It’s something to do with the soil condition that has been created. A condition has been created that’s favorable for the dog fennel to thrive….When that condition has been created in excess then the response comes in excess. I think from mine and others experience that nitrogen is a part of it with dog fennel. I think its responding to an increase in soil nitrogen. I also think that the microbial community is likely playing a role somehow too. The reason I think that is because I’ve been using a good bit of N for years now to try and push out enough winter grain growth to keep up with my high deer density. It’s only been in the last couple years that dog fennel has suddenly boomed. There was another factor at play beside just the N….I think it may have been the amount of fungi present. In the beginning I was likely bacterially dominated and now I’ve swung to a higher make up of fungi…..I think maybe the dog fennel is responding to that as well. If this is true though then the reason you saw it with the sunn hemp I think is likely the N that its returning combined with the type of biomass being returned. Sunn hemp is going to have a really woody stem and likely promote more fungi. These are just educated guesses though so take it with a grain of salt.