Not everyone is going to believe and at the same time, you don't need to convince everyone. Identify your target segment and start building trust slowly.
How did Subway prove to the clients that they use fresh ingredients? - They make your sub right in front of you.
1. Live packing - Take a video of every order that is placed and send it along with your delivery. That's the personal touch they would like to keep.
2. Take videos of your farm and convince your prospects that the flowers are for real.
3. Give a certificate with every order and make them special. (That's what the gold jewelers do when you buy gold)
4. Magnifying lens - Just to make the concept wacky, send a small magnifying glass with every order so that clients could see the details on the petals, stem, etc. Give them a small manual explaining every part and make the whole experience interesting.
You could use this strategy to get your initial set of clients, who your Innovators (2.5%). Take feedback and implement at the earliest so that you could convince the next set of Early Adopters (13%). Once these clients are happy, create a ripple effect by asking them to refer you to their friends and family.
More ideas can be discussed over a call.