When there are too many weeds to pull, advice to "grow a thick, healthy lawn to discourage weeds" incites fury in the most peaceful gardener's soul. At times, you just have to reach for an herbicide to kill persistent, invasive weeds. Consider newer organic and biologic controls before resorting to chemicals. If you must use chemicals, choose an herbicide with appropriate effects and acceptable behavior.
Non-selective herbicides are plant toxins that kill any plant they touch. According to Joseph C. Neal, North Carolina State University's Extension specialist (NCSU), their efficacy depends on the nature of the chemical and method and season of its application. Because non-selective herbicides function by contact or by entering plant respiratory systems, they are used when plants are growing. They are labeled "contact" or "systemic" and "postemergent" herbicides. Diquat and pelargonic acid are postemergent contact herbicides commonly found in home gardening products. Grounds Magazine also lists less commonly available contact herbicides codacylic acid, dazomet, pelagonic acid sold primarily for professional use. Glyphosate and glufosinate-ammonium act as postemergent systemic herbicides. Bromacil, imazapyr, prometon, tebuthiuron and diuron are found more commonly in professional products.
The University of Rhode Island Landscape Horticulture Program (URI) explains that contact herbicides are often most effective against annuals and very young plants, while systemic herbicides are more useful against growing plants and perennials. This characteristic, plus plant physiology and phototoxicity (the toxin's use of light), soil and air temperature and rate of application make many herbicides "selective" in what they kill. Results with selective herbicides are always directly related to how closely applications follow directions. URI lists pendimethalin, trifluralin, benefin (and combinations of trifluralin and benefin), prodiamine and pyridines such as dithiopyr as pre-emergent selective herbicides used primarily for crabgrass control.
Contact selective post-emergent herbicides, often called "broadleaf weed killers," include arsenicals like MSMA and DSMA. Diquat, paraquat, glyphosate and glufosinate are non-selective sprays that can be selective when used for spot treatment. Systemic postemergent selective herbicides are Phenoxy acid-types 2,4-D, MCPP and 2,4-D plus 2,4-DP and the benzoic herbicides triclopyr and dicamba. Pendimethalin, trifluralin, benefin, prodiamine and a combination of trifluralin and benefin are pre-emergent dinitroaniline-type systemic herbicides that limit growth of roots of annual grasses. Siduron, linuron and other substituted urea-type herbicides also limit root growth on germinated seed of annual grasses; siduron is specifically suited to destroy only annual grasses in newly-seeded turf. Atrazine, simazine and metribuzin are systemic phototoxins.
Most alternative weed-eradication methods are preventative or rely upon physical removal of the plant. The Pacific Northwest Weed Management Handbook lists mulching with organic matter, shredded newspaper or geo-textiles as ways to suppress weed growth. Pulling weeds by hand, repeated cultivation and annual burns are "mechanical" methods. A pre-emergent strategy called "solarization" uses a black plastic blanket to force germination with radiant heat then suffocates seedlings by blocking out the light necessary for photosynthesis. According to NCSU, pelargonic acid can be used to formulate "herbicidal soaps" that, like insecticidal soaps, use natural ingredients that do the job with minimal environmental impact. Other natural contact herbicides use solutions of horticultural vinegar, salt or baking soda. Dr. Nick Christians and the Department of Horticulture of Iowa State University have developed a systemic weed killer using corn gluten meal.