For best effect, avoid using plants that will grow too large and hide the jar's interesting features. Keep the plants in the jar simple, using only a few plant types such as the New Guinea impatiens, sweet alyssum, zinnias, and wax begonias. Your strawberry jar can be instantly beautiful if you plant it with flowers in bloom or in bud. If you grow your annuals from seed, give them an early start indoors.
Pick flower colors that complement the red in the clay. For a cooling color contrast, poke such white-flowering favorites as candytuft, sweet alyssum, or begonias into the pockets. Blue or violet will burst into life from ageratum, nierembergia, or lobelia. One of the most cheerful combinations can come from pansies; their "faces" will peek in all directions from the jar.
Your choices also will depend on where the jar will be placed. In a cool, moist, and shaded spot, try baby-blue-eyes, browallia, or mimulus. Out in the sun, marigolds, zinnias, and petunias will do best. In areas where it's very hot, drought-tolerant portulaca or vinca are good performers.