Haworthias need a well drained yet moisture retentive growing medium for the best growth. And these properties are primarily determined by the structure and the components of the growing medium.
When water in the growing medium moves downwards under gravity, a portion of it is held in the pores of 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. 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 the growing medium is under ~2.5 mm, water will form a perched water table (PWT) at the container bottom or above coarse drainage layers, where capillary action overcomes gravity. Perch water can evaporate but will not drain. The smaller the particles size, the greater the PWT height. This saturated water occupies air space and often causes root problems.
Do NOT Add A Coarse Drainage Layer
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 that are not in this range.
Hydroponics is a method of growing plants in physically stable and (bio-)chemically inert mediums, and periodically supplying nutrient solution for plant growth. Such growing mediums have stable structure and therefore consistent drainage. They release few chemicals and allow to accurately control the nutrients and the pH in the solution. Commonly available growing mediums for hydroponics include coarse sand, granite, vermiculite, perlite, pumice and coconut coir. Among them, using 100% pumice as the growing medium is widely adopted 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. The surface of each particle is soft and roots ramify well. It is lightweight yet substantial enough to not float away during watering.
Hoticulture pumice often contains undesired fines due to the mining and crushing processes. Before using as the growing medium, it should be rinsed in water until the runoff becomes clear.