Semi-hydro Cultivation.

Semi-hydroponic cultivation of orchids.

The principles of hydroponic cultivation is that the plant is let to grow in an inert material. The plant continuously stands with its roots in a nutrient solution. The capillary effect (due to adsorption) wets the medium and the roots (it's not as wet as in water, air may circulate). Salts etc. will however not precipitate.

We use washed leca (large and small), Perlite and aquarium gravel as media. The nutrient is a well balanced orchid nutrient with micronutrients added (magnesium is important). A high nitrogen -liquid fertilizer is also used during the growth period.

Nutrient

Higher concentrations of fertilizers than recommended may have beneficial effects during the growth period. The water is important. Rain water may be used. Dissolved nutrients generally cause a drift of the pH-value to somewhat acidic. Some salts may be difficult to dissolve or simply be hard to take up by the plant id the pH is too low. The optimal pH-value is around 7. Rain water (and RO-water) have very little buffer-capacity, compared to tap water. Rain water may also be somewhat acidic. There is thus a risk that the pH is lowered significantly. The pH should be good at normal concentrations (especially when using liquid fertilizers), if one were to overdose slightly, the pH would however be at a negative slope. Tap water can be added to about 10% (volume) of the total solution to add a bit of buffer.

Calcium

A stock solution to enrich the nutrient solution with calcium can be mixed by dissolving 200g of calcium nitrate (Ca(NO₃)₂) in 1L of water (Nilsson et al., 2003). Calcium is important for most orchids, especially some terrestrials. The calcium nitrate will also increase the nitrogen levels. One millilitre (1mL) of the stock solution is added to every litre of nutrient solution.

Magnesium

To enrich with magnesium the same method is applied. 50g of magnesium sulphate (MgSO₄) is dissolved in 400ml water (Nilsson et al., 2003). As with the calcium nitrate solution, one milliliter (1mL) is added to the regular solution.

Placing an orchid in semi-hydro
The roots are cleaned.: All bad and dead parts are removed.
A plastic pot is washed.: Pretty much any plastic container may be used. Transparent pots stimulates the growth of roots. Orchids can photosynthesise via the roots. Never put an orchid in a glass-container. Glass is a heat conductor, far better than plastic.; A plastic container without holes in it can be used. There will be holes needed of course, but these are supposed to sit a few centimetres up on the side from the bottom of the pot/container. When the water level decrease to below the holes, the root system of the plant is aerated.

Wash the leca.: Easiest method to do this is to fill a pot with leca and rinse the leca with water until no more durt is dissolved.
'Plant'.: The pot is filled with washed leca to a few centimeters depth. Place the plant on the leca and fill up with leca around the plant. If the roots are placed a bit up in the pot, they will be able to adapt more easily; growing down for a more moist environment and growing up for a dryer environment.
Watering: The water level should be able to reach the top of the pot. some specimens of orchids require large amounts of water. Let the plant aerate once in a while but keep it in a fresh nutrient solution for most of its time. Different species respond differently, one will have to try out and see what happens.

Hydroponic and common medium Place the pot in a water safe container (glass may be used). Have in mind that the growth of algae is much more a problem in a bright environment. Make a solution of a common NPK fertilizer (containing micronutrients and magnesium) and water your plant so that the holes are covered or so that there at least is a few centimeter of the solution in the outer pot.

Don't feed the plant at first, wait a few weeks after the potting. Some re-fill of the nutrient solution or water is all that is needed. Rinse the pot once a month or so, to be sure that no salts start building up inside the pot.
Salts, dirt, and algae must also be washed away from the outer pot or container.

In a while, the plant has established and the leca keeps it steady in place.

Medium materials

Leca - from heat expanded clay (should be washed).
Perlite - volcanic mineral with a porous structure mostly consisting of silicon dioxide, just like quarts (should be washed).
Diatomite - a mineral with a high silica content (like Perlite). Sedimentary diatoms, produced in Australia (should be soaked for an hour or so, doesn't need washing).
Greenmix - a bit of Perlite and a mix of hydofobic (water repelling) and hydrophilic (water absorbing) mineral-wool.

Important

Perlite, Diatomite and Greenmix dust more or less in dry conditions. Be careful while working with these! Breathing in that dust might be unhealthy for your lungs.

Diatomite and Greenmix must not be let to dry out since they really can dehydrate the plant. These materials absorb water very good.

Problems with semi-hydroponic cultivation

Some sympodial orchids (thin-leafed) produce accordion-like leaves. A possible reason for this is that there's been a fluctuation in the watering and/or feeding of the plant (A. Kastberg, 2004). Another reason might be that the plant require a higher level of moisture (Bertil Nilsson et al., 2003). A solution to the problem can be simply to re-pot the plant in a medium with a higher capillary capacity (better gradient of adsorption).

Links for s/h

Nilsson B., Pettersson C., Miles L., (2003) S/H-Semihydrokultur Ett alternativt sätt att odla orkidéer. (tack till Bertil Nilsson för kommentarer och s/h-kompendiet)

Nilsson B. (2004) S/H – Semihydrokultur, ett alternativt sätt att odla orkidéer Svenska Orkidésällskapet 2004:1