I've found a page answering this and setting out 12 (!) verses:
http://www.acronet.net/~robokopp/english/greengro.htm
I'l l sing you twelve-o
Green grow the rushes-o
What are your twelve-o?
Twelve for the twelve apostles,
Eleven for the eleven who went to heaven, and
Ten for the Ten Commandments,
Nine for the nine bright shiners,
Eight for the April rainers,
Seven for the seven stars in the sky, and
Six for the six proud walkers,
Five for the symbols at your door, and
Four for the gospel-makers,
Three, three arrivals!
Two, two lily-white boys
Clothed all in green-o
One is one, and all alone,
And ever more shall be so
The page also includes a glossary wwhich tends to the Christian.
Incidentally, there's a Scottish folk song of the same name - different words and and tune - with words that may have been written by Robert Burns:
http://mysongbook.de/msb/songs/r_clarke/greengro.htm