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Article 4-3 Orchids

Orchid Culture in Perlite
by Wally Thomas and Barb Thomas
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For a long time perlite has been known as a beneficial soil conditioner and as a component of media mixes. Some 15-years-ago at the West of Scotland Research Station, a technique was developed using pure perlite in growing tomatoes (1). The technique used plastic sacs of perlite sitting in and in continuity with an outer trough of plastic containing fertilizer solution. This provided a water reservoir to maintain a constant fertilizing of the crop and took advantage of the excellent aeration and capillary action of perlite. Studies on tomato crops compared this perlite technique and rockwool culture and showed the perlite to give superior crops and to be simpler to manage. We adapted this perlite technique to growing orchids and later other plants and fruit trees with excellent success. Perlite is a volcanic rock that occurs worldwide, the two largest deposits being in Greece and New Mexico. The raw material is shipped to local factories where the rock is ground into fine particles and heated to 1000°C, turning the small amount of water contained in the rock into steam and so expanding the particles into light weight material. We use the commonly available horticultural size perlite which has particles of about 3mm (1/8in.). Some of the desirable features are: 1) relatively low cost, 2) readily available, 3) light weight, 4) neutral pH, 5) excellent fertilizer control, 6) very easy to leach, 7) easy to dispose of in the garden and 8) it is sterile (Table 1).Two aspects are of particular importance.

  1. Ease of potting - Those who have now tried this technique are in agreement that it is the easiest potting technique they have ever used.
  2. The material is free draining and therefore impossible to overwater. At the same time excellent aeration is maintained. Immediately above a water reservoir the perlite holds 30% of its volume as water, and at 8" (12 cm.) above the reservoir it is still at 19%. Thus excellent aeration and a continuous supply of nutrients are present. Two undesirable aspects of perlite are:
  3. dust
  4. occasional occurrence of compaction

Both of these however, are easily dealt with using the following technique.

Horticultural grade perlite is usually the only perlite that is available and it is commonly packaged in 4 cu. ft. (110 liter) bags of plastic or paper. We find that the plastic bags provide a better dust seal. There are many suppliers of perlite and it is worthwhile trying to find a manufacturer who produces a 'clean' low dust perlite. If possible prepare the perlite outdoors by pouring 1/3rd of a sack into a garbage tub that is half full of a fertilizer/water solution. (To avoid any dust, prior to opening the bag, briefly insert the hose into the bag). The surface is hosed briefly and then the perlite is pushed down into the water a few times. In a matter of minutes the fine material sinks to the bottom and may later be spread in the garden. The cause of the compaction is thus eliminated and what is left floating is a light airy medium that one may scoop out by hand or take out with a colander.

POTS AND POTTING
To take advantage of the water reservoir technique, pots that provide a reservoir are required. These pots provide aeration from below as well as acting as an insect barrier. Plastic food containers and other plastic pots are easily converted into reservoir pots by drilling two holes in the side about one inch (2.5cm) from the bottom. A step drill makes the job quick and easy. The hole size should be 3/8" (0.95 cm.). The size is critical, for a smaller hole inhibits drainage and a larger one allows the perlite to escape. Window-box type plastic containers are also excellent. These pots are 7" (18 cm.) high and across at the top, and 6" (15 cm.) across at the bottom and 22" (55 cm.) long. Three holes are drilled in each end about 1.25" (3 cm.) above the bottom. These are also excellent for daffodils.

POTTING: ORCHIDS
Eight" and 10" (20 and 25 cm.) pots are particularly suitable for bringing orchid seedlings out of flask. Fill the pots with perlite to about 1" (2.5 cm.) from the rim of the pot, plant the seedlings allowing for a topping of washed gravel that should completely cover the perlite. The metal hanger is then placed in the appropriate holes in the rim and a cling wrap material is wrapped around the metal hangers to form a tee-pee with an opening at the top. Maintain the seedlings at 3°C above the temperature that is suitable for the adult plant. The containers are bottom watered once a week using 1/2 strength fertilizer solution. The cling wrap material is removed after four weeks when the pots may be watered in the usual way with regular strength fertilizer.

