An excerpt with permission from a longer article at www.Haworthia.com, the now defunct web site of David Martin. We have made some changes relating to Northwest growing conditions.
Light for your Haworthias
At some times of the year here in the Pacific Northwest, light is in short supply, and I often see fat, green, elongated Haworthias that are distorted beyond recognition by low light levels. However some growers just prefer the big green soft Haworthias that are a result of growing in low light. If this is the case with you, it will work most of the time with most of the Haworthias, but, in my opinion, will result in a great deal of sameness.
In most cases it is hard to give your Haworthia too much light. The exception here is a newly acquired tender plant that needs to harden up. To put a plant in direct sunlight that has never seen the sun will cause a fatal sunburn. If you place your Haworthias in the brightest light you have that is not direct sunlight, you will grow into very nice looking plants. For growing, a greenhouse is ideal; next choose a sunny South facing window; then an East or West facing window. Save your North windows for ferns. If a South facing window has a tremendous amount of sunlight, slowly acclimate your new plants by placing several layers of a thin lacy cloth between the plant and the sun. Then slowly remove a layer at a time at three week intervals to allow the plant to adjust to the hot sun.
Also remember that since a plant in the windowsill is partially protected from the suns ultraviolet rays by the glass, taking a windowsill or greenhouse plant and placing it in the direct bright outdoor sunlight, for even a few minutes, could burn the plant so it is disfigured for years. I recommend that any move to the outside in the summer be under the shade of a tree. If you must put your plant directly in the sun, do it very gradually, just as you would treat yourself when acquiring a suntan in the spring. If your are blessed with skin that does not burn, ask one of your pale friends for advice.
Watering your Haworthias
This is actually much easier than you might think. Here is the rule: "Water when dry, don’t water until the soil is approaching dryness." When you water your Haworthia, go ahead and water it until water flows out the pot’s bottom hole. Then do not water again until the soil is dry. After awhile you can tell by the pot’s weight, but the best way to tell if watering is needed is to dig your finger or a pencil into the soil down to an inch or more and see if the soil is damp.
This method works very well because if you water your Haworthia and then a wet cloudy weather system sets in, it might not require watering for 4 to 6 weeks. However, in the summer, with lots of sunshine, a twice weekly watering might be required. The only caveat here is to be careful during cold wet winters, as a dead plant from rot can occur in days, while a dead plant from lack of water takes months and months and can be revived until the very end.
Fertilizing your Haworthias
If you re-pot every few years (highly recommended) it is actually not necessary to fertilize! Your plants will grow more slowly, but no harm will be done. For optimum growth, fertilization is a good idea. However, the rule here is to fertilized only with a dilute fertilizer, and only when the Haworthias are in active growth, such as the spring and in the fall. We recommend a ¼ - ½ strength solution of fertilizer formulated for African Violets or Cacti. Applying too much fertilizer will encourage too much foliage growth, producing grotesque plants. In the extreme, too much fertilizer will kill your Haworthias. Go easy on the fertilizer, and when in doubt don't fertilize.