I wouldn't say that all postdocs are extremely specific in a field like plant biology. You might require the postdoc to be knowledgable of certain lab practices

I wouldn't say that all postdocs are extremely specific in a field like plant biology. You might require the postdoc to be knowledgable of certain lab practices only for example. In fact, in many cases it is very unlikely that you will find a postdoc that has done work in a cutting-edge niche area that a professor is interested in, so the pool of candidates even globally is not sufficient. Your best hope is to find a postdoc in an adjacent area who you can train up on what you are doing.