You will also probably see these 4 letters imprinted just under the triangle "HDPE" High-density polyethylene www.hdpe.com
Same material that's used to line a landfill, used for Bamboo root barrier, and a array of other gardening products.
When I cut them in half, I make them into Sub-Irrigated Planters. The reservoirs are used to hold water only, no ballast rocks needed. The reservoir top is made from 10mm sign material called Coroplast. Drill 4 holes thru it and insert micro-fiber polyester material for the wicks, this way water can travel upward farther up the soil column. The reservoir is filled using a 3/4" PVC pipe ending just above the soil line and ending just below the reservoirs top.
By drilling 8 to 16 holes thru the sidewalls you can do vertical gardening at the same time, then the top can be planted with more plants.
Caution: Do not let the cut containers that are empty, sit in the sun, or the cut edges will start to curl, I've even had them get almost square looking viewed from the top.
The following photos are similar to a 55 gal barrel cut in half, they are made from the same material HDPE, but are black in color. Each is able to grow 8 vertical planting holes and anywhere from 1 or more plants in the top, depending on the mature size's of the plants
The SIP tub planter, pictured above has 15 salad greens planted in it.
