the greenhouse well ventilated and where possible use a circulating fan day and night.      The photographs below illustrate the methods I have used for many years.        

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