There are some minimum requirements for hubs, but I'm not sure exactly how they relate to posting poetry. From a publication standpoint, if you have an extremely short poem, it is beneficial to add commentary or extra content. You can provide extra information for the reader and it helps search engine ranking. For example, if your poem is about dealing with the death of a loved one, you could provide a short commentary on the feelings behind the loss. You could also provide some links to information about dealing with grief. This provides extra, applicable content for search engine ranking, but more importantly, readers who find the poem because they are dealing with a loss will have additional resources.