The key to the successful use of turf chemicals such as pre-emergent herbicides is understanding their specific properties and behaviour in the soil. Failing to know how these work can lead to problems as shown in the image above. In the image above ryegrass has germinated whilst diflufenican is still active. This has had a detrimental effect on overseeded ryegrass. Our recent blog update covering split applications of pre-emergent herbicides may also be of interest.

 

When not to use pre-emergent herbicides.

If your existing turf is thin or stressed, exhibits a weak root system and you are intending to overseed or renovate then you should think about whether using these is right for you. These operations will directly affect the success of any application.
 
Pre-emergent herbicides prevent germinated weed seedlings from becoming established by forming a barrier at the soil surface. They can inhibit the growth of the root and the shoot. The herbicide must be washed into the soil, and be present when the weed seeds are germinating to be effective. Read more here. The only shoot active pre-emergent herbicide registered in turf is oxadiazon.
 
How well a pre-emergent herbicide works is a combination of its solubility, its binding to clay and organic matter, the prevailing climatic conditions, and the rate of application. Correctly identifying any weed history can also help in making the appropriate decison as to what pre-emergent herbicide to use. They don’t all work well on all weeds! The following is an overview of these factors but if you want a more detailed overview check this out. Remember, all pre-emergent herbicides do not behave the same way!

Factors impacting the successful use of pre-emergent herbicides:

 

Volatilization.

Some pre-emergent herbicides suffer from volatilization. This means they will turn into gas after application if left on the soil surface. The table below gives a pre-emergent herbicide comparison of their tendency to volatilize. Those toward the top of this table have a higher tendency to volatilize compared to those toward the bottom.
 
The consequence of this is that herbicides with a higher vapour pressure will need more urgent watering as losses can begin as soon as the spray has dried. In contrast products with a low vapour pressure do not need immediate watering. For example indaziflam (Specticle®) has a watering window of 21 days after application.
 
Volatilization will be the greatest under windy, hot temperature conditions. Label references to the period for watering are the time after which losses may start to become too high.

Vapour pressure for common pre-emergent turf herbicides.

s-metolachlor (Pennmag)

3.7

Actives with a vapour pressure of greater than 1mPa are generally considered volatile.

pendimethalin (Battalia)

3.34

dimethenamid-P (Freehand)

2.5

oxadiazon (Ronstar/Echelon)

0.67

terbuthylazine (Numchuk Quad)

0.152

Actives with a vapour pressure of less than 1mPa are generally considered non-volatile

propyzamide (Rustler)

0.058

Prodiamine (Barricade/Onset 10GR)

0.0033

diflufenican (Warhead)

4.25 x 10-3

Indaziflam (Specticle)

2.5 X 10-05

Oryzalin (Prolan)

1.1 X 10-07

Persistence.

The rate of herbicide persistence is usually reported as a DT50 value. This value is the number of days that it takes for 50% of the herbicide in the soil to break down. Soil longevity depends upon the soil type, breakdown speed, and application rate with the breakdown rate varying between different soils and environmental conditions.

Average DT50* values for common pre-emergent herbicides.

DT50 Value

dimethenamid-P (Freehand)

7

s-metolachlor (Pennmag)

21

terbuthylazine (Numchuk Quad)

22

0 to 30 Non persistent

dithiopyr (Dimension)

39

Oryzalin (Prolan)

44.8

30 to 100 Moderately persistent

propyzamide (Rustler)

60

pendimethalin (Battalia)

100

diflufenican (Warhead)

105

prodiamine (Dimension/Onset 10GR)

120

 >100 Persistent

indaziflam (Specticle)

150

oxadiazon (Ronstar/Echelon)

502

Very persistent

Not Watering in the Product.

Many pre-emergent herbicides are taken up by the roots of germinating weeds. This means good soil moisture needs to be present. When the soil is dry the weed cannot take the herbicide up meaning it fails to provide good weed control. 
 
Pre-emergent solubility does not mean how easily the product dissolves. Instead solubility is the maximum concentration of a product in a volume of water. Some chemicals readily go into solution but have low solubility. Others are difficult to dissolve but once dissolved have high solubility.
 
