For an informal garden wedding, the groom is wearing a guayabera (Mexican wedding shirt). Is a boutonniere required or necessary?
A boutonniere is not necessary for a very casual wedding where the groom is not wearing a jacket. However, the groom can certainly still wear a boutonniere, especially if his bride-to-be would like to see him with a boutonniere that matches her bouquet.
The boutonniere should be pinned onto the left chest pocket making sure that the pin goes through the shirt as well. This will keep the boutonniere from sagging the pocket.
Another way to incorporate flowers is to wear leis.