Come & Praise Hymn no 134 'I planted a seed' What about the story of Helen Keller? You could 'twist' that to make it about hope? Try the Oxfam website? Or Christian Aid? Bibls story could be that of Abraham and Sarah? They had hope & faith.
The parable about Heaven being a mustard seed - tiny seed growing into one of the largest and most useful plants, is that any use? So it doesn't matter how little you are, how small your actions - they all sow hope.
Err...the story of Johnny Appleseed - conservationist, a bit American but may tick the cultural and historical box?!
Also this is my 10 year old son's suggestion from an assembly he really enjoyed:
I need to write a week's worth of assemblies on the theme of 'planting hope' and I just can't find anything. Saturday is slipping past and I am still sitting here at the computer! I need a bible story, but the one about seeds falling on fertile ground isn't exactly about hope. I also need a good story about new life sprouting out of seemingly dead, dry seeds (= never give up hope?) I would be very grateful for any ideas. Also a suitable hymn.