Hydroponics

PLANT SUPPORT

Plants grown in hydroponics tend to be more prone to falling over than plants grown in soil; and thus frequently need some type of trellis support.  Water culture methods such as NFT and light weight materials such as perlite, vermiculite and rock wool do not provide firm anchorage of roots in the way that soil does.

• Tall growing plants in particular need support.
• Stronger supports are needed if plants are exposed to wind.
• A greater bulk of plant will need a stronger trellis.

There are two types of trellis systems:

1. Horizontal trellis
Here a mesh of wire, nylon or some other material is supported above the plants in one or two layers (depending on the height of the plants and the amount of support needed).
Carnations, capsicums, and other small bushy plants require this type of trellis.

2. Vertical trellis
This may consist of a similar mesh material stretched along a row, or alternatively, single wires stretched along a row with support posts at each end.  Vertical trellis can also be hung from the ceiling in a greenhouse.

• Tomatoes can be grown on wires tensioned and spaced at 50 cm intervals. The stems are tied to the wires as they grow.
• Cucumbers require greater support and are either grown on a vertical mesh, or on wires at 15 to 20 cm spacing. The wires should be tied together every 20 to 30 cm to form a mesh and give additional support.
• In large systems wires need to be connected to turnbuckles so they can be tensioned if they loosen.

PRUNING

Plants are pruned for one of the following reasons:

1. To remove dead or diseased vegetation
This is done so that disease does not spread. Any dead or diseased stems, leaves, fruit or flowers should be cut cleanly from plants and disposed of as soon as it is noticed, irrespective of the crop.

2.  To rejuvenate a plant
Young lush growth is always healthier than tired old wood. A rose for instance can have its life extended and health improved, if old wood is continually removed over a period of years, and replaced by younger wood.

3. To control the direction or shape of growth
A plant can be made bushier by removing the terminal bud of a shoot or stem. When the tip is cut or pinched out, side shoots are forced to develop so that several shoots occur where before there was only one. If you want a taller, less bushy plant, side shoots are removed so that there are fewer growing points (e.g. this is done with tomatoes until they establish and begin flowering).

4. To control the type of growth
Some plants produce flowers and fruit on growth which is in its first year.  Other plants produce flower buds only on older wood (e.g. two-year-old wood).  By knowing the type of wood that flowers occur on, it is possible to prune in order to maximise the fruit or flower crop.  By removing all flower buds, it may be possible to redirect a plan’s energies into vegetative growth.

 

Pruning Examples

Raspberries
Raspberries produce fruit on two-year-old wood.  These should be pruned each winter as follows:

• Remove wood which produced fruit last season (because this will be 3 years old next season and won’t fruit much at all then).
• Leave the strongest growth of one-year-old wood (because those will be 2 years old and will produce fruit next season).
• Leave the strongest half of the new growth which emerged over the past season (i.e. one-year-old wood), because those growths will produce fruit the season after next.

Chrysanthemums
Chrysanthemums produce flowers on the tips of young growth.  The more tips a bush has, the greater the number of flowers it will have, but the smaller each flower will be.

• If you require a large quantity of flowers, you will remove the terminal bud when you plant a young plant, and periodically pinch out the growing points in the developing stages of the plant.  You must cease removing the growing points when flowers start to develop though, or you will be removing flower buds.
• If you require fewer flowers, but larger and of better qualify, you should cease pinching our terminal buds much earlier.

POLLINATION

Some plants can have problems with pollination, which result in a reduced number of fruit.  Corn, strawberries, tomatoes and cucumbers may be affected in this way, particularly if grown in a greenhouse which has reduced air movement and limited access to pollinating insects.  The following methods may be used to help pollination in these and other problem situations:

• Fans to increase air movement
• Vibrating the flowers by tapping them with a stick of shaking them with your hand (not too hard though or you will damage the plants).
• Releasing bees into the greenhouse.
• Moving pollen physically from plant to plant with cotton wool or some similar material.
• Reducing humidity (which can cause pollen to stick on the plant it comes from and note move to where it is needed).

If you are planning to do any of these things, they must be done at the appropriate time, when the plant is receptive (e.g. tomato pollination should be done in later morning, under sunny conditions when the petals of the flowers are curling back).


CARBON DIOXIDE ENRICHMENT

Plants need carbon dioxide in the same way that humans need oxygen.  Without a good supply of carbon dioxide growth will slow.

In some crops and in some localities, yields can be significantly increased by increasing the level of carbon dioxide in the growing environment.  This is of course most suited to a sealed greenhouse.  In an enclosed environment such as a greenhouse, there is a real danger of plants becoming starved for carbon dioxide.  If vents to the outside are closed, the plants will in their normal course of growth gradually deplete carbon dioxide until they reach a level where growth is slowed.

The optimum level for most crops will be around 1000 to 1400 ppm (although natural levels are only around 300ppm).  Nutrition and water demand may increase when carbon dioxide is used.

Methods of supplying carbon dioxide are:

• At the same time heating a greenhouse: burn a hydrocarbon fuel such as kerosene or propane.
• Place containers of dry ice in the greenhouse.
• Release gas from pressurised cylinders.

