are architects responsible for anything wrong?

The answer to your first question is YES. An architect's job is to design a viable structure. Part of his job is to show up on the construction site to be sure his design is implemented and built as designed. I had a very large addition built on my house which was designed by an architect. He showed up during every stage of the construction and corrected the contractor once on something minor, but I felt confident the architect was watching my back. Well, one day I pull into the driveway and the second story framing looks much taller than the blue prints. I called the architect. Someone misread the plans and those nice little dormers were so enormous that I suddenly had a third floor loft and cathedral ceilings where none were planned. My goodness! The builder said it was cheaper for him to just follow through with the mistake and the architect made sure the ceiling joists were reinforced with bigger boards to support the new weight of the structure due to all that extra building material. I made out like a bandit, space wise. But, had the builders carried on without the architects structural support changes, I would have ended up with a very poorly constructed house. It is the architects job to be sure the building is viable for it's safety and design.

About landscaping. I garden quite a bit, so I would have checked the maturity size of the plantings before allowing the work to proceed. I am not sure if landscapers have warranties or can even warranty their design because quite a bit of time has to pass for that tree to grow and wouldn't the warranty expire by then? I would think.