The herbicide we typically use is a contact weed killer. This means that we cannot treat early in the spring because there is not enough weed growth for the herbicide to make contact with the weeds and have a meaningful effect in killing the aquatic plant growth. We have also found in our years of experience that when the treatment is applied too early in May there is significant weed growth by the end of August. By treating several weeks later, we are trying to maintain weed control later in the season and be more effective on controlling the weeds throughout the boating season.