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Orchids Cultivation




Watering and fertilizing of Phalaenopsis in sphagnum moss


Many kinds of media have been used for orchids. Bark and sphagnum moss are major substrates for Phalaenopsis. The two substrates are different in characteristics.

Phalaenopsis is an epiphytic plant and the natural habitat for roots is growth on tree trunks. Roots are exposed to air and absorb moisture from the air. In the ecological world of orchid roots, three types of environments need to be considered: the gas environment, including O2, CO2 and vapor; the liquid environment, water; and the solid environment, composed by roots, medium and fertilizers.

Adequate conditions for the orchid culture medium are as follows:

1. Good aeration ability to provide enough oxygen for respiration of roots.

2. Water and nutrient-holding capacities: to release water and nutrients slowly.

3. Stability to maintain its properties for a long time.

4.  Cost must be inexpensive for mass production of orchids industry.

The mediums for Phalaenopsis culture must have the following characteristics:

1. Storage of water and fertilizers.

2. Drain easily to provide air ventilation.

3. Stable to keep the quality for a long time.

4. No toxic substances

5. Support the plant and sticks easily.

6. Could provide space for roots development and air exchange.

Sphagnum moss cannot meet all these conditions. As a medium for Phalaenopsis, sphagnum moss has good holding ability for moisture and nutrients. It can absorb a lot of water and nutrients quickly and release them slowly. However, compared with other substrates with sphagnum moss, roots can grow faster. Because of the water holding ability, plants need less irrigation.


Optimal irrigation involves the quality and quantity of water and timing of watering.

The quality of water is very important for the Phalaenopsis. The accepted contents in water for Phalaenopsis are sodium, 1.0-3.0 mmol/l; chlorine, 1.0-3.0 mmol/l; bicarbonate 0.5-1.0 mmol/l; and calcium, 1.0-2.0 mmol/l.

Watering aims to supply water for growing and transpiration and to wet the medium to create an adequate environment. The factors to consider are as follows:

1. How often to watering

2.  The water temperature; cold water could damage plants

3. The required time for drying the leaves and stems, especially, the base part of leaves.

According to the required quantity of mixing medium, Anthura b.v. classified four levels of irrigation (Anthurinfo 18(1), 2010). This concept could be applied with use of sphagnum moss.

1. Moistening the top layer of the medium.

2. Small irrigation.

3.  Normal irrigation.

4. Large irrigation.

Moistening the top layer of the medium aims to wet it to avoid the moisture gradient of the medium and prevent the excessive roots moving to the upper layer of the medium.

There are two situations when air humidity of the greenhouse is too dry for Phalaenopsis.

1. The outside air temperature is good but the humidity is too low; as the outside air is inhaled into the greenhouse to cool excess heat from the solar energy, the internal air humidity becomes too low.

2. The humidity of the greenhouse air is too low in winter or during cold nights, because of the heating operation; the top moisture of the medium is evaporated easily, which induces roots to grow to the top of the medium. The top layer of the medium must be moistened. . In this operation, no nutrients are used in the water.

Small irrigation aims to provide some water to enhance root activity. Small irrigation is used when plants are transplanted or recovered from ocean transportation. In above case, the medium is very dry so it cannot absorb enough water. If too much water is supplied, roots will be damaged and lose longevity. Water is supplied to the medium in small quantities. The wetting zone is about a three quarter zone of the medium. This technique could stimulate old roots to recover and new roots to grow.

Normal irrigation aims to provide a normal quantity of water for Phalaenopsis. The best way for normal irrigation with sphagnum moss is to water it twice. The given quantity of water for the first time is over the collar (container) and the second watering, half the container.

Large irrigation is also called excessive watering. This is used when drainage EC values are too high, for example, > 1.3 ms/cm. The medium is rinsing by supplying a large quantify of water with 100 ppm Peter 20-20-20 to flush out accumulated salts.

Large irrigation is used when the water content in the medium becomes non-uniform and some dry points or regions are found. Roots become dark and brown, and the tip of roots are dead, the medium must be watered with a lot of water as soon as possible.

