Celery, onions and fennel cut the base away and plant this on the surface of potting mix. When small leaf shoots start to pop up its time to plant into the vegetable patch.
Carrots can be regrown from the tops in a similar fashion.
Potatoes can be grown from thick peelings, place skin side up on potting mix. Once shoots start to grow, plant into the vegetable patch in a trench.
Sweet potatoes can be regrown from the end of the tuber. Place these on cotton wool in a saucer of water cut side down and grow indoors. Once shoots are about 10 centimetres long pull these off from the mother plant and plant out in the vegetable patch.
Remove leaves from hydroponic lettuce and plant out the stump with roots into the vegie garden.
Small or dried garlic cloves can be planted like bulbs, pointy end up in the garden.