You can also look up recipes for making different stocks. If something is never included, that’s a good sign to be careful. Stocks tend to concentrate a veggies’ flavor.

From your list: Potatoes will thicken the sauce but not add much flavor–you definitely would not want to add any green peels or bits as they can make you sick. Sweet potatoes are good.

Lettuce stems tend towards bitterness, so I wouldn’t add them. Washed mushroom stems are very good. I’d be careful about adding any citrus.

Carrots, onions, and garlic are good, but a whole lot will make your stock sweet. Herbs are good but usually added towards the middle or end.

Spinach is good. I haven’t tried cucumbers. Eggplant that wasn’t bitter would be good, but I’d be reluctant to add the skins because they tend to be more bitter.

If you like the taste of broccoli or cabbage, a little of those can be added halfway. If really over-cooked, members of that family get a sulfur flavor.

Tomatoes are good. I wouldn’t overdo peppers and don’t use the seeds or inner membrane as these tend to be bitter.

I’d be careful about any fruit. I think bananas would get nasty. Avocados get nasty when cooked, too.

Summer squash is good, just be careful the Zucchini isn’t bitter. I love winter squash (acorn, butternut, pumpkin, etc.) bits in a stock, but I wouldn’t put in the stems. I’ve never tried the seeds in a stock.

Corn is good as are fresh corn cobs.

I haven’t tried asparagus; it might be bitter.

Beans and peas are good. I like to use the fresh pea shells from my garden.

Beets will turn your broth bright red and are strong tasting if overcooked, so I don’t use them.

Bok choy, collards, kale,and Swiss chard are good. I wouldn’t use mustard greens or radishes. I would go light with turnips and make sure they aren’t bitter before adding. Leek trimmings would be good.

Basically, if it would get a strong or bitter flavor from cooking a long time, I wouldn’t use it. If it would get a nasty texture, I wouldn’t use it.

In general, a mix of vegetables is best.

Ok, this is all over the place–anyone else got a better summary?

Steve and Shelley, I never end up with dirt in the stock. I wash vegetables (except onions) before I use them so they are clean before I start the stock. Even if I peel carrots, I wash them first. I use onion ends and scraps, but not the brown onion skin because I think it makes the stock a bit bitter. I also trim the root end off the onion so there’s no dirt left.

If I am low on veggie scraps, I will throw in a old carrot, a tough celery stalk (both washed), and an chopped onion (skin and root end removed), to help start the broth. Of course, you wouldn’t want to add anything moldy, but you can also trim the soft spots off old tomatoes and use the rest.

Since they can get a strong flavor, if I add broccoli stems or outer cabbage leaves (once again, washed), I do it when the stock has cooked about half way.

Not all veggie scraps are usable this way. I don’t add anything that might add bitterness since my family is sensitive to that. So I wouldn’t add eggplant skins or broccoli raab stems.

Basically, you are adding vegetables that are not bad, but not good enough to eat as is. If you are just starting, I would try this:

Put in a pot one or two chicken carcasses with a bit of meat on them, an old carrot chopped up and/or a bunch of carrot peels, a chopped onion, celery ends and an outer celery stalk or two, two bay leaves, and 6 peppercorns. Cover with water, put a lid on the pot and simmer for two or three hours. Strain through a fine sieve. Remove excess fat by using a fat separator (see my comment above), skimming it off, or freezing the stock and scraping off the hardened fat. Taste the stock. If it’s not strong enough, add a bullion cube or simmer it uncovered until it is. Add salt (or soy sauce). To finish off the stock, I usually add a teaspoon of garlic powder and a dash of sherry and simmer another 15 min.

Hope that helps.