Haworthias grow best a growing medium that is well-drained yet moisture-retentive, and these properties are mainly determined by its particle size and components. Choosing the right growing medium should be largely based on the local climate and also taken the development phase of individual plants 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.
Do NOT Add Coarse Drainage Layer
Adding a layer of coarse materials (for example, large gravels) at the container bottom does not improve drainage, but actually moves possible PWT higher and closer to roots and increases the risk of root rot.
For mature plants with healthy roots, use particles of 2~6 mm size. For seedlings and plants being rooted, use smaller particles of 1~3 mm size because they hold more moisture. It is recommended to first use sieves to removing the particles out of the desired size range and then wash the growing medium in running water to remove fine dust.
Since Haworthias are perennial, a good growing medium should be physically stable and not decompose within a few years. This requirement excludes many common organic materials like peat and coconut coir, as well as some inorganic materials like vermiculite.
A growing medium can be composed of just a single component or a combination of components. The baseline is that it should approach dryness 1~2 weeks after watering under the local average weather condition. The table below compares the water retention properties of some common materials with the same particle size.
Because Haworthias grown in containers are largely fed by nutrients dissolved in water, a good growing medium should have at least a moderate ability to hold nutrients. The table below can be used as a reference.
In Northern California, I use pure pumice for mature plants. For seedlings and planted being rooted, I use perlite which is lighter and allows newly developed roots to elongate and ramify better.