A Rain of Thorns

Question: Would you allow a low-level Fertility boon that allowed the Scion to 'create' arrows out of any plant material?

Probably not. That sounds incredibly narrowly focused, and, more importantly, it sounds like something we'd let Scions do on their own anyway without a boon. If you've got a dot of the appropriate Craft (or Jack of All Trades), you could already use plants to help you create arrows or javelins by whittling them to point, twisting them to make them stronger, using existing thorns, or whatever else you happened to stunt. Plants are, after all, one of the great raw materials of the world anyway; if humans can turn them into weapons with a little hard work, I don't know why on earth Scions would need a boon to do so.

Such a boon would also be rendered completely irrelevant by the time the Scion was able to get Twist Plants anyway, and putting powers into the game that become literally useless when a Scion gains Legend is pretty counter-productive. It sounds to me like what a Scion really wants in this situation is just a dot or two of Craft (Fletching) (which will help them make arrows out of everything, not just plants!) and then stunt it to go along with their Fertility, possibly along with using Fertility boons to enhance/grow plants to use in arrow-making. That provides a character with more options, not fewer, and doesn't require either losing cool flavor or creating a specialty boon that appears to be mostly pointless.

I'd also consider letting Scions use a Toxic Thorn as an arrowhead if they stunted it well.