SONAR's effect on suscepable species is anything but quick. However, when things do happen, they generally happen all at once. So yes, there could be concerns about a mid-summer fallout of EWM and the potential impact on DO by its decomposition.
However, possibly of greater concern is IF the SONAR would work very well with a mid-summer treatment (maybe not). SONAR's mode-of-action requires active and sustained growth from the targeted plant(s). If, by mid-summer, the EWM has topped-out and its growth rate has drastically declined, SONAR may provide less than optimal results. Best to treat in the spring when new growth is most active.
