The relatively long life span or shelf-life makes it non-time bound as a result of which marketers and media persons can use it at their convenience. The basic content should be sustainable (it has enough subtopics) and non-time bound (it should remain pertinent always).
An example of an evergreen content would be the “How to …” guides which are present in the internet. As long as people have queries, the guide will be successful. For example, Google today has a “How to “page dedicate on everything from “How to tie your shoes” to “How to fly a plane”! Frequently asked questions and glossaries of terms and phrases include other popular go to content.