Killing weeds in the garden with glyphosate

An introduction to how glyphosate works to kill weeds in the garden.

Weeds in a garden can be a real hassle. Perennial weeds will return every year unless they are dug up by the roots. In 1974, the Monsanto Company released a product called Roundup that made controlling perennial weeds a lot easier. Roundup or glyphosate, which is the chemical name, is now the most common herbicide used in home landscapes and gardens. It is a broad spectrum material that will kill a wide variety of plants including annual and perennial grasses, broadleaf weeds, trees and shrubs. Since the patent expired in 2000, other companies are now producing and selling products containing glyphosate.

Some people really like this product because of its effectiveness on so many plants, while others may be concerned about how safe it is to use. This article will discuss some of the interesting information about glyphosate to help you decide if it is right for your situation and to help you use it properly.

How it works

Glyphosate must be applied to actively growing plants. It is translocated throughout the plant where it inhibits the enzymes involved in the synthesis of the aromatic amino acids: tyrosine, tryptophan and phenylananine.

As a result of treatment and disruption of biochemical processes, the plants begin to die. Annuals begin to show symptoms within two to four days while perennials take seven to 10 days. If the weather is cloudy and cool, the symptoms may be delayed.

Formulations and rates

Ortho Kleen-Up contains 2 percent of the active ingredient. Roundup Lawn and Garden is 18 percent and Roundup concentrate contains 41 percent glyphosate. Tractor Supply Company sells a 50 percent glyphosate product called Weed and Grass Killer.

The amount of glyphosate needed to kill specific weeds can vary depending on the species of plants and size at the time of application. For young seedlings, 1 to 2 oz/gal may be sufficient while well-established perennials may require 3 or more ounces per gallon. Read the product label for specific recommendations.

Application conditions

The product can be applied any time the temperature exceeds 60 degrees Fahrenheit and plants are actively growing. Check the weather forecast before spraying. If it is washed off before it has time to dry, it will not kill the plants.

Make applications when the weather is calm; early morning and late evenings are best. If the spray drifts onto desirable plants, they may be injured or killed if not rinsed with water immediately. Drift onto non-target plants may be reduced by adjusting the nozzle on your sprayer to a coarser droplet size.

Spray the plant until the leaves are wet, but not dripping. Take care not to walk on the wet, sprayed area or you may kill grass or other plants from the glyphosate on your shoes. Shrubs and small trees may require repeated applications to kill the plant.

Environmental fate

One of the reasons people like to use Roundup and similar products is because of its low toxicity and it is not persistent in the soil. It is adsorbed onto soil particles and is a very low leaching risk. (Leaching is the movement of a material down through the soil profile.) It is broken down quickly by soil microbes with 10 to 60 percent being converted to carbon dioxide usually within the growing season. The rate of decomposition is affected by the microfloral populations and types.

Since it has little activity in the soil, glyphosate has no pre-emergent activity. In other words, it won’t prevent weed seeds from germinating and growing. Plants are not able to metabolize glyphosate, so it will tend to build up in plant tissues. This can be a problem for non-target trees and shrubs that are exposed to repeated, nearby applications.

The buildup of glyphosate in trees can result in bark cracking and decline in overall health. If enough accumulates in the tissues, the plant may die. When spraying near trees, use some type of shield to protect the trunks, especially if the tree is young and the bark is smooth.


Glyphosate products can be stored for years, but it is best if all pesticide products are not stored in areas where they are exposed to extremes in temperature. As with most liquids, do not allow it to freeze.


Glyphosate is one of the least toxic pesticides to use, but you should always read the label for instructions on what safety equipment to use when handling this chemical. Don’t forget to clean the sprayer after each use and run clean water through the line to avoid damaging plants when the sprayer is used again.

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