I can't tell you more about the potato and onion, but you can also grow a plant from a pineapple top that you cut off the fruit.. And you can grow an avocado tree from a seed. Just wash the seed getting all the stuff off and let it dry or dry it. Then stick it in a pot with another little plant you have to water, or in a pot of it's own in soil and don't forget to water. It will sprout and come up for you. There is a water rooting method but the soil one is easier and always works for me. Have fun. You can also plant pinto beans out of a bag, in soil, and grow a plant.

If you want to grow a crop of potatoes ('Irish' potatoes not 'sweet' potatoes - different family) then you need to put the tuber into some sort of soil.

After a while the tuber puts out leaves, and begins to wither. It also puts out roots and the new potato tubers form on the roots like a string of beads.

In the light they become green and poisonous.

You can divide the potato in half so that each half has a green shoot on it. Let the cut face dry for a day.

Get two 20-25 litre buckets and make three holes in each bucket's bottom about 2cm wide.

Add around 10cm of potting mix and put one half of the potato, cut side down, on the mix. Cover it with more mix until the green sprout is barely showing. Sprinkle about a teaspoon of slow-release fertiliser on the surface. A fertiliser for tomatoes would be fine.

Keep the mix damp but not wet.

As the leaves grow up- add more mix so only 5cm of leaves are showing. Let it grow more, and keep adding mix until the leaves are at the top and the bucket is full. If you have access to straw you can use this instead.

Give the leaves very good lighting for most of the day and keep the mix damp but not wet. Keep the leaves dry when you water.

After a while the potato will flower - either white or mauve. Then the leaves will start to die -perhaps in August or so. This is normal. You can save the seeds and sow them to grow potatoes but they may make different tubers from the sort you started with. The seeds are poisonous.

Tip out the bucket onto a big sheet of plastic and see how well your potatoes have done. You may have lots of little ones, or a few big ones. If they have no green patches on them they are safe to eat.

The onion is a bulb - just like a daffodil. If you were to plant it in soil, in a pot long enough for its roots, and with about half of the bulb showing above the ground then it may flower - a round head of many little flowers usually white; or split into two smaller bulbs - or both. When the leaves die down - perhaps as late as September, there'll be an old flowerstalk and the little bulbs. You can divide them and plant on the offsets to increase your onion crop.

I haven't tried cutting an onion in half and just planting the root shield. It may lead to small onions. Not sure. But taking away the bulk of the bulb is robbing the plant of reserves for leaf and root production, and its flower bud.

If you're growing a sweet potato for its vine, then enjoy it while it's growing and replace it when finished. They require much warmer conditions and a long growing season compared with the 'Irish' potato.

Onions also appreciate potash as a fertiliser while growing. Just a pinch every month.

Hope that helps.