Landscaping disputes generally arise out of poor contractual documentation and/or defective work and/or materials.
Plans often lack detail and specifications are often general and not project specific. Quotations will often omit (deliberately or accidentally) items of work but not expressly state so.
On the other hand, mistakes and errors are a feature of any landscaping related work and some landscaping contractors lack the necessary skills and/or fail to have adequate quality assurance processes in place to ensure they achieve the agreed/specified scope of work to the specified standard, and/or to complete their work by the due date for completion.
Many owners and contractors will request changes/variations to the scope of work and specification as work progresses and no price will be agreed in advance.