Are you struggling to decide how to deal with weeds this growing season? If you’re still unsure whether to use a pre-emergent or post-emergent herbicide, you’ve come to the right place.
This simple and succinct guide will help you make a quick and informed decision and choose the right approach for your pesky weeds.
You’ll find out the difference between pre-emergent and post-emergent and gain a better understanding of which type of herbicide you need to choose for your weed problem.
Contents
- The difference between Pre-Emergent and Post-Emergent Herbicides
- Pre-Emergent Herbicide
- What is a pre-emergent herbicide?
- Types of pre-Emergent herbicides
- How does pre-emergent herbicide work?
- Most common active ingredients of pre-emergent herbicides
- How to apply pre-Emergent herbicide
- When to apply the pre-Emergent herbicide?
- How to determine when weed seeds germinate?
- Weeds Effectively Managed with Pre-Emergent Herbicides
- FAQs – Pre-emergent
- Post-Emergent herbicide
- FAQ – Post-emergent
The difference between Pre-Emergent and Post-Emergent Herbicides
Pre-emergent and post-emergent herbicides are types of herbicides.
In general, many people believe that these two are differentiated only by the timing of their application, but this is not exactly the case. Although the main difference is the timing of application, there are other differences that set them apart.
Understanding the difference between post-emergent and pre-emergent herbicides is vital for anyone dealing with weeds or planning to use herbicides.
For a better understanding of the topic of pre-emergent vs post-emergent herbicides, please see Table 1 which highlights the differences between them.
| Aspects | Pre-emergent Herbicides | Post-emergent Herbicides |
|---|---|---|
| Definition | Herbicide that controls germinating weed seeds. | Herbicide that controls weeds that have emerged above ground at a young stage. |
| Target | Prevents germinated weed seeds from sprouting and becoming visible above ground level. | Kills existing weeds that have come out of the ground. |
| Timing of Application | Applied before weeds emerge above ground level. Early spring and fall |
Applied after weeds have emerged above ground level. Late spring and late summer/early fall. |
| Mode of Action | Creates a barrier between the soil surface and germinated weed seeds, preventing them to grow. | Can kill weeds both systemic (absorbed by the plant) and contact (directly applied to the plant). |
| Soil Residual Effect | Last up to 3-4 months in soil. They are known for having a longer soil residual effect than post-emergent herbicides. |
Lasts from a few weeks up to a few months in the soil. They generally have a shorter soil residual effect. |
| Weed type | More effective against annual weeds | More effective against perennial weeds |
| Active ingredients | Prodiamine, Pendimethalin, Dithiopyr, Trifluralin, Oryzalin, Isoxaben, Benefin | 2,4 D, Dicamba, Glufosinate, Clopyralid |
Pre-Emergent Herbicide
What is a pre-emergent herbicide?
A pre-emergent herbicide is a type of herbicide that is applied to weed seeds when germinating.
This means that pre-emergent herbicides affect only the germinated weed seeds and disrupt their process of growing above the ground.
Types of pre-Emergent herbicides
Pre-emergent herbicides can be classified into three main categories:
- Form: liquid and granular
- Source of the active ingredient: synthetic and organic
- Spectrum of control: selective and non-selective/total
How does pre-emergent herbicide work?
The pre-emergent herbicide works by disrupting the growth of germinating seeds.
It is a process that inhibits cell division in the young root system. Once the seed germinates, the young root begins absorbing moisture from the soil.
When the pre-emergent is applied, the germinated seed also absorbs the herbicide. The pre-emergent herbicide then stops the root or initial leaf from developing further, causing the germinated weed seed to die.
At the same time, the pre-emergent herbicide creates a barrier on the soil surface, preventing the weed from growing above ground level.
Most common active ingredients of pre-emergent herbicides
The most common active ingredients of pre-emergent herbicides include:
- Prodiamine
- Pendimethalin
- Dithiopyr
- Trifluralin
- Oryzalin
- Isoxaben
- Benefin
- Surflan
- Flumioxazin
- Indaziflam
How to apply pre-Emergent herbicide
To apply the pre-emergent herbicide, you should spread the granular herbicide or spray the liquid herbicide all over the infected area.
- Read and follow the label’s instructions.
- Mow the lawn for increased herbicide effectiveness.
- If the soil is too dry, water it for a few days before applying the herbicide.
- Apply the herbicide.
A:
If you use a granular herbicide, use a spreader to spread it as evenly as possible across the area.B: If you use a liquid herbicide, mix the right quantity with water and then put the solution in a sprayer. Spray the area where the weeds are likely to grow.
- Water the lawn again. This activates the herbicide and ensures it penetrates the soil.
