When I first looked this up in Mark it appeared to me that what Jesus was doing was making an analogy in regard to sewing seeds.
And he said, Whereunto shall we liken the kingdom of God? or with what comparison shall we compare it? It is like a grain of mustard seed, which, when it is sown in the earth, is less than all the seeds that be in the earth: But when it is sown, it groweth up, and becometh greater than all herbs, and shooteth out great branches; so that the fowls of the air may lodge under the shadow of it. (Mark 4:30-32 KJV)
So at first glance it would appear that what is being suggested here is that of all the seeds that are planted by sewing them, i.e., tossing them out into the field, the mustard seed was the least or smallest of those-- or the smallest that these people knew about.
There are seeds that are smaller, but I believe that those seeds are not normally sewn in the manner of tossing them onto the ground. So if you were to make a comparison on seeds that were sewn and how they grow into great plants, the mustard seed would be the best comparison. I suspect that if Jesus had said used the comparison using the black orchid, that it would not have been understood. Apparently there was a common saying back then that when you compared things to say that something was little, you would say it was as small as a mustard seed.
At any rate after I formed my own preliminary interpretation I went on the internet and found some sites which discuss the issue.
You analogy is interesting. However, the scripture clearly states that the Mustard seed is the smallest seed. Even if taken in the context of who Christ was talking to, it would have been easier to explain that a small seed, like a mustard seed can grow into something large and strong. But He didn't. He said it was the smallest.