Maintaining a healthy lawn or turf usually involves some form of weed control.
Aside from manual weed removal, herbicides are most commonly used to effectively eliminate various broadleaf and grassy weeds.
You may have heard of simazine and atrazine, but what are they?
Simazine and atrazine belong to the triazene class of nitrogen-based herbicides. These two herbicides are registered for the pre-emergent and post-emergent control of broadleaf and grassy weeds in both lawns and turfgrass systems.
Simazine and atrazine are typically used to prevent weeds in residential lawns and commercial applications such as sod production, golf courses, athletic fields, and parks.
Read on for more information on the pros and cons of using simazine and atrazine to control weeds.
Table of Contents
What Are the Differences Between Simazine and Atrazine?
Simazine and atrazine are classified as General Use Pesticides by the Environmental Protection Agency.
General Use Pesticides include:
- Chemical compounds used to prevent weeds and other common lawn issues
Because of their potential toxicity and adverse effects on the environment, the pesticides atrazine, simazine, and propazine are regulated by the EPA’s Office of Pesticide Programs.
Every 15 years, pesticides are reviewed by the Pesticide Re-Evaluation Division of the Office of Pesticides Programs to determine the safety of their use.
Mesotrione is found in both simazine and atrazine, and it is a naturally occurring herbicide derived from the bottle brush plant.
Despite having similar names and chemical compositions, there are a few differences between these two herbicides.
The main difference between simazine and atrazine is their selectivity.
Simazine is a non-selective herbicide, and it is designed to kill most weeds and grasses without distinguishing between them.
On the other hand, atrazine is a selective herbicide, targeting a specific group of weeds and grasses.
Choosing the correct herbicide according to your grass type is crucial to ensure you do not accidentally cause harm to your lawn.
There are also a few chemical differences between simazine and atrazine, which cause different effects relevant to crop growers.
Sorghum and corn are sensitive to simazine, so atrazine is typically used on these crops.
Cattle and sheep herds are very sensitive to simazine, making it dangerous to use near animals.
There are also different runoff risks associated with these two herbicides because of their water solubility.
Atrazine is more likely to be washed away, so it is not recommended to be used near ponds, streams, or other bodies of water.
Rainfall has more of an effect on atrazine movement when compared to simazine due to atrazine’s higher water solubility.
Because of their differences in solubility, simazine is preferred when the application area is near a water source.
What Does Simazine Kill?
Simazine is formulated to kill a variety of annual and perennial grasses and broadleaf weeds before the seeds have germinated. The broadleaf and grassy weeds simazine controls include:
- Downy brome
- Fall panicum
- Foxtail species
- Wild Oats
- Poa annua
Simazine may also be used to control algae and other submerged aquatic weeds.
However, due to possible copper accumulation and reproductive effects on aquatic wildlife, the application of simazine as an algaecide is an emergency measure.
Is Simazine a Pre-Emergent?
Simazine is a pre-emergent herbicide, which prevents weed seeds from germinating but does not kill already established weeds. Any seedlings which do emerge will turn yellow and die within a short period.
Simazine is commonly used to prevent weeds in fruit orchards, vegetable gardens, vineyards, and other areas where crops are grown.
Sorghum and corn are sensitive to simazine, so their usage on these crops is generally avoided.
Simazine works especially well in preventing late-emerging winter weeds, keeping them from sprouting in the warmer spring months.
Check out our picks for the best pre-emergent herbicides at the link.
How Long Does It Take for Simazine to Work?
Once simazine has been thoroughly watered in for maximum soil penetration, it starts working right away to prevent weed seeds from germinating. Simazine continues to work for three to five months, after which it needs to be reapplied.
The time frame may vary according to weed and grass type and the application rate and soil condition.
As a general rule, simazine is applied in the late winter to prevent weeds from emerging in the spring.
It is then reapplied in the later summer to prevent weeds from germinating in the fall.
Is Simazine Safe on Bermuda Grass?
Simazine is safe to use on Bermuda grass in the late fall when the grass is nearly dormant. It may be applied to actively growing Bermuda grass, but the risk of damaging the grass is higher. Applying too much will hurt Bermuda grass, regardless of whether or not the grass is nearly dormant.
The recommended application rate for simazine on Bermuda grass lawns is 1.5 quarts per acre.
Be sure to follow the recommended dilution rate on the product label of your simazine for the best results to ensure you do not exceed the maximum usage rate.
The dilution rate for Simazine 4L, also known as Princep Liquid, is between 0.75-1.5 fluid ounces per gallon of water to cover 1,000′ square feet.
Take care when using simazine on your Bermuda grass lawn if the primary weed is annual bluegrass.
When simazine is used to control annual bluegrass every year, the grassy weed may develop a resistance to the herbicide.
While atrazine is considered safe for use on completely dormant Bermuda grass, it is usually avoided due to the greater risk of lawn damage.
See the complete list of the best, safe weed killers for Bermuda grass.
What Weeds Does Atrazine Kill?
Atrazine is a selective herbicide effective at killing and preventing various broadleaf weeds and several types of perennial and annual grassy weeds. Common broadleaf weeds and grasses killed and controlled by this include:
- Barnyard grass
- Wild oats
While atrazine will suppress the growth of crabgrass, it will not eliminate it without multiple applications.
Atrazine may also be used to kill post-emergent weeds such as morning glory, cocklebur, buckwheat, and amaranth.
Atrazine is safe for usage on St. Augustine, centipede, and zoysia grasses.
Is Atrazine a Pre-Emergent?
Atrazine is known for being both a pre-emergent and post-emergent weed control product. This means atrazine not only disrupts photosynthesis to prevent seeds from germinating but is also capable of killing certain types of existing weeds.
