Pebble plants or living stones, as they are commonly called are "Lithops." "Lithops" is both singular and plural. These "stone-like" plants are native to Namibia and South Africa. The reason they are called living stones or pebble plants is because the plant consists of two leaves, both of which look like stones. These plants have the unique ability to camouflage themselves, a trick they use to protect themselves from predators.
The key to success when growing living stones, is to remember the conditions in which they were growing in the wild. Although it probably won't be possible to replicate them exactly, the more you can do to provide them with similar conditions, the more likely they are to survive, to grow properly and to look good. In their native wild habitat, they grow in very hot and dry conditions. They also grow very slowly, so keep this in mind.
When people do not have success growing 'lithops,' the reason is likely that they over watered the plant or didn't provide it with adequate ventilation. When these plants are thrust into cool and/or humid places, this becomes an even greater detriment.
In their native African habitat, they grow in places where there may be little or no rain for many months on end. Because they have grown in this type of environment for so long, they evolved in a way that made it possible for these strange little plants to adapt to their surroundings.
Without water, it became necessary for the plant to find some means to keep itself nourished. The shape of the leaves is an ingenious survival mechanism. The reason they are so distorted and misshapen is because that is a protective feature that minimizes evaporation. As with all succulents, the leaves of 'lithops' retain water so that the plant can continue to nourish itself in times of complete dryness.
PROVIDE ADEQUATE LIGHT:
The best growing environment for a living stone is in a south facing greenhouse. In the southern hemisphere, the direction would invariably be reversed. They need as much light as possible, and a southern or western exposure (or both south and west,) will provide this. If there isn't sufficient natural light, full spectrum lights will help. It is possible to create the full spectrum effect by using normal florescent lights. Simply combine equal numbers of cool and warm lights.
When growing 'lithops' under artificial lights or grow lights, it will be beneficial to lower the light fixture so that it's closer to the plant. In addition to providing the plant with more intense light, it will also add some warmth, something that will greatly benefit the plant. When these plants don't get enough light, they begin to grow tall and ultimately lose the characteristic stone like shape.
IDEAL GROWING TEMPERATURE:
Because the native habitat for these plants lies below the equator, they've naturally adapted themselves to tolerate very hot temperatures and very dry conditions. As with many other types of tropical plants and flowers, these succulents only grow during the hot weather which in the southern hemisphere is in summer and fall. (That would be our winter and spring.)
When grown in the northern hemisphere, they will adapt to our summer and fall conditions. They can tolerate cooler temperatures during the winter and spring months because the plant is resting then, preparing itself to grow a new body. Be sure that the plant is never exposed to freezing temperatures or temperatures that are below about 55 or 60 degrees.
Because living stones will die if the soil is too moist, a potting soil is the worst possible planting medium to use. A general purpose compost to which you add a significant amount of grit and some stones to help drainage is ideal. Any type of cactus compost or planting medium will also work. Again, the added grit will help with drainage considerably. The most important consideration when choosing a planting medium is to make sure that whatever you use will not hold water.
WHEN AND HOW TO WATER:
This plant is unique in its watering needs. It will only be necessary to water it during the growing season. Depending on where you live and when the temperatures begin to get cooler and you get your first frost, that will be the time at which to stop watering your living stones. You will not water them again until sometime between May and July.
While the plant appears to be dormant over the winter, it will be generating two new bodies. The leaves are referred to as the plant's bodies. As the new bodies grow, they will absorb moisture and nutrition from the old one. While this happens, the old bodies will be drying out, shriveling up and dying.
As soon as the remains of the old bodies are completely shriveled up and dried out, you can begin to water the plants again. This will happen at different times depending on the variety of the plants. Cut off the stems and remove the shriveled and dead remnants of the old plant from the pot.
When you water your plants, water them enough to soak the the planting mix completely. The new plants will draw water into their leaves or bodies from this and this will sustain them until you water them again. The soil must dry out completely before the plant is watered again. There are a number of ways to check to see whether the soil is dry enough.
The low tech and most cost effective method is simple. Just stick your finger as far down into the soil as you can. Be sure to do this towards the edge of the pot and away from the plant so you don't disturb the delicate leaves or root system. The soil may seem dry towards the surface but moister down below. If this is the case, it's not time to water it.
The very accurate high tech method involves using a long probe water meter. Stick this into the soil as far down as you can. The meter will detect the level of moisture in the soil and this will let you know if it is time to water the plant again.
The importance of allowing the plants to dry out cannot be understated. The biggest reason that plant growers fail in their attempts to grow 'lithops' is that they over water the plants. Rather than risk over watering the plant, it would be better to err on the side of caution.
Remember, these plants have the capacity to store water in their leaves, and this will provide them with the nutrition and moisture they need. A little neglect may be advantageous when growing 'lithops.' Just be sure they get sufficient light and warm enough temperatures.