This is generally in February or March for many regions. See your frost dates. Cold-weather greens such as chard and lettuce can also be started in milk jugs by early March.
Some flower examples include yarrow, asters, blanket flower, black-eyed Susans, butterfly weed, coneflower, blazing star, and purple hyssop. To prepare the planting: Use either quart or gallon jugs. Slice them horizontally on three sides to make an opening. Fill the bottom with four inches of potting soil.