Table 1. Comparative list of features for four different growing media for orchids.



CHARACTERISTIC
BARK
PEAT
ROCK-WOOL
PER-LITE
pH OF MEDIUM
FERT. CONTROL
LEACHING
AERATION
DISPOSAL
HEALTH HAZARD
STERILITY
MANAGEMENT
SIMPLICITY
WEEDING
AVAILIBILITY
COST
EASE OF POTTING
REPOTTING TIME
NUTRIENT
OVERWATERING
REWETTING CATION
EXCHANGE
BUFFERING
SL ACID
GOOD
EASY
GOOD
EASY
CARE
NO
f-EASY
FAIR
f-EASY
GOOD
VAR-IABLE
GOOD
1-2 YRS
MIN.
YES
FAIR
YES
SLIGH
ACID
GOOD
FAIR
FAIR
EASY
CARE
NO
f-EASY
FAIR
f-EASY
V-GOOD
VAR-IABLE
GOOD
1-2 YRS
MIN.
YES
FAIR
YES
ACID
SL ALK
GOOD
FAIR
FAIR
HARD
CARE
YES
f-EASY
FAIR
f-EASY
FAIR
HIGH
GOOD
1-2 YRS
NIL
YES
v-POOR
NO
NO
NEUT
v-GOOD
v-EASY
v-GOOD
EASY
CARE
YES
v-EASY
v-SIMPLE
v-EASY
v-GOOD
COMPETATIVE
v-GOOD
2 YRS ++
NIL
NO
EASY
NO
NO


REPOTTING
While young orchid seedlings may be transplanted at any time, larger plants are best moved into perlite from other media when they show new growth. The old medium should be thoroughly removed from the roots and all dead roots cut off with sterile clippers. When repotting, the plant should be held in such a position that the new growth has room in front for two years growth. The wet perlite is easily scooped into the pot and plants should be potted slightly deeper in this media than with other media, to allow for a large root system which may push the plant up a bit. The perlite is then leveled and gently poked down with the fingers to firm in the plant and the surface is then completely covered with a layer of pea gravel so that no perlite is visible.

The gravel layer has four functions.

  1. It holds plants firmly in position.
  2. It prevents washing out the perlite.
  3. It markedly decreases surface evaporation.
  4. It inhibits the growth of moss and algae.

In order to provide uniform conditions, we use community pot growing until plants are ready for individual 4" (10cm) pots. In repotting from perlite to perlite, tip out and save the gravel, then lift out the plant and if no trimming is required and the roots look healthy, place it in the second pot with perlite remaining attached. Additional perlite is then scooped in and gravel applied to the surface. Plants will move from perlite to perlite with virtually no setback. If necessary, plants may be left in the same pot for three years. However, to achieve the best growth, repotting should be done every two years. We wash and use the old perlite for growing potatoes in plastic pails but most of it is spread in the garden as a fine soil conditioner.

Beds and pots are watered once a week or less during the winter and more frequently during the summer. We are currently considering a trial using a recirculating watering system, but because orchids are long lived plants and are susceptible to several viruses, one must be cautious.

It is important that the reservoirs do not go dry, although unlike rockwool, the perlite is easily re-wetted. We fertilize/water heavily each watering so that there is very significant overflow drainage and periodically check the electrical conductivity (EC) of the overflow.

We did a trial comparing rockwool and perlite and at the end of a year there was no significant difference as measured by weight and leaf length. Odontoglots, Oncidiums, Cochliodas, Masdevallias, Lycastes, Cymbidiums, Miltonias, Dendrobiums and Paphiopedelums have all shown excellent growth with the reservoir technique. Phalanopsis however do not seem to like the reservoir. If you have the time available for watering when needed the reservoir may be omitted. Plants we have treated this way have done well, though we have not carried out a comparative trial. If you are using ordinary pots which frequently have fairly large drainage holes, gravel may be used in the bottom to hold in the perlite.

