With anything started in water you must replenish water as needed and change on a weekly basis.
Leeks, Spring Onions, Scallions and Fennel
Press root ends into moist soil and let them grow.
Romaine lettuce, Celery, Bok Choy, and Cabbage
There are several ways to do this. Cut and set in a cup of water on a windowsill and watch it grow havesting as needed. Or start them in water and when roots appear transplant to moist soil. Or press into moist soil, keep the soil moist and watch them grow – my personal favorite as I forget about things. Spritz with water weekly.
Wyzyrd: Bok Choy and Napa Cabbage
I’m playing with the core of the iceberg lettuce, so we’ll see what happens – it has sprouted.
Celery & Small Onions
Ginger
Be sure it has some nubs on it. Press into the soil similar to planting an iris. Keep moist but not damp. When you need it in the kitchen again, just cut off a chunk leaving nubs to regrow.
Potatoes
Cut chunks of potatoes out that have one or two eyes and leave to dry for 1-3 days then plant in rich soil about 4 inches deep with 4 inches above to keep covering it as it grows. You can transplant into the garden when weather permits.
Garlic
Take leftover cloves and press root side down into the soil and keep moist. I personally haven’t done this and the instructions vary from letting the top sprout and grow to nipping off the top so that all the energy goes into the root to regrow the bulb.
Sweet potatoes
My Kitchen Scraps Garden
Sweet potatoes vines are often grown as an ornamental in northern climates, but will also produce new sweet potatoes. Cut the potato end so that you have eyes. Stick toothpicks in and start it in glass of water. When roots develop and eyes are leafing out, plant in rich soil and keep watered.
Onions
Ever buy a bag of onions and have small ones in it? Just stick in soil and keep moist and they will grow larger. When you cut off the root ends of onions, just press into the soil leaving the cut end up and uncovered and keep moist as they too will regrow.
Lemongrass
Put in a glass of water and watch it grow! You can repot into moist soil. Harvest as needed.
Pineapples
Cut off the top leaving no fruit on the plant. Look for the roundish root buds. Cut leaves to within an inch of the top. Stick in a pot with rich soil and lots of moisture. Once the roots begin to develop and the top begins to grow reduce watering to once a week – this may take 2-3 months. Harvest a fresh pineapple in 2-3 years – yes, YEARS.
Mushrooms
Mushrooms are tricky. Cut the stem off and place it in pot of compost rich, moist soil. They tend to like warm filtered light during the day and cool nights and you will know in just a few days if it takes and will regrow a new head or rots in the pot. Just set it on the floor at night in a cool room.
Carrots
Yes you can regrow carrot tops, but the root will not grow back. Great for greenery or if you have rabbits or chickens.
Great little 3 minute video on replanting some of these.