The golden rule for watering Haworthias during their growing season is to wait until the growing medium around the roots is dry before re-watering. Spring and autumn are the two peak periods of the growth.
Haworthias start growing in autumn and slows down during the coldest part of winter (when the daytime temperature is below 7 ºC) and pick up growth rate again during the early part of spring before shutting down for dormancy during summer (when the night-time temperature is above 33 ºC). A shriveled plant during the growing season is a sign that it needs water. A shriveled plant during the dormant and slowing down season is a sign of resting and the watering frequency should be stopped.
The best time of day to water is early morning during cold days, and late afternoon or evening during hot days. The idea is to have a moderate temperature (15~25 ºC) for several hours after watering, so that the excess water in the growing medium can evaporate. A low temperature slows down evaporation, and a high temperature increases the chance of pathogens infection. They both can lead to root rot.
When to Re-Water
With a well drained growing medium, water the plant from the top down until ~20% of water applied exits the drain. This process of flushing prevents salts carried by fertilizers and water from accumulating in the growing medium. As salts accumulate, the plant becomes difficult to take up water and the nutrients dissolved in water. The bottom up ‘soaking’ approach, by contrast, is less efficient of taking salts away.
Ideally the whole plant should be flushed to remove salts and other dusts on the body. However in some areas tap water or well water can contain high mineral content and leave water stains on the leaves of a Haworthia. The solution is to use rain water, distilled water or filtered water (for example, reverse osmosis water). Water softeners are not recommended as they often contain sodium chloride and cause the accumulation of salts.