We also use the perlite/pea gravel/reservoir technique to grow daffodils, strawberries, potatoes (in dog biscuit buckets), tomatoes and many garden annuals and herbs. Dwarf fruit trees also do well. The trees are grown in large plastic barrels which are cut in half and 4 3/8" (10.2 cm) holes are drilled about 3" (7.5 cm) up from the bottom. After seven years they continue to thrive and be productive in the same perlite.

WATER
About 75% of orchids are epiphytes and in the wild grow clinging to tree branches or onto rock surfaces. They derive their nourishment from the low level of dissolved nutrients in the rain water that runs over them. They grow so slowly that they only require and are able to tolerate about 1/4 the fertilizer concentration that is needed by most plants. In order to provide the full range of nutrients, the water itself must have a low background concentration of salts. The other 25% of orchids, found mainly in the temperate climates, are terrestrial and grow in soil. However, all orchids respond well to hydroponic growing. The discussion of water may be broken down into two parts:

  1. total quantity of salts qualitative analysis of the salts

QUANTITATION OF SALT CONTENT
The salt content of water may be measured by testing the EC. Pure water conducts an insignificant amount of electricity and the reading will be near zero. However when most salts are added the water will conduct electricity. There is a direct relationship between the concentration of any particular salt (or mixture) and the EC. An EC meter for measuring the concentration of salts consists of a battery (replaceable), a digital readout and two wires (electrodes) which are inserted into the solution. The amount of current flowing between the electrodes shows up on the readout. The EC is measured in units that are either called mhos (this is ohms (resistance) in reverse) or as Siemen (S). They are different names for the same measurement. For orchids, the level of conductivity you wish to measure is extremely small and is measured in millionths, called micro and expressed by the symbol 'u'. Thus, one would express a reading of 1000 as 1000umhos or 1000uS. Most plants use a higher concentration and it is measured in thousands called MilliSiemans (1mS=1000uS) or mhos (1mmhos=1000umhos), in Europe, the Siemens is the unit of measure for conductivity. In Australia conductivity is often measured as Conductivity Factor (CF) 10CF=1mS=1mmhos.

There is another type of meter which is calibrated to read in units called Total Dissolved Salts (TDS). This meter reads in TDS units and is calibrated so that it will give a reading in parts per million (PPM) which is milligrams per liter (mgm/L). However, this is misleading for each salt varies in its ability to conduct electricity (some such as urea do not even conduct electricity). Thus, none of the meters really give the true salt concentration but rather, having made up and established the conductivity reading of the fertilizer solution that you wish to use, the meters will tell you if you have made it up correctly next time. It will also allow you to follow the changes in concentration of the solution in pots or in a circulating solution.

For orchids, a small hand held meter that reads from 0-1999 uS is easiest to use but one reading in mS is useable for orchids and much better for growing most other plants.

Modern meters compensate and correct for water temperature. This is necessary for a low temperature significantly reduces the conductivity and so gives a false reading. Test solutions are also readily available to check the calibration of the meter.

Although the above description sounds very complicated once you have a meter in your hand it all follows quite easily.

As mentioned earlier, since orchids require such a low level of nutrition, it is important that the water supply also has a low level of salt. Water with an EC level of under 100uS is very good and even up to 200 uS is acceptable. Certainly orchids are grown in water that is far from ideal, but they respond much better in water with a low salt content, be it naturally (rain water) or artificially produced, to which the appropriate fertilizer is added.

QUALITATIVE ANALYSIS
Not only should one know the total salt content of the water supply but also the proportions of the various salts that make up the total. All of this information is readily available from the local water authority or often local orchid and garden societies are able to supply their members with this information. In some areas, the water is very hard (and alkaline), meaning that it has a high content of salts, most often Calcium and Magnesium in carbonate form. The further addition of these salts in the fertilizer is not likely necessary for orchids and could be toxic.

