Haworthias grow best a well-drained yet moisture retentive growing medium. These properties are primarily determined by the structure and the components of a growing medium.
Growing Medium Structure
When water in a growing medium moves downwards under gravity, a portion of it is held in the pores of the growing medium by capillary action. A given volume of growing medium with smaller particles has a bigger surface area than those with larger particles, and therefore holds more water. The optimum drainage is achieved through pure non-absorbent particles of ~4 mm size. Any larger particles increase very little to drainage and become mere obstructions to root growth.
When the particle size of a growing medium is under ~2.5 mm, water will form a perched water table (PWT) at the container bottom or above any coarse drainage layers, where capillary action overcomes gravity. Perch water can only evaporate but will not drain. The smaller the particles size, the greater the PWT height. This saturated water occupies air spaces and often causes root rot.
Do NOT Add A Coarse Drainage Layer
With the same growing medium, PWT heights are the same in all containers. Adding a coarse drainage layer (for example, gravels) at the container bottom does not improve drainage, but moves PWT higher and closer to roots.
The uniformity of particles is also important. Mixing large particles with small is often ineffective to drainage because the smaller particles fit between the large, increasing surface area and consequently capillary action.
For Haworthias, a growing medium with particles in 2.5~4 mm size range provides the ideal balance between drainage and moisture retention. Sieves can be used for removing the particles out of this range.
Growing Medium Components
Hydroponics is a method of growing plants in physically stable and chemically inert mediums, and periodically supplying nutrient solution for plant growth. The structural stability ensures consistent good drainage, and the inert property allows to accurately control the nutrients and the pH in the solution. Mineral components with these properties are more suitable than organic components for growing Haworthias.
Pumice is a solidified frothy magma. It contains a large percentage of cavities and can hold substantial quantities of moisture. Its particle surfaces are soft and the roots ramify well. It is lightweight yet substantial enough to not float away during watering. With many advantages and no apparent drawbacks, it is the mostly recommended growing medium.
The mining and crushing of pumice often produce undesired dust. Before using, pumice should be sifted (and rinsed in water) to remove the dust.
Scoria (lava rock) forms from basaltic magmas, while pumice forms from rhyolitic magmas. Scoria has many similar properties with pumice and can generally be used as a replacement for pumice. It has some disadvantages though. Scoria often has sharp edges which can wound the roots when a container is squeezed. Its large cavities can prohibit root elongation if a root tip is trapped in. For a young seedling with a few roots, this can result in growth stunt.
Perlite is an amorphous volcanic glass. Its lightness makes it unstable as a single-component growth medium to support the plants. Perlite crumbles easily and can be dusty. It is often mixed with other components like crushed granite to reduce the weight of a growing medium.
Vermiculite is a hydrated magnesium aluminum silicate. It is extremely absorbent and can lead to root rot. It breaks down relatively quickly which further increases the water retention. This property, on the other hand, makes vermiculate an excellent rooting medium.
Grits like crushed or decomposed granite and coarse sand are heavy in weight. They absorb little water and require more frequent watering to keep the fine roots alive. They are often mixed with more water retentive components like perlite to balance water retention.