Organic materials tend to rot one way or the other. I would go with a bag of dyed ground foam. The only organic material I prefer is decomposed granite known as "chicken grit" which makes for an inexpensive and realistic ballast.
In all respect, I have build and displayed layouts under a very wide variety of environmental conditions for a very long time, and I have zero evidence that organic materials on the layout cause any problems. Ever.
Do you ever use Homasote? Do you model using bristol board or cardboard or vellum? How about foam core? Non-toxic paint? Paper? Lichen? Wood? All of these extremely common modeling materials are (or contain) organic materials and I venture to say that you will never find a significant layout that doesn't employ many of them. The "don't use organics" bogy is not a credible argument.
I will step off of this particular soapbox now.
