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. Many mineral components have such properties, including pumice, coarse sand, granite, and perlite. They are widely used by Haworthia growers.
Pumice is a solidified frothy lava. It contains a large percentage of cavities at its surface and inside, 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.
The mining and crushing of pumice often produce undesired dust. Before using, pumice should be sifted (and rinsed in water) to remove the dust.