I think poll/list questions would be acceptable in this SE and this SE should not always behave like other SE to only accept question with solid questions.
It is natural for gardeners to ask which plant is suitable for them and of course every answers are equally valid in some aspects. In the mean time, the questions can become a community wiki and the answer get voted by users. For example, this question/wiki about gardening tools were accepted.
Back to the choosing plant problem. Every answer is equally valid only if it is valid and correct as an answer, so there are less problem of too subjective. The only matter is the effectiveness of the answer, and that would be judged by the votes. For example, this, this. And how to defeat pest: this, this.
And btw, this question is not closed too and I think this is a decision by the community to not vote to close it. Although a question regarding labelling a metal is of course can be consider as off-topic.
As for the opinion of yoda, I think Botany.SE is far from beta phase. My opinion is that Gardening.SE is more suitable for this because we have more users who are interested in planting different plants, whatever of their purposes. You cannot ignore that if you simply search google about "air clean plant", the top 10 results are all plant suggestions for gardeners.