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'