Since you probably already have the date nailed down, consider these seasonal themes.
Spring: Vibrant pinks and yellows show off the brilliance of spring. Consider;Anemones, Daffodils, Freesia, Hyacinth, Iris, Sweet Peas
Summer: Beautiful country garden flowers, hot pink and shades of blue represent a hot lazy summer. Consider; Agapanthus, Campanula, Delphinium, Galdioli, Larkspur, Roses, Stocks, Sunflowers
Autumn: Carry on your favorite summer theme of tie in the seasonal change with rich russet shades. Consider; Celosia, Chinese Laterns, Gerbera, Hydrangea, Willow
Winter: Use cool whites with rich reds or even beautiful berries to keep everyone warm from the flurries outside. Consider; Amaryllis, Ginesta, Mimosa, Orchids, Spruce, Ilex berries