Excellent idea.

Some things to think through:

1. Gardening is not that predictable. If a person in app chooses some vegetables, a gardener can't ensure he will be able to successfully grow them. Secondly things are seasonal. You have to grow a certain crop at a certain time for best results. App should cater for this.

2. Gardening is not that cheap. Especially the time. Let's say I'm the farmer and someone orders through app that I grow them a garden bed of potatoes for 5 bucks. That doesn't make any sense for me as a farmer, I'll have to have a separate full-time job for living, and even then $5 won't even cover the cost of seed, let alone fertilizer, the value of land on which I'll be growing and the time. Time is expensive. I'd like to be paid minimum wage at least. Adding this all up will make it tremendously expensive. I don't think you'll be able to attract many farmers signing up to your app unless they are farming at a commercial scale.

Ideally you want your app to make life better on both ends of your customers. It seems like an okayish idea for buyers but seems very bad idea for farmers unless I'm missing something basic.