Three persons have uploaded photos and as near as I can tell Kelli has the right plant and the other two do not. I see an entry in the db for a cultivar (Centaurea melitensis 'Aloha') that look similar but no info or other photos of it. Have a look and see if we can sort this out.

Centaurea americana?

http://www.wildflower.org/plan...

https://www.google.co.uk/searc...

Aloha Rosa ..

http://www.vandijkenco.com/pro...

I've removed Centaurea americana Spreng. as a synonym from the C. melitensis entry since it refers to a different plant than listed on the sites Janet referenced (Centaurea americana Nutt.) and because it is an illegitimate name.

Plectocephalus americanus is synonymous with C. americana Nutt. if that helps.

I see the confusion!

http://www.theplantlist.org/tp...

Centaurea americana Nutt. is an unresolved name*

http://www.theplantlist.org/tp...

http://www.ipni.org/ipni/plant...

http://www.ipni.org/ipni/idPla...

As unresolved it may be, it is 'legitimate' on Tropicos.

http://www.tropicos.org/NameSe...

First in time takes priority.

Collections on the NYBG ..

http://sweetgum.nybg.org/vh/sp...

http://sweetgum.nybg.org/vh/sp...

http://plants.usda.gov/core/pr...

Legitimate but no conclusion yet, per both Tropicos and the GCC.
Centaurea americana Nutt. validly published-1821
http://www.tropicos.org/Name/2...

[Centaurea americana Spreng., Illegitimate-1826
http://www.tropicos.org/Name/1... ]

Plectocephalus americanus (Nutt.) D. Don is validly published--1830
http://www.tropicos.org/Name/5...

Click on the Accepted tab in each of the two legitimately published entries and the other is referenced.

The same is true at the GCC except they note each as a synonym for the other.
Centaurea americana Nutt.
http://compositae.landcarerese...

Plectocephalus americanus (Nutt.) D.Don
http://compositae.landcarerese...

Edit: So a plant floats in limbo and we have no entry for it.

First in time takes priority, the reason the name is undetermined we don't know. It could be a simple answer such as nobody has got around to doing the determination.

Centaurea americana is the earliest named, and it has more references on Tropicos as well as images. The IPNI only lists this, there is no reference on the IPNI for Plectocephalus americanus.

The Plant List is a conglomeration of scientific resources, it has changed the reference to Plectocephalus americanus ....

Earlier versions

This name was in version 1 of The Plant List, record gcc-14994.
•this name record has changed
•names related to this name have changed

http://www.theplantlist.org/tp...

I would take the record from the IPNI as the correct one, look at the source references of both:

Plectocephalus americanus ..

Published In: The British Flower Garden, . . . series 2 1: t. 51. 1830. (Jun 1830) (Brit. Fl. Gard., ser. 2)

Centaurea americana ...

Published In: Journal of the Academy of Natural Sciences of Philadelphia 2(1): 117–118. 1821.

The indecisiveness could be that DNA tests are required to determine if they are actually the same plant, taxonomists are now renaming many plants due to DNA where originally the only thing they had to go on was physical structure.

I see no reason why Centaurea americana can't be listed.

The links are to illustrate why The Plant List notes C. americana as unresolved (and why C. americana Spreng. is ill.). I'm prone to agree that C. americana be included in the database with P. americanus as it's synonym. Hopefully searching for Plectocephalus americanus helps in providing confirmation that the images posted by Lesliray are in fact C. americana Nutt.

The book by Robert Sweet which Plectocephalus americanus is listed in, plate 51.

https://books.google.co.uk/boo...

It's been added. See The thread "Very confusing synonym?" in Plant Database forum
Maltese Star Thistle (Centaurea melitensis)

It makes sense.

It felt like a deja vu but I couldn't remember why.

I don't know what it is but it is beautiful!