The Farm Terrace Allotment Group held an open day to spread the word of its campaign against the council's plans to build on the allotment land.
The allotment land is part of plans to build the health campus, a new hospital and 600 new homes in West Watford.
The brunch morning on Sunday featured a barbecue, as well as cake, plant, and book stalls, and activities for children.
Sara Trebar, from the group, said: "It was brilliant considering the foul weather, we had more than 100 people and a great feeling of community and purpose."
Last year plot-holders were told the land was needed to make the project economically viable for developers.
In December Watford Borough Council passed a plan for the health campus that included the Farm Terrace land.
The final say over whether the allotment land can be used in the project rests with the secretary of state for communities and local government, Eric Pickles.