2,4-D is one of the more useful herbicides available to the homeowner. This herbicide is selective for broadleaf weeds, meaning it will not affect grasses, even grasses that have wide blades. This makes it useful to control such weeds as dandelions, plantains, knotweed, pigweed, and many others. It is a systemic herbicide, which means that it is absorbed into the plant tissue and transported throughout the plant, including into the root system. 2,4-D works well on many deep-rooted weeds, though certain weeds with very deep roots, such as field bindweed (the morning glory weed) or Canada thistle, may require repeated applications or a different herbicide. For those weeds spot applications of a glyphosate herbicide (e.g. Roundup) or dicamba (e.g. the Trimec products) may work better. 2,4-D will not affect grasses (including weedy grasses), but it can damage or kill other desirable plants such as garden vegetables, flowers, and ornamentals. So care must be taken not to let 2,4-D drift onto those plants, or to accidentally spray them.
Several common broadleaf weeds are resistant to 2,4-D, even though the label specifies it will control them. For this reason, glyphosate or herbicides that contain dicamba, such as the various Trimec products, may be more effective to kill black medic, mouseear chickweed, and creeping bellflower, and sometimes field bindweed.
Two advantages of 2,4-D are that it is of relatively low toxicity to humans in the formulations available to homeowners, and it breaks down quickly in the soil. The toxicity rating still requires following the safety requirements on the label, however; wear long pants, a long sleeved shirt, unlined rubber boots, rubber gloves, and goggles. Avoid breathing the vapor. The residual activity of 2,4-D in the soil lasts 1 to 4 weeks, depending on soil type, rainfall, and other factors. If crops or ornamentals are planted too soon after 2,4-D has been used, herbicide damage can occur to the new plants. When used according to label instructions 2,4-D usually does not move through the soil. However, if you have sandy soil, or use more herbicide than is recommended, it can leach through the soil or be carried by runoff into groundwater or surface water; this is especially true in home landscapes because of the frequent watering those areas receive. So care should be taken to follow label instructions. A disadvantage of 2,4-D is that in some cases it can make grasses more susceptible to leaf, crown, and root rots. This usually happens only after repeated applications.
A video and companion bulletin titled "Nine Steps to Safe Pesticide Use; A Guide for Homeowners" is available on loan from your county Extension agent, listed in the phone book under you county's government listings. The video is 13 minutes long and includes such topics as how to get pest problems identified, how to select the appropriate pesticide, mixing and application of the pesticide, laundering clothing, and storage and disposal. The video can also be purchased.
References: