Even if you love that great succulent look or drought has led you to these plants, you may already have had trouble growing them. The cause of death is usually overwatering, after which rot sets in, and the plants melt down into a gushy rotten mess. The problem is knowing when they want water and when they do not.
To understand the "when to water" question, you need to know how succulents are different from ordinary plants. The chief characteristic is specialized cells that hold moisture ready rather than deriving new moisture daily from the soil. After they are watered, succulents take up moisture until every succulent cell is fully hydrated. Over time, they utilize this stored moisture, gradually reducing the hydrostatic pressure inside the plant's cells and tissues. The roots prefer dry airy soil in between water applications which is why they are typically grown in porous potting soils.
Problems arise in wetter or more humid climates when soils do not dry out between water applications. They also occur when the drain hole in a pot becomes blocked or too small to begin with. Pots designed for succulents often have numerous holes for this reason.
To know when to water succulents, you must learn how to tell when hydrostatic pressure is low.
When your newly watered succulent is fully hydrated, it is in the "hard body" stage. Squeeze it gently to get a feel for how hard the individual plant becomes, and let this serve as a basis for future comparison. When the internal moisture is used up, the whole plant will gradually soften due to pressure loss. They actually yield to your touch. Beware of those with only some parts softening, as this can indicate invisible rot is spreading into healthy tissue.
When fully hydrated, every succulent plant will stand up to its full height and form, which combine to create an attitude. However, many alter their overall attitude when internal pressure drops low enough. For example, Pachypodium's long thin stalk will literally fold in half, then once watered, it straightens right back up again as hydrostatic pressure returns. Others will show slight wrinkles on the skin caused by reduced interior tissue volume. Drooping leaves, sagging tips, or subtle leaning are all attitude changes caused by low internal moisture.
There are many problems caused when succulents are watered from the top down like ordinary potted plants. Water travels down the soil mass edges and out the bottom, leaving too little behind for adequate uptake. Succulents often rot right at the soil line due to this kind of watering. Set them in a pan of water for easily handled pots so the soil inside can wick up the water through the drain hole. When the moisture wicks up to the soil's surface, take the pot out of the water and let it drain. This method ensures that the entire soil mass is thoroughly moistened, not just the edges.
Remember that most succulents originate in habitats with occasional heavy rain events followed by extended periods of drought. Fortunately, they speak to us in the silent language of touch and appearance, which tell you clearly when they are dry enough for a drink.
- Succupedia: Browse succulents by Scientific Name, Common Name, Genus, Family, USDA Hardiness Zone, Origin, or cacti by Genus