Gardeners and salespeople alike refer to granular lawn treatments that contain both herbicides and fertilizers as “weed and feed” products. This two-in-one product's effectiveness and utility is questionable; Canada passed restrictions in early 2010 that will halt the production of weed and feed products by the end of 2012. Officials point out that the effective application times for herbicides and fertilizers are different, wasting the excess chemicals and contributing to ecological disruptions. Nevertheless, weed and feed products continue to be sold in the United States.
The basic trinity of most traditional lawn fertilizers remains the same in weed and feed products. Nitrogen, phosphorus and potassium are the three major nutrients, and manufacturers often add trace minerals, like magnesium, iron and zinc, according to their company's proprietary blend. Nitrogen, usually in the form of urea, makes up the lion's share of most lawn fertilizers, with phosphorous (as phosphate) and potassium (as potash) added in predetermined ratios. The ratios are expressed as a series of three numbers, such as 28-3-3.
Weed and feed manufacturers add granular herbicides, such as 2,4-dichlorophenoxyacetic acid, dicamba and (4-chloro-2-methylphenoxy)acetic acid, abbreviated MCPA, to their formulas. These agents harm broadleaf plants while grasses metabolize the chemical, leaving lawns unharmed. According to Cornell University, 2,4-D breaks down over a relatively short period, and only trace amounts remain two months after the initial application. Another broadleaf weed killer, dicamba, is more persistent in the soil and may remain in soil for a year. Dicamba is water soluble and easily leaches into groundwater, so follow manufacturers' directions carefully. Trees and ornamental plants can take up excess dicamba from nearby lawns. MCPA, like 2,4-D, does not last long in soils but is water soluble like dicamba. These herbicides interfere with cell division and growth in actively growing broadleaved plants and ornamental trees.
To spread the granular weed and feed product effectively, manufacturers mix the nutrients and herbicides heavily with fillers such as sand, lime or other processed materials. This dilutes the formula to the appropriate application rate, allows for even coverage and reduces fertilizer burns.