Mowing and routine lawn, garden, shrub and tree maintenance need no Arizona Registrar of Contractors license under the gardener exemption, A.R.S. §32-1121(A)(16). If a maintenance visit turns into an install, such as new irrigation, pavers, walls or a landscape job of $1,000 or more, that work should be done by an ROC-licensed contractor, typically R-21 or CR-21. Ask for the ROC number and verify it at roc.az.gov.
A gardener doing maintenance may apply herbicides without a license (A.R.S. §3-3613), but the exemption ends for pre-emergents or sterilants, for companies that advertise weed management, for crews of two or more, and above set daily amounts; then an Arizona Department of Agriculture Pest Management Division business license is required (§3-3615). For pre-emergent treatments, or any company that advertises weed control, ask for its Arizona Department of Agriculture Pest Management Division license and verify it. Exempt gardeners must give you treatment records showing the address, date, product, amount and applicator.
This is general information, not legal advice.