I cannot, in good conscience, recommend an online course for design.

You're looking for a single course? Or an online learning certificate...

yes any thing for an introduction to landscaping and it doesnt have to have a certificate

Doesn't exist. There are tools and technical matters, but design? No.

Spend the money and go to school.  At the least find a summer discovery program. There's your intro.

Read a book.

Marc: School doesn't teach you how to design, its meant to nurture you and give you possibilities. Design IMO is innate and is encouraged. I suggest saraarafa reads design theories and tries to under his/her surrounding.

Sure, op cold read a book or look at YouTube videos independently. Given that op seems to have tried the latter and is looking for courses...

No to mention Dirr is incredibly dense and at time obtuse, it's much better taught than just read.

Design IMO is innate and is encouraged.

That encouragement is best served in the form of constructive advice from well-qualified advisors. Success in design is 75% good feedback, 25% history. 

You'll never get past 25% online. Look at poor Richard.

Online courses are a waste or time...Try your local botanical gardens for classes...the larger ones usually have classes that you can take...If you are looking to get into the field professionally, an online class or even the courses at the botanical gardens will not be enough imo.  I would suggest looking into an LA or LD program at a university...

This idea that you always have to have a formal course in anything is curious. Abraham Lincoln taught himself surveying before plotting out several communities in the Midwest. That was before he taught himself law and passed the law exam. Frederick Law Olmsted firmly believed landscape architecture should be self taught. Olmsted never went to college at all, His first landscape commission was New York City's Central Park. He is still the unequaled landscape architect of all time.