Can I use public domain poetry for commercial purposes?

You can use anything in the public domain for any purpose, But "dead writer" isn't synonymous with "public domain," and the current copyright law is expressly designed to benefit the heirs of authors by extending until years after their death.

For these dead writers, with their works published way before 1978, so they're covered by the old copyright law, and even with maximum protection under original and renewed copyright, they're both in the public domain.

Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.

You need a copyright attorney to research whether Auden's poems are in the public domain. He died in 1972. Depending on whether copyright notice and renewal formalities were adhered to, his poems could still be under copyright until 2042 (70 years after his death). Also, be careful: others may have already created songs using the lyrics to the "If I Could Tell You" poem -- see, for example, < http://bit.ly/7At7N >.

Before you create a work that incorporates someone else's work or if you create a work that's based on someone else's work you need to speak with an intellectual property attorney. The rules for using other people's works are quite complex (and sometimes applying those rules does not lead to a clear cut answer).