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What is the difference between sedum and succulent and cacti? Kalanchoe seems to have many different types. Is Kalanchoe a succulent because it is not winter hardy? Is Dr. John Creech a sedum because it is from the Mountains and is winter hardy?

This question makes an apples-to-oranges comparison, and some basic botany principles are useful in answering it, so bear with me.

Scientific plant identification is called Plant Taxonomy. The entire hierarchy for identifying plants consists of Kingdom, Subkingdom, Superdivision, Division, Class, Subclass, Order, Family, Genus, Species and Cultivar. Fortunately, as gardeners we normally only need to be concerned with the last three classifiers: genus, species and cultivar. Sometimes it’s useful to go up a level and explore Family as well.Genus is a group of closely related species clearly defined from other plants. Species are sub-categories of the genus.

An example of genus and species would be maple trees: Acer is the genus for all maple trees. Under that genus are the species Acer japonicum (Japanese Maple), Acer rubrum (Red Maple), Acer succharinum (Silver Maple) and so on.

A cultivar is a plant or grouping of plants selected for desirable characteristics that can be maintained by propagation. Most cultivars have arisen in cultivation but a few are special selections from the wild. Continuing with the Maple tree example: Acer japonicum ‘otaki’ identifies a specific cultivar of the species Japanese maple.

Most gardening references and plant tags will provide one to three names. If there are three names given with the first capitalized, the second in lower case and the third in single quotations you have the genus, species and cultivar in that order. Example: At your local nursery, a purple coneflower plant would likely be labeled: Echinacea purpurea ‘Magnus' (genus, species, cultivar) If there are only two names given, the first is the genus and the second is the species. If only one name is given and it doesn’t sound like English (usually it’s Latin), it is usually the genus (in this case Echinacea).

If the single name is English, it’s probably the common name, which in this case would be Purple Coneflower.

If you want to read more about taxonomy, here’s a link to a good description of taxonomy from Colorado State University Extension:

Okay -- So with that out of the way, let’s look the specific plants you’ve asked about:

The term succulent is used to describe plants having some parts (leaves, roots, or stems) that are more than normally thickened and fleshy, usually to retain water in arid climates or soil conditions. But plants are not either succulent or non-succulent. Within the same genus and family there might be plants with thick leaves and normal stems as well as plants with very thickened and fleshy leaves or stems. So deciding what is a succulent is often arbitrary. “Succulent” is a term of description, not a category in formal plant classification.

Cactus - Botanically, all cacti are succulents, but not all succulents are cacti. (In other words, all cacti have the characteristic of retaining water in thickened and fleshy parts of their structure, but not all plants with succulent characteristics are classified as cacti.)A cactus is a member of the plant family Cactaceae, within the order Caryophyllales.. Cactus are native to the Americas, ranging from Patagonia in the south to parts of western Canada in the north. There are also cactus that grow in Africa and Sri Lanka.

is a large genus of flowering plants in the family Crassulacae, members of which are commonly known as stonecrops. The genus has been described as containing up to 600 species of leaf that are found throughout the Northern Hemisphere varying from annual and creeping herbs to shrubs. The plants have water-storing leaves (there’s that succulent characteristic!). The flowers usually have five petals, seldom four or six. There are typically twice as many stamens as petals.

So, as you can see, cactus and sedum are each in a different genus of plants coming from two different families within the plant hierarchy, yet both can be described as “succulent.”

is a genus of about 125 species of tropical, succulent flowering plants in the family Crassulacea. They are mainly native to the Old World, but with a few species now growing wild in the New World following introduction of the species. Most are shrubs or perennial herbaceous plants, but a few are annual or biennial.. Note that Kalanchoe are in the same Family (Crassulacea) as sedum, but not in the same genus. Since Kalanchoe are generally native to warm climates, they’re not hardy here in Ohio, so we enjoy them as house plants. But regardless of their hardiness or lack thereof, they are still “succulents” that store water in their fleshy leaves.

Dr. John Creech is a cultivar of sedum. (family Crassulaceae/genus Sedum/Cultivar Dr. John Creech) Dr. John Creech is cold- hardy in zones 3-9 to minus 40 degrees, yet it is in the same family and genus as Kalanchoe, which is a tropical! Genetically the plants are related, and again, both have succulent characteristics. 

Photo:  Sedum spurium 'Dr. John Creech'