If I do not have enough for a batch of stock, I simply freeze and keep adding to my scrap bag until I have enough. As I peel veggies and cut up veggies, any waste goes in my freezer for a stock as well.
Making vegetable stock each week will give you plenty of stock for soups and other recipes.
Slow Cooker Stock - I also like to put the water and veggies in my slow cooker at night and let it simmer on low all night. Let it cool and place in freezer containers to freeze.
When I cannot salvage, I compost it...Love our compost bin. This is the one we have (I only paid $69 as it was on a daily deal sale):
keep an eye on sales for compost bins in early Spring and again in the Fall.
Or make your own. Here are more tips on Composting.
~ Ruth Ann