Choices for motifs to adorn wedding invitations are almost limitless, but one option is to use an image inspired by nature.
One plant alone -- used once or in repetition -- will make an impact. An orange bloom in a variety of sizes packs a graphic punch. You might also try aspen leaves, pine trees silhouetted in bronze (for both the invitation and reply card), a flowering vine that grows from top and bottom, or an elegant white lily-of-the-valley engraved on green stock for a formal affair. Other considerations include a stylized drawing of a poppy placed boldly across the center of the invitation and daisies.
Consider a monogram, such as a simple single letter encircled with a floral leaf and foliage continuing in different forms on the reception and reply card; or first names in a whimsical typography.
Patterns are another idea, utilizing such decorative motifs as Japanese patterned paper; blossoms on a sewn-on origami paper-backed invitation and reply card; a stippled floral pattern; or a foliage design on a band encircling the invitation.
Try a symbol of nature in flux, such as wheat, pumpkins, or other harvest icons; raspberries, or dandelions whose flowers have turned to white seeds.