Here is what I found out from the book and and Plant Delights Nursery, Inc.
Here is a quote from Plants Delights website:
http://www.plantdelights.com/S...
(aka: S. reflexum 'Lemon Coral') Sedum mexicanum 'Lemon Ball' is a great garden and container plant, which could explain why it's been illegally renamed so many times for marketing purposes. Sedum 'Lemon Ball' is a selection of Sedum mexicanum (not reflexum or rupestre) that is a brighter yellow and more heat-loving counterpart of the popular Sedum 'Angelina'. The soft, golden, rubbery, linear evergreen foliage is great for creating fun color combinations...and wait until the fireworks-like heads of bright yellow flowers top the patch in late spring! Expect a 6" tall x 3' wide mound in 3 years. What a gem in the full sun rock garden.
No wonder there has been so much confusion over this plant.
Interesting way to tell if it is a S. rupestre, the bloomstalks are reflexed, bending over, until time to bloom. They they stand up straight.
