For drawing, here's the usual textbook: https://www.amazon.com/Landscape-Graphics-Section-Perspective-Drawing/dp/0823073335
Some starting Youtube videos: https://www.youtube.com/watch?v=24rnfO8s0hU
https://www.youtube.com/playlist?list=PLxotExjpp_T90rJTELFbWRFW968Xu5yvZ
For basic design for client's needs and uses, here is a book: https://www.amazon.com/Essential-Garden-Design-Workbook/dp/0881929751
Definitely get to know the local plants by visiting nurseries. Learn about the native plants in your area; they are locally adapted to the climate and soil situations and can be very successful after planting with less coddling than usual garden plants.
Visit local arboretums or horticultural gardens to get a feel of spaces.
These look a lot like landscape architecture graphics. If you like the aesthetic, there are many reference books on the subject. I think this is the one I had in school: https://www.amazon.com/dp/0823073335/ref=cm_sw_r_awdo_Q28PXAF31G6X3ZNW6G1A
This book is invaluable: http://www.amazon.com/Landscape-Graphics-Grant-Reid/dp/0823073335
Also, line weights will add a much needed depth