Carbon dioxide enrichment can benefit the crops below as follows:

• Commercial tomato growers in northern parts of the USA claim crop increases of 20 to 30%.
• Lettuce and cucumber yields increase up to 30% in cool climates.
• Carnations crop faster, have stronger stems and will give production increases of up to 30%.
• Roses under 1000 ppm carbon dioxide levels over winter in cool climates have increased production, improved quality and shorter cropping times.
• Chrysanthemums have stronger and longer stems and crop faster.

TRANSPLANTING

Whenever you transplant a seedling into hydroponics there will be some “shock” effect which is detrimental to the plant and will result in either:

• Some dieback on the root system – some of the root hairs and perhaps root tips may die; or
• A break in the growth – growth will slow or cease for a period.

This shock effect will normally be unnoticeable with hardy and easy to grow plant varieties, but can be significant in some situations.

Transplant shock can be minimised by following these rules:

• Don’t disturb roots any more than is necessary.
• Use young plants growing in hydroponic media such as Growool where possible, so that the propagating medium can be retained and most of the roots do not have to be exposed.
• Roots should be exposed to the air for the absolute minimum amount of time
• Don’t transplant in hot, windy or dry conditions.
• If planting into NFT, have the system running before you start transplanting.
• If planting into aggregate, apply nutrient solution before transplanting.
• If planting onto rockwool slabs, soak the slabs with nutrient solution before transplanting.
• If you damage or prune the roots of a transplant, cut a corresponding amount of the top back before transplanting.
• Plants which are more susceptible to transplant shock can be sprayed with a mist of water (not nutrient solution!) just before transplanting.
• Irrigate plants immediately after transplanting.

 

MANAGING PLANT HEALTH

Diagnosis of Problems
Problems fall into three possible categories:
1.  Nutritional – either too little or too much of one or several particular nutrients is available.
2.  Environmental – The environmental conditions are not suitable.
3.  Pathological – One or more organisms are interfering with the health of the plant.  Such organisms are called “pathogens”.

It requires a great deal of knowledge and expertise to be able to diagnose plant troubles.  Do not expect to develop such ability quickly.  The first and perhaps most important skill to develop is an ability to inspect a plant and look for the tell-tale symptoms which can provide an indication of what might be wrong.


PLANT DISEASES IN HYDROPONIC CULTURE

Plant disease is generally distinguished from insect and other pest problems.  Plant pests actually eat the plant, or break the plant by standing on it (as does the human pest).  Plant disease is far more subtle, disturbing the microscopic physiological processes within the plant.

When a plant is diseased, it may be affected by one, two or more different problems.  It is often difficult to identify what is wrong with a plant clearly, because the problem is in fact a combination of problems.

A possible scenario is outlined in the example below:

• The plant is weakened by poor nutrition.
• Excessively wet conditions create an environment conducive to the growth of an infectious fungus.
• The plant, weakened by poor nutrition, is infected by the fungal diseases that develop in wet conditions.
• The roots begin to rot through fungal attack.
• Because the roots are damaged, the plant does not take in water and nutrients as well as it would normally.
• The leaves of the plant are infected by a second disease because of the above situation, which has made the plant weak and less able to repel infection.

Disease organisms
Disease organisms usually fall into one of the following groups.

Viruses
Viruses are very small microscopic particles composed of nucleic acid and protein. They exhibit many, but not all, characteristics of living organisms, and are therefore sometimes called a life form. Viruses can mutate. They cause many serious diseases, frequently causing variegation or mottling of leaf colour. Some viruses are considered beneficial because of the variations they provide in leaf colour. Whether considered beneficial or not, viruses cause a general weakening of plants they infect, making the plant more susceptible to other problems, and frequently stunting growth to some degree.

Bacteria
These are single-celled organisms, some of the smallest living things.  They enter plants through stomata or wounds. (They cannot break directly through cell walls or the ‘surface’ of a plant).  Bacteria can cause rots, blights, spots, galls, scabs and other symptoms. (N.B. Fungi can also cause many of these.)

Fungi
Fungi are chlorophyll-less members of the Thalophyte plants. They are either parasites (living on live tissue) or saprophytes (living on dead tissue). Over 15 000 species are known, and many are responsible for major plant diseases. Fungi are thread-like organisms which grow amongst the tissue from which they derive their nutrition. The individual threads are known as mycelium. To reproduce, fungi grow fruiting bodies from a mass of mycelium, and spores are produced in these fruiting bodies.

Nematodes
These are microscopic worms which feed in the intercellular spaces, causing breakdown of cell walls. They generally enter plants through the roots, through wounds or stomata (different nematodes have different standard methods of entry). Nematodes are much less of a problem in hydroponics than in soil.

