Planting in plastic bags is cheaper than purchasing plastic or clay pots for your garden. Start seedlings in bags for transplanting, or grow vegetables directly in the bag until you are ready to harvest the plant. While bags of compost or potting soil are the simplest plastic bags to use, any sturdy bag is capable of being used as a planter--including heavy-duty trash bags.
Choose plastic bags for your plants. Pick heavy-duty bags that are the proper size for the amount of plants you plan to sow. A compost or potting soil bag holds approximately three adult plants, and a regular-size garbage bag holds five plants.
Fill the bag with rich growing mix. Make your own by mixing one part peat moss with one part compost and adding one or two handfuls of perlite. Tie the end of the bag closed securely once filled so the compost doesn't leak out. Shake bag to loosen the mixture.
Lay the bag on the ground horizontally and move the compost inside until it is evenly distributed and the top of the bag is flat. Poke small holes every 2 inches around the bottom edge of the bag for drainage.
Cut out 3-by-3 inch squares from the top of the bag, spacing them 6 inches apart. Leave one edge of the square attached to the bag to form flaps, if planting from seed.
Sow a seed or seedling in each square according to the instructions on the seed envelope or plant stake. Water thoroughly but do not soak the soil.
Close the flap over the seeds to help speed germination, if applicable. Cut the flaps off once sprouts appear.
Keep the soil moist, but not soaking. Stick a finger in the soil before watering to check moisture levels. Soil should be damp, but not so wet it cakes onto your finger.