whether to say xxx or xxx flowers is complicated. It depends what xxx is. If they are primarily known and grown as flowers, then you can omit the 'flowers', e.g. I like ixias. If they are (say) a tree, then you need the 'flowers'. It was interesting. I've never heard of plumeria, so I googled it, and the picture was in fact a Frangipani. Assuming that's what you meant, and based on my knowledge of Frangipani, I would say that they are not primarily a flower, and if you said "I have two plumerias" I would assume you meant trees. "I plant a rose bush" usually.

Let's just take "plumeria" as example to see the usage of certain names of plants. 囍 Plumeria 1. as uncountable noun, refering to a genus of trees(plants) I like PLUMERIA. I like PLUMERIA trees. I like PLUMERIA flowers. I like the sweet PLUMERIA fragrance. 2. as countable noun, a plumeria tree or a plumeria flower I bought a plumeria yesterday morning. [ possibly either a tree or a flower, depending on the context ] 囍 Which to choose? 1. for common ornamental flowering plants, such as rose and violet, the name is often used as countable noun in everyday language, refering to the flower, the tree or the shrub, although in scientific context, the basic meaning may still be the plant as uncountable. For example, a rose, a rose tree. 2. If you anticipate that your listeners may be unfamiliar to the plant, or even not know what you are going to say is a plant, you may firstly use xxx tree/shrub/flower, and then you may say an xxx or whatnot.