The difficulty in caring for plants is making sure they get enough water but not too much, particularly if you are growing a large amount in a greenhouse. If you plan on going out of town, a self-watering system of trays will ensure that your plants only get the water they need while you are gone, but buying them can get expensive. making them yourself, however, is simple.
Invert the plastic storage basket and draw an outline of the rim of the basket on the floor of one storage tub. Cut out the inside of the outline with the pocketknife and fit the basket through the bottom of the tub, but let the rim catch so it doesn't fall through the hole.
Fit the tub with the hole in the bottom into the other storage tub. Draw an outline around one of the black tubes on the floor of the tub next to a corner or wall, and cut at the outline with the knife. Insert the tube through the hole and lean it against the wall.
Draw two more outlines of the black tubes on the side walls at the bottom on either side of the upper storage tub. Cut out the holes through both the top and bottom tub. This is both to aerate and ensure the water has a place to overflow if need be. Slice and turn holes throughout those two black tubes and slide them through the holes across the tubs.
Lay the landscape fabric across the bottom of the upper storage tub and the basket. Fill the basket over the fabric with potting soil and start to fill the upper tub. Do not cover the tube next to the wall of the tub coming up. Plant your plants in the watering tray.
Move the watering tray into your green house and pour water down the tube next to the wall of the tub. One of the great parts about this design is that it allows a minimum of the water to evaporate, which makes it ideal for a greenhouse to keep the water in the plants, rather than letting it get into the air. The potting soil wicks up the moisture so the plants only take the water they need and the tubes running through allow air to get to the roots.