"Weed and feed" describes a range of products offered by several fertilizer manufacturers that combine herbicide and fertilizer in one bag. While multi-tasking appeals to homeowners scrambling to get all the home maintenance chores done and still have some weekend left, there are some drawbacks to weed and feed products.
Lawn herbicides fall into two categories. Pre-emergent herbicides retard the development of germinating seed. They are non-selective, meaning that they retard the development of all seeds, whether they are a desirable or undesirable plant. Post-emergent herbicides kill actively growing plants, while not harming most turf grasses. Pre-emergent herbicide has to be applied before seeds germinate to have any effect. Post-emergent herbicide has to be applied to actively growing plants, or it is wasted. Both types of herbicide are commonly used in combination products. Grabbing a bag of "weed and feed" off the shelf without knowing what type of herbicide is included might not have the desired effect.
Pre-emergent herbicide needs to be applied early in the spring. Crabgrass, dandelions and other undesirable weeds start germinating as soon as winter draws to a close. The pre-emergent effect lasts for eight to 10 weeks, but doesn't do a thing to plants already growing. In other words, it's hard to be too early and it's a waste to be too late. On the other hand, turf grass rarely needs encouragement to grow in the spring. The University of Missouri Extension Service recommends that the first application of fertilizer for a lawn should wait until the lawn's natural growth spurt slows down--usually late spring to early summer.
Bad Application Method
Post-emergent herbicides kill the plants by soaking into the plant, traveling to the root and disrupting the normal growth cycle. They are best applied as a liquid. They are also best applied in spot treatment--sprayed directly on the weeds--rather than broadcast over the entire yard. Granular weed and feed with post-emergent herbicide applies far more herbicide than needed in most instances and must be watered after application to move the herbicide into the plants' foliage. The timing and amount of water used are critical to success, but difficult to gauge.
Warm-Season Grass Damage
Homeowners in areas of the country where warm-season grasses are used in lawns need to be particularly careful with weed and feed products. The commonly used post-emergent herbicides in weed and feed products can kill some turf grasses, too. You must be familiar with your lawn and carefully read the label to determine that the product won't kill your grass.