Tenants are obligated to ensure basic garden maintenance such as ensuring it is free of litter and that it is not severely overgrown. Tenants cannot be required to complete tasks that require specialised skill.
It is recommended that you outline these responsibilities in your rental agreements to avoid confusion.
Typically landlords are responsible for the maintenance of the garden. If the landlord passes gardening responsibilities onto the tenant, they should provide the tools and equipment required to maintain the garden.