What are the requirements for karpas? It needs to be fruit of the earth, not of the tree or vine.

Where does it come from? From the Greek word karpos, meaning fresh raw vegetable. Karpas also comes from similar words for the color green in a number of different ancient languages. For example, karpas was the denotation for the color green in Farsi, and was the name for the color green according to Rashi (old French). Karpos was one of the original courses in the Greek symposium that many consider to be an influence on the shape and format of the seder. Even today, in Italy, there is a  tradition of starting a meal with pizimonio, a raw vegetable antipasti course.

Karpas  and "carpet" have something in common - and no, carpet is not the Sefardi pronunciation.

We all know  karpas  כרפס is the vegetable - often parsley or celery - eaten as a sort of appetizer at the Pesach Seder. What is the origin of the word?

There are those that claim it comes from the Persian word karafs   (or  karats, according to Klein), meaning parsley. Others claim that it derives from the Greek  karpos, meaning "fruit of the soil."  Karpos  originates in the Indo-European root kerp, meaning "to gather, to harvest." Other words from the same root include "harvest", and "carpet", because it was made of unraveled, "plucked" fabric.

One very similar word that does not appear to have any etymological connection (some interesting drashot notwithstanding) is the word  karpas  appearing in the Book of Esther (1:6), meaning "fine cotton or linen". I won't go into detail about that meaning of  karpas , since a big post on cotton should be coming up soon. However, Mar Gavriel presents an interesting theory here, that the pronunciation of  karpas  the vegetable was influenced by  karpas  the fabric:

According to Prof. Guggenheimer (in his book The Scholar's Haggadah), the words karpas (fine white linen) and karafs (celery) are both Farsi. Whoever provided the vowel-points for the mediaeval song "Qaddêsh u-Rechatz" only knew the consonants KRPS from the Meghilla, so he vocalized them as he had found them there.

haggadah Section: Karpas
Source: http://www.balashon.com/2006/04/karpas.html