Haworthias grow best a well-drained yet moisture-retentive growing medium. The drainage of a growing medium is mainly determined by its particle size and components, of which choices should be largely based on the climate, but also taken the phase of plant development into consideration.
When water 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 particles of ~4 mm size. Any larger particles increase very little to drainage and become mere obstructions to root growth. And when the particle size is under ~2.5 mm, water will form a perched water table (PWT) at the container bottom or above any layer of more coarse-textured, where capillary action overcomes gravity. Perch water can only evaporate but will not drain. It occupies air spaces and can cause root rot. The smaller the particle size, the greater the PWT height, regardless of the sizes and shapes of containers.
Avoid Coarse Drainage Layer
Adding a layer of coarse materials (for example, large gravels) at the container bottom does not improve drainage. This actually moves any possible PWT higher and closer to roots and increases the risk of root rot.
For mature plants with healthy roots, 2.5~4 mm is the ideal range of particle sizes. For seedlings or plants being rooted, particles of 1~2.5 mm size are preferred because they make newly developed roots easier to elongate and ramify. Sieves can be used for removing the particles out of a desired range.
Since Haworthias are perennial, the components of a growing medium should be physically stable and not decompose within a few years. These requirements exclude many organic materials like peat and coconut coir, as well as some inorganic materials like vermiculite.
The choice of materials should allow the growing medium should approach dryness 1~2 weeks after watering under the local average weather condition. The below table lists the water retention property of materials with the same particle size.
As Haworthias grown in containers are largely fed by nutrients supplied with water, a good growing medium should have at least a moderate ability to hold nutrients.
It is recommended to use materials that are easy to be obtained. Using the same growing medium for all plants enables a more consistent watering schedule. In northern California, I use a 2:1 mix of pumice and fir bark.