Would voting help?
What if you had the gardening topics listed and asked which week members would prefer if they are planning to participate in that specific gardening topic or plant. The week with the most votes wins for that particular topic. Others could still participate if they wanted to show and talk about yet to peak plants in their garden or share photos and info from the previous year. It would seem that this would be most successful if a "majority" of folks wanting to contribute to each topic were included. I don't know how many folks grow all these different species of plants? There are many that I don't grow, so I would likely not be contributing. (I'd just be reading.)
