Glyphosate is an effective herbicide for many kinds of weeds, but the chemical harms most types of plants and has other drawbacks. A weedkiller for annual and perennial weeds, glyphosate destroys all plant parts by traveling from the leaves to the roots. It is useful for controlling perennial weeds, which regrow from their roots every year, and for controlling weeds without having to dig them up, but glyphosate doesn't work well on dormant weeds. When applying glyphosate, wear protective clothing, including a long-sleeved shirt, long pants, gloves and safety goggles.
Glyphosate controls weeds by affecting the systems they need to live and grow, but it has the same effect on desired plants. Some herbicides damage only the top growth of plants, but glyphosate prevents all plant parts from making certain proteins they need to function. So any glyphosate that accidentally contacts desired plants can cause severe damage in them. Applying glyphosate on a still day is the best way to prevent it from drifting, and cover desired plants or spray glyphosate only where no desired plants are nearby.
A major concern about using herbicides is their toxicity, but glyphosate has low toxicity in most situations. Human skin absorbs glyphosate poorly, and if glyphosate enters the body through the skin or mouth, it passes out of the body through urine or feces within one day. Glyphosate also has low toxicity to fish and other wildlife, and it binds tightly to soil so is unlikely to contaminate groundwater. Other chemicals included in glyphosate products to aid their sticking, penetration and other effects, however, can cause eye and skin irritation, and irritate the nose and throat if breathed. Swallowing a glyphosate product can cause nausea, vomiting and diarrhea, burns to the mouth and throat, and increased saliva, and pets and other animals can be at risk if they touch or eat plants treated with glyphosate products.
Actively Growing Plants
Plants must be actively growing for glyphosate to be effective on them. If weeds aren't growing, glyphosate remains on their leaf surfaces and doesn't move into the plants. Stress factors such as pest damage, very low humidity, poor soil and drought can slow down or stop weed growth. Plants also often stop growing in midday heat, and many are dormant during winter. Most plants grow in spring or early summer, and perennial weeds often store energy in their roots in fall. So spraying with glyphosate at those times is likely to have the best effect. In summer, glyphosate is most effective when applied in early morning or late afternoon. In winter, glyphosate should be applied in the middle of the day, when plants are most likely to be growing. Glyphosate doesn't prevent new weeds from emerging.
Some weeds are naturally resistant to glyphosate, and others develop resistance over time. Weeds naturally resistant to glyphosate include field horsetail (Equisetum arvense), which is perennial in U.S. Department of Agriculture plant hardiness zones 3 through 11, and morning glory (Convolvulus spp.); the morning glory also called bindweed (Convolvulus arvensis) is perennial in USDA zones 3 through 10. Additionally, weeds have wide differences in their genetic makeup, and that variation means any population of weeds may include a few members that have a natural resistance to glyphosate. When those weeds survive being sprayed with glyphosate, they produce seedlings that are likely to carry the genes that provide glyphosate resistance. With regular use of glyphosate, the population of resistant weeds increases.
- National Pesticide Information Center: Glyphosate -- General Fact Sheet
- University of Arizona Cooperative Extension, Arizona Crop Information Site: Maximizing the Use of Glyphosate
- Purdue Extension: Facts About Glyphosate-Resistant Weeds
- ZipcodeZoo.com: Equisetum Arvense
- ZipcodeZoo.com: Convolvulus Arvensis