In message , Hibiscus
writes

How do you differentiate "sea holly vs thistle"???


Sacha has inferred that you mean globe thistle (Echinops) rather than
all the other plants known as thistles, which makes sense as it is
Echinops that shares with Eryngium the trait of globular flower heads.
Another plant with a similar jizz is teasel (Dipsacus).

I tell Eryngium from Echinops by jizz (they don't look the same), but
that won't help you.

Eryngium is an atypical umbellifer, and Echinops an atypical composite.
This means that Echinops has fused anthers, but that won't help you,
unless you carry around a high-power hand lens.

Looking at my image files, Eryngium has a whorl of bracts below the
capitulum, while these are lacking from the capitulescence of Echinops
(part of what is atypical for Echinops as a composite is that the
capitulum is reduced to a single flower - even wall lettuce has five
flowers in a capitulum - and the capitula are arranged into globular
heads.
--
Stewart Robert Hinsley