How to use pre-emergent herbicides. 11

What is a pre-emergent herbicide?

A pre emergent herbicide prevents weeds from becoming established. This is in contrast to a post emergent such as a sulfonylurea herbicide that kills weeds once they have grown.The key to the successful use of pre-emergent herbicides is understanding how they work and behave in the soil as failing to know this can lead to poor results.

In the above image ryegrass is germinating while the pre emergent diflufenican is still active. Our recent blog updates on split applications of pre-emergents and reasons for pre-emergent failure may also be of interest. 

How do pre emergent herbicides work?

The first thing to remember is that pre emergent herbicids have zero effect on weed seeds as they prevent germinated weed seedlings from becoming established by forming a barrier at the soil surface. After being applied, they all mostly inhibit root growth. In contrast the only shoot active pre-emergent herbicide registered in turf is oxadiazon which is the active ingredient in Echelon® and Echelon Duo®. To work properly the herbicide must be washed into the soil, and also be present when the weed seeds are germinating so soil mositure is vital to allow these to work. 

Are pre emergent herbicides safe?

Table showing the modes of action of turf registered pre emergents.

When not to use pre-emergent herbicides.

Factors affecting the successful use of pre-emergent herbicides:

Volatilization of pre-emergents.

Table showing the vapour pressure of common pre-emergents.

The persistence of pre emergent herbicides.

Table showing the Average DT50* values for common pre-emergents.

Not Watering in the Product.

Table showing the solubility of common pre-emergent turf herbicides.

Soil properties.

• First, in coarse soils such as those with high sand contents that have little ability to hold onto any chemicals;
• Second, in low organic matter soils as less herbicide is likely to become “tied up”;
• Lastly, in sloped and areas of thin turf cover where water movement over the surface can wash away material.

The sorption coefficient (K_oc) is a measure of how much a chemical binds to soils. Chemicals having a high Koc are likely to bind tightly to soil and organic matter i.e. pendimethalin. In contrast low K_oc herbicides i.e. metalochlor are less likley to bind to clay or high organic matter soils and are likely to be mobile.   

So if we rank from those least likely to bind to those with the highest binding: Metalochlor < Dithiopyr < Oxadiazon < Pendimethalin = Prodiamine.

Table showing the average adsorption coefficients for common pre-emergent turf herbicides.

Chemical Breakdown of pre-emergent herbicides.

Incorrect pre-emergent herbicide timing.

Poor timing has a major impact on any results and in my opinion this is the main reason for herbicide failures. So, lets clear something up. If you can see weeds, most of the time its going to be a waste of money applying a pre-emergent.

As an illustration, the table below shows critical soil temperatures for common grass weeds (measured at 10cm depth) courtesy of North Carolina State University.

Because crowsfoot germinates at a higher temperature than summergrass it tends to germinate 2-8 weeks later in the spring and knotweed germinates even earlier. This means that if you are planning make a pre emergent application for crowsfoot a smart move is to make the application around 2 weeks after you see summergrass.

Photodegradation

This is the breakdown of chemicals due to the action of sunlight at the soil surface. Yellow coloured herbicides like prodiamine and pendimethalin, can all decay in sunlight if not watered in after application.

Getting the best results:

The following are some general tips to give you a better chance of getting great results from using these chemicals.

Make sure your application equipment is calibrated properly.

Correct calibration is important if you use a granular or liquid as failing to do this means it will be applied unevenly and then you get poor results.

Choosing pre emergents based on price.

First and foremost buying something purely on price never guarantees good results. Instead, you should select the right product for your particular situation. There are five factors to consider:

• Firstly what weeds are you trying to control? Different pre emergents work better on different weeds and the cheapest option might not even work on your target weed.
• Secondly what is the cost of applying the pre emergent? Factors such as rate, longevity and bag size should all be considered.
• Thirdly what is your turf type? Not all pre emergents are safe on all turf types. There isn’t much point in buying the cheapest pre emergent on the market if its going to kill your turf.
• Next when buying your pre emergent are you buying it from a supplier who can give you good agronomic advice on how to use it? 
• Lastly how long do you want it to last? Longer control isnt always the best especially if your overseeding etc.

If the active ingredients are the same, look at how easy is it to apply and handle. For instance, both Barricade® herbicide and Onset 10GR® contain the same active but one is a liquid and the other a granule. As discussed in the next section there are pros and cons to both of these.

Are Granular or liquid applications best?

Influence of granule size on summergrass control 6 months after treating with pre-emergent herbicides. (Kelly and Coats, 1999)

Successful use of pre-emergent herbicides; Generic vs branded?

​A generic product performs as well as other registered products and if you don’t believe me look at the labels for generic and branded products.

Isomers.

Your left and right hands are isomers as they are the same but one is the mirror image of the other. During the manufacture of metalochlor it creates a blend of four isomers. These all have the same chemical formula but have a different molecular make up.

Pennmag® contains 88% of the S and 12% of the R isomers; Meteor® contains roughly equal amounts of the S and R isomers. Research shows that a higher amount of the S isomer works better. This doesn’t mean that the R isomer doesn’t work but it may not work as well. In fact both the R and combination of R and S formulations have around 50% less activity. Interestingly, on the launch of S metolachlor in the USA, the application rate was 30% less than other metalochlor products. 

Delay weed resistance by changing the chemistry

To avoid the development of herbicide weed resistance to pre emergents you should rotate with products with different modes of action every third or fourth year.

Mixing pre and post emergent herbicides to delay resistance

It has been suggested that a mixture of both a pre and a post emergent works better than rotating chemistry for slowing down the development of herbicide resistance. This approach also:

• Has a larger application window which allows for changes in the timing of winter grass germination due to weather variations;
• Produces a better playing surface and turf quality than applying either alone;
• Gives increased flexibility and allows for weather extremes such as :
  • Heavy rainfall that can leach and breakdown herbicides;

Using pre-emergent safely.

​Turf safety and reseeding interval vary depending on which pre emergent you use. If the chemical is still active in the soil, any seeding work will fail, so make sure that seeding is after the reseeding interval on the label. This means you need to plan ahead and be aware of any future use.

Table showing pre-emergent safety on turf.

Root pruning

Many pre-emergents have negative effects on root growth which is called “root pruning”. Good examples of these are Barricade® and Specticle®. So if you have stressed turf or it already has a poor root system avoid using these as they will only send the turf backwards.

The amount of root pruning that occurs is a direct result of the particular chemical and the turf type.

2018 work into how pre-emergents affect roots on couch grass establishment.

• Specticle® did not achieve a 50% couch cover by the end of the trial;
• Only Ronstar® did not increase the number of days required to reach 50% cover;
• All treatments reduced root mass 6 weeks after treatment (WAT);
• By 10 WAT, Barricade®, pendimethalin, and Specticle® had less root mass;
• At 4 WAT, all treatments other than Ronstar® had shorter roots;
• 10 WAT, only Dimension®, S-metolachlor, and Specticle® had shorter roots;
• The conclusion was that Specticle®, Dimension®, and S-metolachlor are not safe on newly established hybrid couchgrass and should be avoided during establishment.