Haworthias need a well drained yet moisture retentive growing medium for the best growth. And these properties are primarily determined by structure and components of the medium.
While water in medium moves downwards under gravity, a portion of it is held in pores by capillary action. The greater the surface area, the stronger the capillary action. A given volume of larger medium particles has a less surface area than those with smaller particles, and therefore holds less water. Optimum drainage is achieved through pure non-absorbent particles of ~4 mm size. Larger particles increase very little to drainage and become mere obstructions to root growth.
When medium particle size 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.
Do NOT Add A Coarse Drainage Layer
For Haworthias, medium particles range in size from 2.5 to 4 mm size provide the ideal balance between drainage and moisture retention.
Hydroponics is widely used among Haworthia growers. It is a method of growing plants in physically stable and (bio-)chemically inert mediums and periodically applying nutrient solution. Such mediums keep a stable medium structure and therefore consistent drainage, and allow accurate control over the nutrients and the pH in the solution. Commonly available mediums include coarse sand, granite, vermiculite, perlite, pumice and coconut coir. Among them, using 100% pumice is a popular choice.