The concept is that it keeps plants off the ground away from pests and by hanging upside down the plants will naturally grow upward toward the sunlight. With the water flowing downward onto the plant itself it also gets moisture and nutrients onto its leaves, thus producing a hardier plant yield.
Here's a generic version that can be used for a variety plants, from tomato's, green beans or any vine type plant.
I use it to make a Hanging Herb Garden just outside my kitchen door for a year round supply of fresh herbs to cook with. Living in So Cal most growing can go year round but we occasionally get those too cold days and rather than let my herbs die it only takes a minute to grab them and stash then on hooks in the garage until the weather warms up.
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Step 1: You will need:
A young leafy plant, such a Tomato, Green Bean, Cucumber or your favorite Herb. I used Italian Parsley for this project.
A Two Liter Green Soda Bottle
A Hole Punch
A Roll of Duct Tape
Scissors and/or a sharp Knife
Enough soil to fill your bottle at least 3/4 full
A little Garden Mulch or Bark
Something sturdy to hang your planter with, such as twine, leather string, a cut coat hanger, etc.
A weather resistant hook to hang your planter on
Note: I had stated before that using a green bottle aids in photsynthesis, I stand corrected as you will find explained in the comments below by Dwygrshpr.