teignage - I applaud your efforts to find a biodegradable, sustainable bag for waste. But I do encourage you to use resources that do not necessarily rely upon altering the environment, such as forestry. Although plastic is something that does not break down as easily as wood, it can be re-used and recycled - even the bags you put food in. Just rinse them out and return them to your grocery store.
I live in the Pacific Northwest. I see what 'sustainable forestry' does to the environment, especially on private land where clearcuts are still allowed. And we don't even need to talk about South America, do we? Let's work towards alternatives to wood products.
Compost your food, recycle and re-use your plastic bags. Skip the paper and save a tree.