Remember, it is crucial to water the pre-emergent herbicide properly after finishing spraying it.
When to apply the pre-Emergent herbicide?
You can apply the pre-emergent herbicide in the early spring or fall. These are two great times of the year to use a pre-emergent herbicide.
In the spring – applying the herbicide in mid-March can effectively prevent summer weeds. However, it is important to check the soil temperature before application, as simply relying on the calendar month is not enough. The soil temperature should be 55-60°F, which is when weeds start to germinate.
In the fall – applying pre-emergent herbicide around September or before the first frost can prevent cool-season weeds. To ensure maximum effectiveness, the soil temperature should be below 70°F before application.
How to determine when weed seeds germinate?
A pre-emergent herbicide is effective when the weed seeds start to germinate. But how can you determine when this happens?
Here are three ways to figure out when the weed seeds germinate:
- The first way is to observe the weather and soil conditions. If the weather starts to get warm or has been warm for at least 7-10 days, then there is a good chance that the weed seeds will germinate.
Most weed seeds start germinating when the soil temperature is above 55°F. You can use a soil thermometer to check the temperature.
- Another way is to recall what happened last year. This is because there is a high chance that history will repeat itself.
If you remember the time when the weeds started appearing above ground, then you can make an idea of when to apply the herbicide.
- You can also conduct a seed germination test to test your soil directly. Take a sample of the soil, keep it in a dark, warm place and if you see sprouted seeds, then it means that the weed seeds have germinated in the soil.
Weeds Effectively Managed with Pre-Emergent Herbicides
There are several that respond better when controlled in the phase of germinated weed seeds and therefore are better controlled with pre-emergent herbicide application.
Weeds that are better controlled with pre-emergent herbicide applications include:
- Crabgrass
- Foxtail
- Annual bluegrass
- Dandelion
- Common chickweed
- Lambsquarters
- Pigweed
- Purslane
- Shepherd’s purse
- Spurge
- Wild mustard
FAQs – Pre-emergent
You can choose a pre-emergent herbicide by first identifying the type of weeds you want to control.
Once you have identified the targeted weeds, choose a pre-emergent herbicide that has those types of weeds listed under the “weeds controlled” section on the label. Additionally, factors such as soil type and pH can also be important when choosing a pre-emergent herbicide.
You can apply a pre-emergent herbicide up to two times a year in early spring and fall.
Yes, you can use atrazine as a pre-emergent herbicide to control a variety of weeds like broadleaf and grassy weeds before they emerge from the soil.
You can overseed in the fall after using a pre-emergent herbicide in the spring.
If you think your lawn needs to be reseeded, you can skip the pre-emergent herbicide application or apply the herbicide and wait for four months before overseeding the lawn. However, overseeding in the summer is not recommended due to the hot temperature, so it’s best to overseed in the fall.
It is too late to apply a pre-emergent herbicide when the weeds have already grown above the ground and become visible to the eye. The pre-emergent herbicide needs to be applied before the weeds emerge from the soil. As long as the weeds are not visible and above the ground, it is not too late to apply a pre-emergent herbicide.
If you apply a pre-emergent herbicide too late, meaning that the weeds have already grown above the ground, then the herbicide will not work. You will have reduced or no results because the herbicide was not applied at the correct time.
A pre-emergent herbicide can last up to 3-4 months in soil when applied properly.
Yes, there are organic pre-emergent herbicides. Most of them contain corn gluten meal.
Post-Emergent herbicide
What is a post-Emergent herbicide?
A post-emergent herbicide is a type of herbicide that is applied to weeds and other unwanted plants after they have already emerged from the soil.
In simpler terms, post-emergent herbicides target undesirable vegetation that has already grown above the ground and becomes visible.
Types of post-Emergent herbicides
Post-emergent herbicides can be classified into three main categories based on different factors:
- Form: liquid and granular
- Mode of action: contact and systemic
- Spectrum of control: selective and non-selective/total
How does post-emergent herbicide work?
Post-emergent herbicides work differently based on their type and the specific active ingredient they contain. To better understand their differences, let’s take a closer look at each type.
1. Systemic post-Emergent herbicide
Systemic herbicides work by being absorbed by the plant through its leaves, stems, or roots.
Once absorbed, they are transported throughout the entire plant system, including the root system, which can lead to the death of the plant. Unlike contact herbicides, systemic herbicides are effective in killing the entire plant from the inside out.
2. Contact post-emergent herbicide
This type of herbicide works by affecting the exterior of the plant, specifically the foliage and cellular membranes of the plant cells. It does so by coming into direct contact with the unwanted plant.
