Passive hydroponics, semi-hydroponics or passive subirrigation is a method of growing plants without soil, peat moss, or bark. Instead an inert porous medium transports water and fertilizer to the roots by capillary action. Water and fertilizer are held in a reservoir and conducted to the roots as necessary, reducing labor and providing a constant supply of water to the roots. In the simplest method, the pot sits in a shallow solution of fertilizer and water or on a capillary mat saturated with nutrient solution. Since routine maintenance is much simplified, passive hydroponics can reduce the labor required to maintain a large collection of plants.
The various hydroponic media available contain more air space than more traditional potting mixes, delivering increased oxygen to the roots. Allowing air to reach the roots is particularly important in preventing root rot in epiphytic plants such as orchids and bromeliads, whose roots are exposed to the air in nature. Passive hydroponics also may add additional ambient humidity through evaporation. It is important in passive subirrigation to wash out the system from time to time to remove salt accumulation. Plants that require drying between waterings or a dry dormant period may fail to thrive under the constant moisture of passive hydroponics.
Many media are available for passive hydroponics, but the most common are expanded clay pellets, coconut husk chips, perlite, vermiculite, diatomite, charcoal, and rockwool. These are frequently used in combination.
Passive hydroponics is one of many methods to grow plants without soil. Growing medium is inert and wicking. It delivers water with fertilizer to the roots by means of capillary action. Substrate contains many small air pockets and can thus deliver oxygen to the roots. The method has been applied to orchids.
Orchids can be planted in any non glass container. The 3-4 drainage holes are placed not at the bottom but 3-5 cm up, at the sides of the pot. The idea is to provide a water reservoir at the bottom of the container from which the medium wicks moisture to the roots.
LECA (Lightweight Expanded Clay Aggregate) - expanded / fired clay pellets or clay pebbles , perlite, vermiculite, diatomaceous earth, gravel, charcoal, rockwool, coconut husk chips and their combinations.
Substrate is flushed with tepid water solution when reservoir is nearly empty. Translucent pot may help to see when it is.
Orchids are fertilized with 1/2 to 1/4 of recommended strength of balanced inorganic fertilizer with every
watering. Container is flushed with plain water every month to prevent harmful salt build up.
Most popular orchids will more or less thrive in hydroponic culture: Paphiopedilum, Phragmipedium, Masdevallia, Phalaenopsis, Cattleya, Cymbidium, Oncidium, Dendrobiums, Epidendrums, Miltoniopsis, Pleurothallid and Zygopetalum.
Its simplicity and effectiveness. No guessing about watering and fertilizing, no media decomposition, practically no root rot, healthy plants, fine blooming, no moving parts, low cost, reusable media.