In Oregon, any project that exceeds $2000 requires a contract.
I’m assuming you meant “written contracts”. That applies to those licensed as a landscape contractor much like the CCB requirements for written contracts over a certain dollar amount. This does not apply to “landscape designers” because there are no licensure requirements and laws & rules that apply explicitly to licensed occupations do not apply explicitly to non-licensed occupations. Having said that, in the litigated nature in the AEC industry, I would recommend using a written contract signed by all parties to the contract regardless of the contract value. Unrecorded verbal agreements are useless. You want some record and a written contract is the best way to go.