Peter, my condolences on your loss. It's wonderful that you plan to honor your grandmother by composing music for that poem!

Though there appears to be a number of 19th century poems about 'forget me not' flowers, I've not found the one you posted.

However, just as a matter of interest-though not for the music/song you are composing, here's a copy of what appears to be the most often cited 'forget me not' poem:

"The Scottish poet William McGonagall (1825?-1902) rhymed a romantic medieval legend that is is supposed to explain the origin of the flower's common name, & I cannot resist quoting the entirety of his poem "Forget Me Not":


FORGET ME NOT
(William McGonagall)

A gallant knight & his betroth'd bride,
Were walking one day by a river side,
They talk'd of love, & they talk'd of war,
And how very foolish lovers are.

At length the bride to the knight did say,
'There have been many young ladies led astray
By believing in all their lovers said,
And you are false to me I am afraid.'

'No, Ellen, I was never false to thee,
I never gave thee cause to doubt me;
I have always lov'd thee & do still,
And no other woman your place shall fill.'

'Dear Edwin, it may be true, but I am in doubt,
But there's some beautiful flowers here about,
Growing on the other side of the river,
But how to get one, I cannot discover.'

'Dear Ellen, they seem beautiful indeed,
But of them, dear, take no heed;
Because they are on the other side,
Besides, the river is deep & wide.'

'Dear Edwin, as I doubt your love to be untrue,
I ask one favour now from you:
Go! fetch me a flower from across the river,
Which will prove you love me more than ever.'

'Dear Ellen! I will try & fetch you a flower
If it lies within my power
To prove that I am true to you,
And what more can your Edwin do?'

So he leap'd into the river wide,
And swam across to the other side,
To fetch a flower for his young bride,
Who watched him eagerly on the other side.

So he pluck'd a flower right merrily
Which seemed to fill his heart with glee,
That it would please his lovely bride;
But, alas! he never got to the other side.

For when he tried to swim across,
All power of his body he did loss,
But before he sank in the river wide,
He flung the flowers to his lovely bride.

And he cried, 'Oh, heaven! hard is my lot,
My dearest Ellen! Forget me not:
For I was ever true to you,
My dearest Ellen! I bid thee adieu!'

Then she wrung her hands in wild despair,
Until her cries did rend the air;
And she cried, 'Edwin, dear, hard is out lot,
But I'll name this flower Forget-me-not.

'And I'll remember thee while I live,
And to no other man my hand I'll give,
And I will place my affection on this little flower,
And it will solace me in a lonely hour.' "

http://www.paghat.com/forgetmenots.html

****

Also, see these excerpts from the Wikipedia entry:

"In the 15th century Germany, it was supposed that the wearers of the flower will not be forgotten by their lovers.

Legend has it that in medieval times, a knight and his lady were walking along the side of a river. He picked a posy of flowers, but because of the weight of his armour he fell into the river. As he was drowning he threw the posy to his loved one and shouted "Forget-me-not". This is a flower connected with romance and tragic fate. It was often worn by ladies as a sign of faithfulness and enduring love.
...

In 1948 the little blue Forget Me Not flower, or badge, was adopted as a Masonic emblem at the first Annual Convention of the United Grand Lodges of Germany, Ancient Free & Accepted Masons. The flower, or badge, is now universally worn as a Masonic emblem in the coat lapel to remember all those that have suffered in the name of Freemasonry, and specifically those during the Nazi era."

http://en.wikipedia.org/wiki/Forget-me-not