A Defra representative said De Mauley's portfolio, which included Kew, plant health and the ornamentals round table, will continue but Lord Gardiner will not be given a ministerial role.
Stewart will cover environment, floods and water, resource and environmental management and rural affairs. He will have lead responsibility for the Environment Agency, Natural England and the Forestry Commission.
Lord Gardiner's lead responsibilities will include biosecurity and landscape.
Eustice will cover food and farming, animal health and welfare, marine and fisheries, science and innovation and better regulation.