A large table saw, radial arm saw, and even a large miter saw. It largely depends on how you want to make those cuts. If you are not cutting anything lengthwise, then a miter saw sounds like a good choice for your desired tasks.
You can source a decent consumer-class 12" compound miter saw for around $200-$250. You can get a 10" much cheaper, but I suggest a 12" if you intend working with framing timbers or similar size wood stock for your flowerbeds.
I would also suggest taking a trip to Sears instead of Home Depot, but that is largely a personal choice. I used to work for Sears, so I have a decent insight into its big-ticket hardware product lines and warranty/return policies. Take a look at Sears.com, as there are couple of 12" models currently on sale.
For what its worth, I could do those basic cuts with a good old handheld circular saw as well, but YMMV depending on skill level with a circular saw. If you don't already have one, get a circular saw ASAP. Not only is a circular saw great for short cuts, but it will be also good for your lengthwise cuts, such as lattice panels for trellising. Freehanding can be difficult, but you can clamp a level or similar straightline to your work for a guide. Around $60 to $100 will source you a decent model for occasional residential projects.