People often create content that is easily forgotten by the public within weeks and eve day of release. When this happens, that content is hardly ever going to be searched for and it becomes obsolete for SEO purposes. When you create evergreen content, you are posting information that is universally valuable and not just trendy.
If you want to make evergreen content, you need to focus on any content relating to your product that you know is going to be useful to people indefinitely. If you sell gardening tools, an evergreen topic would be how to take care of your gardening tools, while a trendy topic would be talking about a famous gardening tools company closing their doors.