The best choices for a pot lying on its side are plants that will seem to be spilling from its opening. Choose plants that spread horizontally, such as ground covers, mounding or trailing annuals and perennials and soft-textured grasses that will seem to be flowing from the pot's mouth. Avoid upright plants or plants that grow taller than about 6 inches.
The deep jewel colors of Million Bells (Calibrachoa spp.), ivy geraniums and trailing petunias are a good choice for a bright burst of color spilling from a sideways pot. Nasturtiums, another trailing annual with flowers in orange, yellow and red, also works well. Trailing perennials such as sun rose (Helianthemum spp.), campanula, ornamental oregano, golden variegated sage and prostrate rosemary are other good choices.
Ground covers are a natural choice to create the look of a sea of plants emerging from a pot's mouth. Arabis and aubrieta, which cover themselves with magenta, white or purple flowers in spring, offer this effect. Saxifraga, lady's mantle and mosses are options for shady spots. For sunny areas, try trailing raspberry or strawberry, groundcover daisies (Bellis spp.), creeping phlox or succulents.
Japanese forest grass, which comes in varieties with striped gold, green and pink, as well as white and green and solid gold, is a flowing grass that grows about 18 inches tall and gives the impression of fountaining up out of the pot mouth. As it moves in the breeze, it simulates the movement of water. Its blooms are light, airy seedheads. Other choices are black mondo grass, which spreads by rhizomes and has tiny white blooms followed by black berries, and blue moor grass (Sesleria caerula), which has blue-green leaves and a loose, trailing habit. Its blooms are bronze.