There are a number of items that can be regrown from the scraps you throw away. Below are several examples.
Green Onion
Green onions grow fast and all they need is a little water and a lot of light. With very little effort, you can have a steady supply of green onions, grown right in your kitchen! The next time you pick up a bunch or two of green onions at the grocery store, don’t discard the white ends after you've used up all the greens. Instead, stick the white root end into a glass with an inch or two of water then place it in a sunny window. Make sure to trim most of the green end off before you place it in the water, or they’ll just get droopy! Also change the water every few days. Within a day or two, you’ll notice the green shoots starting to regrow, and the roots in the water getting longer. In less than a week, they’ll be back to their original size, and you can just trim off as much as you need, whenever you need it.
**Method also works for lemongrass, leeks, fennel, and spring onions
Romaine Lettuce
The best and most amazing thing about lettuce, besides eating it, is that if the stump is intact it will regenerate and regrow new shoots. To regrow romaine, put each stump in a half inch of water, change the water every couple of days, and wait for tender new growth. The perfect environment for regrowing leafy greens is a cool and bright one.
**Method also works for bok choy, celery, and cabbage.
Garlic
Simply plant garlic cloves individually. If they've started
to sprout, even better! Otherwise plant them with the pointy end up. Make sure
they are in a sunny location and soil isn't too damp. You can plant multiple
cloves four inches apart either in your yard or in a pot. It will fare well
even if planted among other flowers or vegetables. Then just sit back and
wait for your garlic to grow. If it's potted, water it occasionally but don't
drown it. As your garlic grows, it will sprout leaves. Once the leaves turn
brown and die, it's time to harvest your garlic. (Don't harvest any earlier or
your cloves will be too small!) Once you harvest your garlic, hang the
bulbs in a cool, dry location to dry them and prevent rot. Your garlic should
dry in about a week, at which time you can simply brush off the dirt and start
cooking with the cloves or plant some more!
Pineapple
I've seen many different examples but the best, most detailed one that I know of can be found
here.
Note: Pineapples take between 1 and 2 years to bear fruit.
Ginger
Plant a small chunk off of your piece of ginger in potting soil with the newest buds facing up. Ginger enjoys non-direct sunlight in a warm moist environment. Before long, it will begin to regrow shoots and roots. Once the plant is established and you’re ready to harvest, pull up the whole plant, including the roots. Remove a piece of the ginger, and re-plant it to repeat the growing process.
Onion, Mushroom, Potato & Sweet Potato, Avocado, Carrots, Lemon, and Apple coming soon...
Important Notes:
It pays to spend just a dollar or two more to buy an organic form for reusing. Also, for the items that are shown regrowing in water... Keep in mind that these should be transferred to soil after they root. Soil has nutrients that the plants need to grow and be healthy. And what's the point in growing it if it has no value to your nutrition as well?