Watering is a basic factor for successful culture. Excess watering gives more water than the medium needs. The leaching from the bottom of pots could enhance the air exchange of the medium and wash out accumulated salts.

The key point in watering for sphagnum moss or barks is let it dry and water it again. The drying of root environment could stimulate root activity. However, a dry environment is dangerous for roots. If roots lack water, no sign of stress was found until the lower leaves turn yellow.

Some methods used to justify the moisture conditions of the medium are with experience or weights plants in-pot. The adequate way is to detect the moisture content of the medium with a TDR (Time-domain refectmeter) (Figure 1). This commercial meter can detect the moisture content of medium quickly and accurately. However, the cost is high. As well, the EC values of the medium could be detected using an EC meter in situ and real time (Figure 2).

The factors affecting watering frequency include high light intensity, high air temperature, low relative humidity and rapid air movement. The small size of pots and coarse, porous medium also require frequent watering.


Figure 1.  A TDR meter was used to detect the moisture content of the medium


Figure 2.  An in situ and real time EC meter was used to detect the EC values of the medium


The required concentration of fertilizer is affected by orchid variety and environmental conditions. Many researchers have found little or no effect of fertilizer type on growing and flowering, perhaps because most orchids require only a little fertilizer. All types of fertilizer are composed of the required macro and micro elements for orchids.

The C/N balance is the key point for the application concentration of fertilizers. At high temperature and high light intensity, more carbon is fixed during the photosynthesis process, so higher concentrations of fertilizer are used (for C/N balance). For a safety EC level, 0.7 to1.4 ms/cm is good for most varieties.

Salt damage needs to be checked carefully. Salt damage is a common stress for orchids. Growers need to check the tip of roots frequently and make a mark on the pot surface to show the position of the tip of roots for subsequent observation. Salt accumulation can be found at the tip of roots and at the end sides of leaves.


Figure 3. The mark of root tips for successive observation.

The recommend EC levels of water are different in several culture guides. The EC values according to Taiwan growing experience for sphagnum moss are classified by cultivation stages. The EC values are for growth stage (2- to 4-inch pot), 0.6-1.2 ms/cm; cooling stage, 1.0-1.4 ms/cm; and flowering stage, 1.0-1.2 ms/cm.

The recommend EC levels in water from the culture guide of Anthura b.v. for mixing medium are: for growth stage, 1.0-1.2 ms/cm; cooling stage, 1.2-1.3 ms/cm; and flowering stage, 1.0 ms/cm. The EC level is lower in flowering stage to avoid damage to roots.

The EC levels recommended from Floriculture b.v. are as follows:

1. Fertilizing with 20-20-20 (N-P-K) and EC 1.0 ms/cm during growing stage (day temperature, 28; night temperature, 26; light intensity, 4000-7000 lux).

2. The EC levels not exceeding 1.2 ms/cm and pH value do not exceeding 5.5. The highest nutrient is with pH 5.0-5.5. The safety pH range is 5.0-6.0. The pH < 5.0 or > 6.2 will damage roots.

3. During the winter period, replace 20-20-20 with 71-5-34 to reduce the N level for the C/N balance.

Several methods have been proposed to measure pH and EC in water, such as 1:1.5 method, 1:4 method and drain method. No true method can be found. The key is to use the same method all the time

Drain method is recommended as the standard method. This technique is introduced as follows:

1. Place the pot on the collecting cup.

2. Seal the cup and pot to prevent water from entering the collecting cup (Figure 4).

3. Wet the medium completely, usually watering twice.

4. Collect the drain water in the collecting cup.

5. Measure pH and EC immediately,

6. The actual EC value is always higher than measurement value. However, the values with the drain method could be compared and evaluated.

Figure 4. The seal of cup and pot to prevent water from entering the collecting cup


Manufacturer information:

 1. Delta-T Devices

WET-2 - WET Sensor

http://www.delta-t.co.uk/product-display.asp?id=WET-2 Product&div=Irrigation and Horticulture

 2. Spectrum Technologies, Inc.

FieldScout Direct Soil EC Meter