Scotts Miracle Gro has pulled out of the £7,500 sponsorship after several years support.
HTA, RHS, BALI, Arboricultural Association and Landscape Show have discussed clubbing together to fund the 29 November reception at the Houses of Parliament.
Organisers Newington Communications (formerly Bellenden) hope Defra secretary of state Andrea Leadsom will speak at the event.
The APPGHG group formed in 2000 and holds an annual reception for 150 people at the Houses of Parliament, usually with a minster speaking. Defra minister George Eustice was speaker in 2015 and Alan Titchmarsh is among former speakers at the lobbying event.
The group is co-chaired by Baroness Fookes DBE and Ian Liddell-Grainger MP.
Horticulture industry lobbying has also been hit by Defra’s concentration on Brexit, with the Ornamentals Round Table losing its Defra representative Kathleen Kelliher, who has moved to a Brexit role within the department.
Issues on the committee's agenda include Sunday trading legislation, skills shortages, overseas labour, tree and plant procurement for large scale infrastructure projects, the impact of the introduction of the living wage and promoting innovation in the sector through investment in research and development.