Academic journals are often seen as the pinnacle of scholarly research. Academic journal articles are usually peer-reviewed, which means they have been reviewed by experts in the field. The authors of these articles are also experts in the field - usually professors, doctors, or researchers.
In many fields, including agriculture, scholarly peer-reviewed journal articles are considered primary sources. They are the origin point of a particular experiment or research project.
In horticulture, academic primary sources usually have sections like literature review, methods, analysis, discussion, and references.
Note: These lines are a little messier in real life. Academic journals sometimes contain secondary sources or sources that are not peer-reviewed, such as book reviews or letters to the editor. Ask me if you ever have questions or want to verify what kind of source you're using!
These sources are all magazines specifically for horticulture and greenhouse management. These types of sources are often called "trade publications". They're not nearly as in-depth or technical as most academic sources, but they're usually still written by horticulture experts and contain factual information. Trade journals are usually secondary sources because they analyze, interpret, or discuss primary academic articles.