Check out the university's website, each professor should have a page with the courses taught and syllibi with required textbook. Unfortunately the texts cost around $100 a pop and more, but you can pick them up used for less. Older editions cost even less because they are not used any more for the courses. But there is a problem,... you will probably have to buy the new edition if you take the class or risk missing the new information it contains when you are tested on it. Once you find out the botanical topics the department teaches (get that from the faculty pages, not the university list of courses because the university catalog is full of courses that are almost never taught). Once you know the name-topic of the courses, search online for lectures professors at other universities have posted for their students.
Reading lists for individual upper level courses usually consist of the textbook and maybe some scientific journal articles, which are usually only found in a university science library. These articles rely on the lecture notes/and discussion sections of the course to get much out of them. For horticulture courses it is even more difficult because the readings may be gray literature (technical reports etc) not widely published.
Also check out all the required courses for the degree you seek to get some idea of what you are in for.