Contact herbicides are effective in killing the portion of the weed that comes into contact with the herbicide. However, they only kill the part of the weed that was sprayed, and not the entire plant.
3. Selective post-emergent herbicide
Selective post-emergent herbicides work by targeting and killing specific types of plants, while leaving other plants unharmed.
4. Non-selective/total post-emergent herbicide
Non-selective post-emergent herbicides work by killing any plant that comes into contact with the herbicide.
Info: Selective post-emergent herbicides can be found as either contact or systemic herbicides. Also, non-selective post-emergent herbicides can be both systemic and contact herbicides, depending on their active ingredient and mode of action.
Info: The mode of action of a post-emergent herbicide, whether it is a contact or systemic herbicide, is determined by the active ingredient.
Most common active ingredients of post-emergent herbicides
The most common active ingredients of post-emergent herbicides are:
- 2,4 D (systemic)
- Dicamba (systemic)
- Glufosinate (contact)
- Clopyralid (systemic)
- Glyphosate (systemic)
- Fluazifop-p-butyl (systemic)
- Oxyfluorfen (contact)
How to apply post-Emergent herbicide?
Post-emergent herbicides can be applied using two methods: blanket treatments or spot treatments.
How do you know which method is right for you? It’s very easy, you just have to consider:
– Weed density
– Weed distribution
– Location
– Sensitivity of the plants nearby
– Size of the surface
1. Blanket treatments
| Factor | Description |
|---|---|
| Weed density | High |
| Weed distribution | Evenly distributed |
| Location | There are no desirable plants nearby that could be affected by the herbicide |
| Sensitivity of the plants nearby | There are no sensitive plants nearby that could be harmed by the herbicide |
| The size of the surface | Medium to large |
If the weed density is high and evenly distributed, then you should use the blanket treatment method. This method guarantees a uniform and even herbicide distribution over the entire area infected by weeds.
Here are all steps for applying a post-emergent herbicide using the blanket treatment:
- Read the label and follow its instructions.
- Wear the appropriate personal protective equipment.
- Ensure the weather conditions are suitable for herbicide application, including the right temperature, wind, soil moisture, and no rain in the forecast.
- Calculate the correct amount of herbicide per unit of the area using the label’s instructions.
- If using a granular post-emergent herbicide:
- Use a granular spreader.
- Fill the granular spreader with the herbicide.
- Adjust the spreader to the right application rate based on the size of the granules.
- Walk at a steady pace with the spreader, spreading the area evenly. It’s recommended to walk in straight lines for easier tracking.
- Water the entire area to activate the herbicide. Check the label for the recommended water amount.
- If using a liquid post-emergent herbicide:
- Use a hand-held sprayer, backpack sprayer, ATV sprayer, or tractor-mounted sprayer, depending on the size of the surface.
- Mix the herbicide with water according to the label’s ratio in a container.
- Pour the solution into the chosen sprayer.
- Spray the entire area evenly and uniformly, making sure all targeted plants are covered in the solution.
- Water if necessary. Some liquid herbicides do not require watering after application, but check the label for further instructions.
Tips for liquid post-emergent herbicide application. Use a marking system such as flags or stakes to help guide you and ensure you don’t miss any spots. You can also add a spray indicator dye to the herbicide solution.
2. Spot treatments
| Factor | Description |
|---|---|
| Weed density | Low |
| Weed distribution | Concentrated in specific areas |
| Location | There are desirable plants nearby that could be affected by the herbicide |
| Sensitivity of the plants nearby | There are sensitive plants nearby that could be harmed by the herbicide |
| The size of the surface | Small to medium |
Low weed density and concentrated weeds in the specific area tell you that you can use the spot treatment method to apply your herbicide.
Here are all steps for applying a post-emergent herbicide using the spot treatment:
- Read the label and follow its instructions.
- Wear the appropriate personal protective equipment.
- Ensure that the weather conditions are suitable for herbicide application, including the right temperature, wind, soil moisture, and no rain in the forecast.
- Calculate the correct amount of herbicide per unit of the area using the label’s instructions.
- Prepare the herbicide solution by mixing it with water and transferring it into a hand-held sprayer or backpack sprayer.
- Spray each weed with the herbicide solution.
- Make sure each weed is completely covered in the solution.
- Move on to the next weed and spray it as well, continuing with the next weed until all weeds have been sprayed.
When to apply the post-Emergent herbicide?
You can apply the post-emergent herbicide during late spring through late summer in the morning.
Depending on the area where you are located, you could also apply it in the early fall if the soil temperature is still above 55°F.
Now, let’s break down this short answer.
