Ask a Master Gardener: All about succulents

In this issue, we discuss succulent plants. From a botanical standpoint, a succulent is defined as any plant that possesses morphological and/or physiological adaptations that allow it to survive severe drought conditions. There are about 55 plant families that contain succulents of one form or another, with the Cactaceae plant family being the most familiar. All cacti are succulents, however, not all succulents are cacti. It is estimated there are over 10,000 species of plants that can be classified as succulents.

A good example of a physiological adaptation is found in the Crassulaceae plant family. Members of this family open their stomata at night when conditions are less conducive to water loss via transpiration. Carbon dioxide taken in by the plant is stored in the form of organic acids. After the sun arises the next morning, the acids are broken down so that the carbon dioxide they contain along with water and the sun's energy can complete the photosynthetic process.

Succulents can be categorized furthered according to the nature of the adaptation that causes them to be classified as succulents. The three major categories include leaf succulents, stem succulents and root succulents. Leaf succulents store water in enlarged, specialized spongy portions of their leaves. In certain cases, leaf succulents lack stems, as is the case with Lithops spp., also known as "living stones." Most leaf succulents also have thick cuticles to help delay water loss.

Stem succulents generally lack leaves and carry out photosynthesis in their green, succulent stems. Many members of the Cactaceae plant family (e.g., Cerus peruvianus, or Peruvian apple cactus) are stem succulents. As is the case with leaf succulents, most stem succulents possess thick cuticles and other adaptations that help to conserve water.

Finally, root succulents store water in a thick, enlarged structure known as a caudex which is formed at the base of the plant between its roots and stem. In addition to the water storage structure, most root succulents also have small leaves with reduced surface area to minimize water loss. As a group, root succulents are relatively rare in nature.

Succulents make good houseplants and are relatively easy to care for. Like all plants, they need certain basics such as light, a growing medium, water, plant nutrients, and proper temperature to thrive. While all are important, light often is the most limiting factor for successful succulent growth.

When grown indoors, succulents should be placed in brightest location available. Ideally, they should receive ten or more hours of bright, indirect light. When indoor light levels are inadequate, supplemental lighting is beneficial. High-output, full-spectrum light from fluorescent and LED fixtures works well in the home. Situate the fixtures six to twelve inches above the plants and keep them illuminated from between fourteen to sixteen hours daily. If succulents are moved outdoors for the summer, initially avoid direct sun which can harm plants not acclimatized to high light levels. After several weeks of indirect sunlight, the plants can be moved to sunny locations without harm.

Succulents require a very porous growing medium that dries quickly and does not hold too much moisture. In nature, these conditions are provided by very well-drained, sandy soils. This type of root environment should be duplicated when growing succulents in containers. Typical potting mixes retain too much water for long periods of time and can cause root rot and death of succulents. Alternatively, a self-formulated growing medium of one part potting mix and one-part coarse sand, turkey grit or perlite works well. To test a potting medium for moisture retention, moisten the mixture and squeeze it in your hand. Upon release, the soil should fall apart.

Proper watering probably is the most important and often misunderstood factor when it comes to the care of succulents. After selecting a well-drained growing medium, the best approach to watering succulents growing indoors is to set up a wet-dry cycle. Begin the cycle by watering plants thoroughly, making sure the entire soil volume is fully wetted. Water should run out of the drainage holes in the container. Following watering, allow plants to dry thoroughly, making sure the entire volume of soil is dry before watering again. When in doubt, wait to water. Succulents do not tolerate wet growing media for extended periods of time. Therefore, take care not to allow water to sit for more than a few hours in trays, sleeves or anything placed underneath the pot to catch drainage water.

As a rule, succulents are not heavy feeders and require relatively little fertilizer. Only when plants are actively growing should they be fertilized. This normally is in the spring and summer months. When the succulents are actively growing, application of a water-soluble fertilizer at one-quarter to one-half the rate listed on the label every three or four waterings normally is satisfactory. Avoid the temptation of applying fertilizer to succulents in the winter months when they are not actively growing.

Temperature is another important consideration for success with succulents. Although most succulents are native to hot climates, they do not require above-average temperatures. Temperatures maintained in an average home are adequate for nearly all succulents. When growing indoors, succulents often prefer to have cooler temperatures at night and warmer temperatures during the day. In general, keeping succulents between 55 degrees and 75 degrees is best. However, many species will tolerate temperatures as low as 45 degrees and as high as 85 degrees.

Succulents are adapted to tolerate the low humidity conditions typical of a home environment, especially during winter months. If homes are equipped with humidifiers, good air circulation around succulents is needed. Air circulation helps the potting mix to dry, lowers humidity, and reduces the risk of insect pest infestation such as mealy bugs and spider mites.

Finally, a word of caution is in order. Most cacti along with a few other succulents possess spines. In nature, these spines serve as a defense mechanism that deters herbivores from feeding on succulents. In the home, they can pose a risk to children or pets. Therefore, succulents with threatening spines should be placed out of harm's way in the home.

(Source: Missouri Environment & Garden Articles, Dr. David Trinklein)

Dhruba Dhakal, PhD is a University of Missouri Extension Horticulturist, serving to Missourians about a decade in Central Missouri. Dhruba can be contacted at [email protected] with gardening questions.