I am taking a few this fall at the University of Maryland. They are pretty basic level courses that I'm sure you've had (Insects of Ornamentals & Turfgrass, and Pesticides). I have to take these to get my pesticide applicator's license. I'm just starting, but so far they seem like they'll be helpful.

There are some other colleges in Maryland that also offer some hort classes online, but they cost more. If you want details, let me know.

Depending on what you need it for, you can often get a lot of the same knowledge for free just by reading books from your local library or good web sites. Are you working in the field now, or looking to get into it, or is it just for your personal enrichment?

Try working part time at a local nursery. My friends did that while we were in college, and I couldn't believe how much they knew about plants- plus they were getting paid for it!