Many varieties of petunias (Petunia spp), which are part of the Solanaceae family, grow in a "wave" fashion across a horizontal space. Some varieties -- including Petunia "Trellis Pink" -- grow upward when given a vertical support. When you have kids or pets running through your garden, the vertical option might keep your colorful flowers from being trampled. It's simple to make a trellis using minimal of tools and supplies.
Lay three 3-foot pieces of bamboo about 1 inch thick on the ground with the bottom ends touching.
Fan the top ends of the bamboo out so that there is about 18 inches of space in between the two outside pieces.
Lay an 18-inch piece of bamboo across the top of the "fan," 2 inches from the top, allowing the piece to hang over the outside pieces slightly.
Lay a 12-inch piece across the fan, about 6 inches from the 18-inch piece, allowing the piece to hang over the outside pieces slightly.
Lash the bottom of the trellis where the three pieces touch together with a 5-foot piece of cord that is slightly tacky so that it "sticks" to the bamboo. Ask for tacky cord or sinew-like cord at your local hardware or garden store -- a burly twine will do in a pinch. Wrap the cord around the left-most bamboo piece; wrap it around the right-most piece and then wrap it around the center piece. Wrap the cord around all three pieces and then over, under and around the pieces again until the cord is much shorter. Tie a double knot with the two ends to secure the cord in place.
Lash the cross pieces of the trellis to the support pieces of bamboo with pieces of cord that are about 3 feet long, wrapping around one piece and then the other and then over and under both pieces. Tie each lashing with a double knot.
Press the trellis into the back side of the pot, sinking it 8 to 12 inches into the soil. The "Trellis Pink" petunia grows up to 24 inches tall, so having 2 feet of trellis above the soil should be adequate.