FERTILIZER
All plants need Hydrogen, Oxygen and Carbon, as well as 12 or 14 other nutrients. The plant derives the first three from the atmosphere through pores in the leaves ( though oxygen is vital to the roots) and the other 12 almost entirely by the roots. Table 2 gives the salt content of the fertilizer that we use and assumes that the natural salt content is minimal.

The problem in supplying all of the nutrients in one solution is that if the Calcium in Calcium Nitrate, and the Sulphate in Magnesium Sulphate meet in a concentrated solution, Calcium Sulphate will rapidly precipitate out as a white mass at the bottom of the bucket. Hence, it is important that these salts only come in contact when they are diluted so that this reaction is slowed.

Thus, nutrient salts may be divided into two groups, the first containing the Calcium (usually Calcium Nitrate), and the second all the other salts.

These salts may be put together two ways:

  1. Diluting them to the appropriate EC required and then using the combined solution.
  2. Diluting the concentrated solutions of the two groups of salts by means of an injector unit. Two Dosmatic units may be used in series and they give accurate dilutions at varying pressures. We set ours to give a dilution of 1/100. Two Hozon units may be used in parallel but the EC at the nozzle must be monitored as it will vary a great deal according to the water pressure. For example, raising the end of the nozzle four or five inch (1.5 m) may result in a decrease in the fertilizer concentration of 30 or 40%.

pH
Perlite is neutral (i.e. pH=7), thus the acidity or alkalinity of the fertilizer solution determines the pH of the growing environment. The pH may be measured by a meter similar to the EC meter. It requires more frequent calibration. The pH meter also corrects for temperature even though the reading is only minimally affected by changes in the temperature of the solution.

FERTILIZING
We use two parts of Calcium Nitrate by weight in one bucket and three parts of 7-11-27 in the other bucket. The salts are weighed so that at a dilution of 1/100, we get an EC of 600 uS at the nozzle. We fertilize/water heavily (so that the water flows freely out of the drain holes) for five waterings and use plain water at the sixth watering. We check the EC of the overflow from random pots. Our water supply has an EC of 30 to 40uS and a pH of 6.4. The above fertilizers when used at an EC of 600 uS give a pH of 5.8 which is excellent for most orchids and other plants. The pot reservoir water pH goes down over a period of months and should be checked and readjusted. Alternatively the pots may be heavily flushed out with fertilizer solution. Orchids do surprisingly well even with a pH as acid as four.

Element
ppm
Element
ppm
N
P
K
Ca
Mg
SO4
54
14.7
70
37
11.5
15
Fe
Cu
Mo
Zn
Mn
B
0.31
0.0136
0.027
0.094
0.27
0.08

NOTE:

These levels may be achieved using, by weight, two part Ca(NO3)2 to 3 parts 7-11-27 Plantprod (Plantex) to yield an EC of 600 microSiemens (uS).

Although the various hydroponic solution requirements of various plants is well established, we use the same fertilizer and ratio of salts with our other hydroponic growing but increase the EC to a level of 1200 uS.

Many other suitable fertilizers that are excellent and easy to use in large or small quantities are available at hydroponic stores.

SUMMARY
Perlite has many advantages as a medium for growing orchids and many other plants. The most important features are total fertilizer control, ease of potting, repotting, simplicity of management and maintenance and with excellent aeration, for one cannot overwater. The roots are easily washed clean for scientific examination and analysis or for agricultural inspection. No other media encompasses so many outstanding qualities.

In addition to our excellent growth with this technique we have reports of similar results from a number of fanciers covering most of the major orchid genera.

REFERENCES

1. Hall, D.A., Hitchon, G.M., and Szmidt, R.A.K. 1988. Perlite culture: a new development in hydroponics. Perlite Culture, ISOSC Proc. Pp. 177. 2. Pierce, J.W. 1983. Perlite culture of orchids, Amer. Orchid Soc. Bull. 52:(8) pp. 806.

Wally Thomas is a retired Lab-oratory physician who has grown orchids for 40 years, Barb Thomas his daughter, has a Ph.D. in Forest Genetics and has been growing orchids for 15 years.

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