Using a weed and feed product that combines a lawn fertilizer with an herbicide can seem like a good idea, a way of simplifying lawn care and getting two chores done at the same time, but there are also some negative effects to consider. Both fertilizers and herbicides are granular and can be applied with a spreader, but that's the extent of the similarity between them. Buying them separately might be a better option.
The fertilizer portion of weed and feed products is a high-nitrogen inorganic formula that makes grass grow more quickly. As with all fertilizers, choose one based on the needs of your type of grass in your climate. Consult your local extension service for advice.
The herbicide included may be 2,4-D, dicamba, mecoprop, metsulfuron, atrazine or other chemical. All kill broad-leaf plants without affecting grass. They may also contain a pre-emergent herbicide to prevent weed seeds from growing. Dicamba, metsulfuron and atrazine also can be absorbed through the roots, with possible damage to trees and other ornamental plantings.
For the herbicide to work, it needs to stick to the leaves of the weeds, so weed and feed products need to be applied to wet grass. The lawn should not be watered for at least two days afterward. Best times for applying fertilizer may not coincide with best times for applying herbicide, however. Get information on local conditions from your extension service.
As with every pesticide and fertilizer application, the first consideration is always whether it is truly needed. If a lawn is reasonably green, without thin spots, and growing well you may not need to fertilize at all, or only lightly. If you have weeds in only a quarter or a third of your lawn, applying herbicide to the whole lawn is wasteful and expensive.
Any product containing an herbicide is a toxic substance and is subject to local laws and regulations. Read the label and know the chemical you're buying, exactly how it is approved for use, and the possible dangers of getting it on your skin or inhaling it.
Herbicides are poisonous if enough is ingested, or if exposure is prolonged. Children are particularly susceptible to damage from herbicides such as 2,4-D when applied to a lawn. They may roll on the grass, put fingers in their mouths, or even chew on a grass blade. Their developing organs are less able to detoxify pesticides. Weed and feed products may also be tracked into the house and linger there without the normal breakdown process facilitated by microorganisms.
Mecoprop is of medium toxicity to birds, and other herbicides have the potential to accumulate in groundwater, often the source of local drinking water. Dicamba, 2,4-D and mecoprop are among the most frequently found weed killers in streams.