Different flowers are given different messages and meanings according to the language of flowers, a system of communication that was most popular during the Victorian era. At the time, the way to show someone your feelings without loudly proclaiming your emotions was to gift them with a bouquet of flowers, each with its own meaning.
Freesia is the most straightforward of the flowers that indicate trust. Trust and innocence are the only implications this flower has. It's used in a variety of products for its fragrance, so a similar message of trust could be sent by giving the person a gift of freesia scented soap or perfume. The freesia flower is extremely delicate and requires much care in cultivation and in harvesting. Its message of trust makes this flower popular in wedding bouquets.
Lily of the Valley
Lily of the valley is a flower native to Great Britain, making it very popular for smaller weddings there. It's not limited to small weddings, however; Kate Middleton's bouquet at her wedding to Prince William was made primarily of these flowers with a few field maples signifying humility. Though it's not a direct message of trust, the lily of the valley indicates sweetness and a trustworthiness about the person.
Freesia is the only flower that specifically indicates trust, but many imply similar messages and connotations. The white carnation is one example. Its message is one of purity but also of faithfulness, implying that the person may be trusted as loyal. Variegated carnations have a meaning related closely to trust and ask the recipient of the flowers to put their faith in the giver. Violets have a variety of meanings, such as modesty, but the blue violet in particular means faith.
Lavender means the opposite of trust. This meaning doesn't come from the lavender flower itself, however -- an unusual fact within the language of flowers. Instead, the implication of distrust comes from the fact that many cultures believed that snakes nested beneath lavender.