The earliest time to apply the post-emergent herbicide is in late spring when the weeds are already above ground and still in the growing phase, which is the phase when they absorb the most nutrients and herbicide.
However, make sure that the temperatures are above 60-65°F for at least a week before the herbicide application.
I suggest applying the herbicide in the morning because weeds grow faster during this time of day. As the day progresses, they slow down, which means they don’t absorb as much herbicide.
Between late spring and late summer, you can apply the herbicide whenever you see weeds that you want to get rid of.
The latest application you can do is in late summer or early fall. By this time, the weeds are quite mature and not growing as much, which means they absorb fewer nutrients and herbicides. It could also be harder to kill them, or you may need more than one application.
Always consider the weather conditions before applying the herbicide. So, remember:
- No rain in the forecast
- Temperatures between 65°F and 85°F
- Avoid applying during powerful winds
- Soil moisture should be moderate.
FAQ – Post-emergent
You can choose a post-emergent herbicide that’s right for you by taking into account factors such as the type of weed you want to control, the size of the weed, and the sensitivity of other plants in the area.
When dealing with different types of weeds, it’s important to choose a post-emergent herbicide that can effectively control the targeted weed. Check the labels of the herbicides and make sure that the weed you want to control is listed.
Consider the size of the weed and its location when selecting a herbicide. If the weeds are young and located in an area with desirable plants, you may want to use a selective post-emergent herbicide. For mature weeds or areas with no desired plants nearby, a non-selective herbicide may be more appropriate.
Lastly, consider the mode of action of the herbicide. If you want fast-acting results, a contact herbicide may be a good option. If you prefer longer-lasting results, a systemic herbicide may be more suitable.
Yes, post-emergent herbicide can burn your grass if you apply it:
- in temperatures above 85°F
- in excessive amounts.
To avoid burning your grass, it is important to check the temperature before applying the herbicide and to follow the recommended application quantity specified on the herbicide’s label.
Yes, post-emergent herbicides are systemic.
When classified based on their mode of action, post-emergent herbicides can be both systemic and contact. The systemic post-emergent herbicides are absorbed by the plant, travel through the plant’s system, and end up killing it from the inside out.
The latest time to apply post-emergent herbicide is during late summer or early fall, typically between August and September.
This depends on two factors like climate of your location and the type of weed.
Yes, you can apply too much post-emergent herbicide, so there is always a risk of over-application of herbicide.
If you apply too much post-emergent herbicide, you risk damaging your lawn and even non-target plants. To avoid over-application, you can consult the label and follow the written instructions. Each herbicide’s label states the ratio of herbicide to water and how much quantity you need per acre.
To plant seeds, you should wait between 10 to 14 days after using post-emergent herbicide.
This is an average waiting period suggested before taking any action on the area that has been treated.
However, it’s important to consult the label of the herbicide you’re using, as some may recommend seeding earlier than 10 days, while others may require a longer waiting period.
A post-emergent herbicide can be applied twice a year, typically in late spring and late summer/ early fall.
However, the frequency of application depends on the severity of the weed infestation. It’s recommended to apply the herbicide once in late spring and wait for results. If weeds persist, spot treatment can be used to target specific areas.
To prevent weeds from reemerging in the next spring, it’s also a good practice to apply herbicide in late summer/early fall.
It’s important to check the label of the herbicide before use, as some have a maximum amount that can be used per area per year. No matter how often the herbicide is applied, the maximum recommended amount should not be exceeded.
Glyphosate is a post-emergent herbicide that is applied to weeds that have already emerged above ground level.
It works by being absorbed through the leaves and traveling down to the roots to kill the entire plant.
Yes, 2,4-D is a post-emergent herbicide that controls and kills a variety of weeds after they have grown and become visible above the ground surface.
Yes, there are organic post-emergent herbicides available.
Most of them are made using vinegar, citric acid, and essential oils. These herbicides are generally considered to be safer for the environment compared to synthetic post-emergent herbicides
A post-emergent herbicide can last from a few weeks up to a few months in the soil.
However, some post-emergent herbicides tend to break down quickly and have a short soil residual effect. Therefore, the duration of their activity in the soil can vary from a few weeks to a few months.
Let’s take a look at some specific examples: the post-emergent herbicide 2,4-D lasts from 1 to 14 days in the soil. Glyphosate, another post-emergent herbicide, can last for up to 6 months in the soil. Dicamba herbicide can also last up to 14 days in the soil.
Bibliography:
– NC State University / weeds.ces.ncsu.edu/weeds-teaching/landscape-weed-management/preemergence-herbicides/
– Texas A&M AgriLife Extension Service / agrilifeextension.tamu.edu
