Sub-irrigation is a technique that conserves water by creating a water reservoir beneath plants. This technique is best for use in grow boxes that are kept outdoors and receive some amount of natural rainfall. You can build your own sub-irrigation grow box using a few common household items; and you can grow nearly any type of plant in the grow box. A homemade sub-irrigation grow box takes less than an hour and costs little to nothing to make.
Poke holes throughout one side of each of the ½-gallon plastic milk jugs. Poke about 20 holes in the side using a screwdriver, knife or other sharp tool. The milk jugs will become your water reservoir.
Cut a slot into each of the milk jugs--in the side opposite the one with the holes. Make the slotted hole about 2 to 3 inches long and a ½-inch wide.
Cut a hole in one of the milk jugs (in the same side with the holes) the same size as the mouth of the water bottle or PVC pipe.
Cut the bottom off of the water bottle and ensure that the bottle is long enough so that it can reach 1 inch above the top of the 18-quart or larger container. If you're using a PVC pipe instead of a water bottle, cut the pipe so that the length of it is 1 inch taller than the height of the larger container.
Lay the milk jugs into the bottom of the 18-quart or larger watertight container with the sides with the holes facing up. Arrange the milk jugs so that they cover nearly the entire bottom of the 18-quart or larger container.
Insert the mouth of the water bottle or one end of the PVC pipe into the milk jug that you cut the hole into. This will become your fill pipe.
Pack potting soil in the bottom of the 18-quart or larger container around the milk jugs. Tamp the soil around the milk jugs firmly.
Cut a small, ¼- or ½-inch overflow drainage hole into one side of the 18-quart or larger container that is right above the level of the milk jugs. Cut another drain hole at the same level into the opposite side of the larger container.
Add your potting soil and whatever plants you desire. Ensure that the plastic water bottle or PVC pipe remains free of soil and is accessible above the soil surface level.
Fill your grow box's reservoir with water by pouring water into the fill pipe (the plastic water bottle or PVC pipe). The reservoir is full when you see water begin to dribble from the drainage holes in the sides of the 18-quart or larger container.