Atrazine is widely used to control weeds in sorghum, sugarcane, and corn crops, as well as commercial turfs and residential lawns.
Atrazine is also used in cow and sheep farms, where simazine usage is dangerous.
Since atrazine is a selective herbicide, it is less likely to cause damage to certain grasses such as St. Augustine, centipede grass, and zoysia grass when applied correctly.
It is not advised to use atrazine on Bermuda grass unless it is completely dormant, and the grass may still be damaged if the maximum usage is exceeded.
Due to the high potential for runoff and toxicological effects to aquatic wildlife, avoid using atrazine near ponds, streams, or other bodies of water.
How Long Does It Take for Atrazine to Work?
Atrazine is a slow-acting herbicide, and it will take between 2-3 weeks for it to kill established weeds. As a pre-emergent herbicide, atrazine will stay in the soil and work for up to 6 weeks.
Atrazine works slowly because it must reach the roots of the weeds to begin killing them.
Since atrazine does not bind well to the soil, it does not stay active as long as simazine when used as a pre-emergent.
Because atrazine is so potent, it should only be applied a maximum of twice per year in any given area.
Is Atrazine Bad for Trees?
While atrazine works well to kill and prevent weeds, it may be harmful to your trees and shrubs. The small feeder roots of trees and shrubs will easily pick up the chemicals in atrazine, causing them to become burned.
As the roots absorb the atrazine, there will be yellowing of tree leaves, and the tree or shrub will have stunted growth.
Larger trees may survive the harmful effects of atrazine by flushing the chemical from the roots, but younger, smaller trees and shrubs may eventually die.
Damage from atrazine may not be noticeable right away, and it could take more than a year for the tree to fully recover.
How Are Simazine and Atrazine Applied?
Simazine and atrazine are diluted with water, and they are applied with a hand-held backpack or pump sprayer. A broadcast spreader may also apply the herbicides to a larger area. It is essential to determine the square footage of your lawn, so you will know how much simazine or atrazine you need.
Once the herbicide is mixed correctly, apply evenly to avoid oversaturating areas and causing damage.
After the simazine or atrazine has been applied, it needs to be watered in so the herbicide can thoroughly penetrate the soil.
Always follow the directions on the herbicide label for the correct application rate and the recommended maximum usage.
Most herbicides should only be used twice per year due to their potency and potential for environmental toxicity.
Wear protective clothing such as safety glasses, gloves, non-absorbent shoes, and long sleeves when applying simazine and atrazine.
Can You Mix Simazine and Atrazine?
Simazine and atrazine may be mixed to take advantage of atrazine’s post-emergent weed control. When mixing these two powerful herbicides, it is crucial to be mindful of the application.
Some crops and animals are especially sensitive to simazine, and atrazine should not be used near bodies of water due to its high potential for runoff.
In addition, certain grasses such as Bermuda grass are not tolerant of atrazine application, and damage may occur.
It is safer to mix simazine with a less-potent form of post-emergent herbicide if you need to kill established weeds.
Always read the label on your herbicide to determine if it is safe to be mixed with another herbicide.
What Are the Major Disadvantages of Simazine and Atrazine?
While simazine and atrazine work well for weed control, there are several disadvantages such as environmental toxicity, long-term health effects for people, and increases in restricted usage. Atrazine is particularly toxic to aquatic wildlife because of its high potential for runoff.
The EPA has recently issued guidelines for decreasing the application rate and frequency due to increasing concern over toxicity.
Whenever the Environmental Protection Agency determines a pesticide is harmful to an endangered species or its critical habitat, they must consult with the U.S. Fish and Wildlife Service.
The EPA must then draft a biological evaluation to review the use of the pesticide in question.
Similar guidelines have been issued for simazine, with recommendations to ban usage on residential lawns, turf, parks, and recreational fields.
Simazine will only be allowed for usage on golf courses and sod farms once these changes occur.
As recently as November 2021, there have been heavy restrictions on purchasing and using many brands of simazine and atrazine.
Only licensed professionals will be able to buy or use these products.
The neuroendocrine effects of simazine and atrazine have also been found to cause developmental toxicity, prostate cancer, breast cancer, and reduced sperm count in males.
Due to water runoff, there have been increased reports of reproductive toxicity and copper toxicity among fish and other aquatic wildlife.
The European Union has completely banned the usage of simazine and atrazine because of environmental toxicity concerns.
Commonly Asked Questions
How long does simazine last in the soil?
Simazine will work as a pre-emergent herbicide for three to five months.
However, the residues from the herbicide may stay in the soil for up to three years.
When used to kill algae or control other underwater weeds, it takes anywhere from 50-700 days for the herbicide to dissipate in the water.
Do I need a license to buy atrazine?
Atrazine is considered a Restricted Usage Pesticide by the Environmental Protection Agency.
A license is required to purchase or use atrazine in all 50 states.
The main reason for the restricted use of atrazine is its history of groundwater contamination.
In some locations, atrazine concentrations measured up to 48 micrograms per liter over 60 days.
Atrazine is thought to cause more toxic effects to human health than glyphosate because it is an endocrine disruptor.
Many environmental and human health advocates call for the complete ban of atrazine usage.
Atrazine usage continues to be debated among environmental, human health, farmworker, agricultural advocates, and the chemical industry.
Some alternatives to atrazine include dicambia, mesotrione, S-Metachlor, and terbuthylazine.
When should I apply atrazine?
The best time to apply atrazine is in the early morning when temperatures are between 65-90° degrees Fahrenheit (32° C).
Since it has to be watered in, apply atrazine at least one hour before it rains unless you manually water the area.
Atrazine is usually applied a maximum of twice per year, either before or after planting.