Common Terms Used to Describe Diseases

• Rot: Decomposition or decay of dead tissue.
• Spot: Well defined grey or brown tissues surrounded by purplish margins (or margins of some other dark colour).
• Shot hole: Dead tissue in a spot cracks and falls, leaving a hole in the leaf.
• Blotch: Fungal growth appearing on the surface of a dead spot.
• Blight: Quick death of complete parts of a plant. The disease pathogen develops very quickly, e.g. leaves die and fall.
• Wilting: Drooping of leaves and/or stems.
• Scorch: Similar to blight but leaf veins are not affected. Leaf tissue dies between the veins, or along the margins.
• Scald: Whitening of surface (or near surface) cell layer on fruit or leaves.
• Blast: Unopened buds or flowers die suddenly.
• Die back: Death of growing tips, moving down through the plant (i.e. the terminal buds dies, followed by death of the stems and lower parts of the plant). Die back can occur to just part of the plant or, in severe cases, can continue moving through the plant to the roots.
• Damping off: Sudden wilting and falling over of young plants due to tissue being attacked by fungal disease near the soil line.
• Mummification: Diseased fruit dries up, becoming wrinkled and hardening as it shrinks.
• Canker: Death of a restricted area of woody tissue, usually a callus of healthy growth forms around the edge of the canker.
• Bleeding: A substance is exuded from a diseased part of the wood.  Only refers to exudation which is not resinous or gummy.  
• Gummosis: Bleeding where the exudation is resinous or gummy. Gummosis on conifers is called resinosis.
• Firing: Leaves suddenly dry, collapse and die.
• Rosettin: Spaces between leaves on step do not develop: buds and leaves become squashed together within a short section of stem.
• Mosaic: Mottling of yellow and green on leaf surface.
• Dwarfing: Plants do not grow to full size.
• Fasciation: Round plant parts such as stems become distorted, broadened and flattened.

PHYSIOLOGICAL PROBLEMS

There are a number of environmental factors which can damage a crop if not properly controlled. Frost or sun may burn fruits or foliage, fruit can crack and leaves can discolour. Some of the more common problems are detailed below.

Cracking
Lack of water or excess water can cause the skin of various crops to split. Freshly harvested carrots sometimes split. Tomatoes that are suffering from lack of water and exposed to high temperatures may split.

Blossom end rot
A common problem with tomatoes is where the bottom of the tomato turns brown or black and leathery in appearance. This typically occurs where there a low supply of calcium combined with irregular growth, causing stress in the plant. Irregular and variable water supply and variable temperature conditions are also associated with this problem.

Crooking
This is where fruit becomes distorted (eg. when cucumbers become excessively curved). Crooking has been attributed to poor control of temperature, moisture or nutrition.


COMMON DISEASES AND THEIR CONTROL IN HYDROPONICS

Alternaria
This blight commonly affects leaves, sometimes stems. Symptoms are usually spots, often developing concentric rings as they enlarge. There are many types of alternaria. Most are controlled with Zineb. A copper spray will control some.

Anthracnose
There are two different groups of anthracnose diseases, distinguishable by their symptoms:
1. Symptoms are dead spots.
2. Symptoms are improper development of some part of the plant (e.g. a raised border around a depressed central area of undeveloped tissue.

Anthracnose can be controlled by various fungicides. Some types are controlled by copper sprays, others by Zineb and other chemicals.

Botrytis
A grey fluffy mouldy growth occurring on stems, leaves, flowers and leaves, botrytis occurs in wet, humid conditions. Affected parts should be removed and burned immediately.
Preventative measures include increasing airflow and reducing humidity. Thiram and dichlofluanid fungicides can be used to control infections.

Downy mildew
The upper leaf of a plant with this disease shows yellowing or dull patches with a greying mould growing underneath. It occurs under most conditions, and is controlled with Zineb.

Fusarium
Symptoms can include foliage yellowing, stunted growth, wilt and leaves dropping. Hygiene will usually control fusarium.

Phytophthora
There are several forms of Phytophthora ranging from disease that the stem of young seedlings to others that impair the uptake of nutrients in very large plants. Symptoms are frequently dramatic and can cause sudden death of the plant.

Remove infected parts and sterilise infected areas. Fongarid will effectively control some types of phytophthora and slow the spread of others.

Powdery mildew
This disease occurs in warm, moist, humid conditions The main symptom is a white powdery growth on leaf surfaces. Sulphur sprays or dust will usually give control.

Rhizoctinia
Symptoms are brown or black dead spots or rot, normally on leaves or stems. This disease can be controlled with terraclor.

Scab
Patches of discoloration develop into spongy, blister-like scabs which can affect leaves, stems or underground parts. Remove or burn infected parts. Spray with dithane (Maneb).

Smut
Symptoms are black spots with cracks developing in the spots to reveal a sooty black powder. Control with Maneb.

Verticillium wilt
The disease infects a variety of crop plants. Symptoms may be as mild as slight paleness in foliage to drooping of leaves, stunted growth, browning between the veins of a leaf, and death. Symptoms can be slow or reasonably fast to develop. Infected plants should be removed and burnt, and infected areas should be sterilised before replanting.

Virus
Symptoms can be any type of abnormal growth, such as discolouration of foliage, twisted or stunted growth etc. Once a plant has a viral disease, it is virtually impossible to eliminate it from that plant. Prevent spread by controlling sucking insects (in particular aphis). Some plants (e.g. strawberry) deteriorate with the virus over a period of years. Control in such instances involves replacing plants every few years with verified virus-free stock.

 

 

 

 

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