This week, I listened to a brief 2012 KYUK radio piece on the threat of invasive white sweetclover (Melilotis alba) to Y-K Delta berry patches. It was suggested that the weed, which at publishing had yet to be noticed off the road system, had the potential to invade wild blueberry and cranberry stands in the Interior and lure pollinators away. If blueberries and cranberries received less pollinator visitation or the incorrect pollen, fruit set could be affected.
Research conducted by UAF on 20 test plots near the Steese, Elliot and Dalton Highways found that flowering sweetclover actually encouraged pollinator visitation to berries. Cranberries and blueberries saw at least 3 times the pollinator activity they normally would and cranberries actually had improved fruit set when flowering sweetclover was present. However, researchers couldn’t quite link the results solely to the sweetclover.
Another study was done in a more controlled setting (sweetclover was introduced to test plots around UAF):
“During a rainy June in the first year, conditions seemed to draw pollinators away from native berry plants that were a moderate distance away. During a sunny June in 2012, conditions were good enough that all the plants seemed to benefit.”
More research is necessary, but it was suggested that certain geographic areas might be more susceptible to sweetclover competition (especially those where the sweetclover and berries have highly overlapping flowering periods). Competition for space, however, might be the real issue for native berries:
“Mulder said white sweetclover towers over berry plants and adds nitrogen to the soil, which are factors that could cause it to slowly crowd native plants out of their turf.”
HortAlaska Berries 