Herbicides with low solubility (see Table below) generally need larger water volumes to wash them in. They also need very good moisture conditions after application and for the period of desired control.
 
Those with high solubility are easy to water in within 2 or 3 days. However, even herbicides that bind tightly can still be lost if heavy rainfall occurs immediately after application.

Solubility of common pre-emergent turf herbicides.

diflufenican (Warhead)

0.05

Prodiamine (Barricade/Onset 10GR)

0.013

pendimethalin (Battalia)

0.33

oxadiazon (Ronstar/Echelon)

0.57

Oryzalin (Prolan)

1.13

Dithiopyr (Dimension)

1.38

Low solubility (0 to 49mg/L @ 20C). Likely to require moist conditions for incorporation and uptake.

Indaziflam (Specticle)

2.8

terbuthylazine (Numchuk Quad)

7

propyzamide (Rustler)

9

s-metolachlor (Pennmag)

480

High solubility (> 501mg/L @ 20C).

dimethenamid-P (Freehand)

1499

Soil properties.

Organic matter and soil texture (the ratio of sand, silt, and clay) affect the binding ability of pre-emergent herbicides. Clay soils and those with high amounts of organic matter will adsorb more herbicide. This results in higher application rates to achieve weed control due to less being available for uptake by germinating weeds. Conversely, pre-emergent herbicide movement out of the profile will increase in:

  • Coarser soils such as those with higher sand contents.
  • As soil organic matter decreases.
  • Sloped areas and
  • In areas of thin turf cover.

 

The sorption coefficient (Koc) is a measure of the tendency of a chemical to bind to soils, corrected for soil organic carbon content. Chemicals with a high Koc will have less chemical available for weed uptake and microbial degradation.

Commonly used pre-emergent herbicides for their ability to bind to the soil (from least to highest):  Indaziflam < Dithiopyr < Oxadiazon < Pendimethalin = Prodiamine.

 

Average adsorption coefficients for common pre-emergent turf herbicides.

pendimethalin (Battalia)

17491

Koc > 4000

Likely to bind tightly to soil and organic matter

Prodiamine (Barricade/Onset 10GR)

12710

Non-mobile.

oxadiazon (Ronstar/Echelon)

3200

diflufenican (Warhead)

1622

indaziflam (Specticle)

1000

Koc 500 to 4000 slightly mobile

oryzalin (Prolan)

949

propyzamide (Rustler)

840

dithiopyr (Dimension)

801

terbuthylazine (Numchuk Quad)

230

Koc 75 to 500 moderately mobile

dimethenamid-P (Freehand)

218

s-metolachlor (Pennmag)

200

Koc 15 to 75 Mobile

Chemical Breakdown.

Herbicide breakdown in the soil occurs mainly by microbial degradation, with chemical hydrolysis also playing a lesser role. Faster breakdown occurs with warmer temperatures, excessive soil moisture, adequate oxygen and nutrients, and neutral pH.

Poor timing.

Incorrect application timing has a major impact, as once weeds have germinated, it is generally too late. As a general guide applying when you can see weeds is a waste of time as there are probably a larger number of weeds hidden in the turf canopy. The table below shows critical soil temperatures for common grass weeds (measured at 10cm depth) courtesy of North Carolina State University.

 

Weed         Temperature °C
Poa annua / winter grass21
Crowsfoot (Eleusine indica)16-18
Summergrass12-15

 

 

Because of higher temperature requirements for germination, crowsfoot normally germinates two-to-eight weeks later in spring than summergrass. As a rule of thumb, aim to apply a pre-emergent herbicide two weeks before summergrass germination.
 
If you can’t apply before weed germination, there are a few options that have a wider window of application to provide consistent results. A good example of this is Dimension® (dithiopyr) which can control summergrass before it emerges and in the early post-emergent stage.

Dimension pre-emergent herbicide offers a larger window for application thus optimizing your pre-emergent herbicide application.

Getting the best results:

 

Make sure your application equipment is properly calibrated.

Correct equipment calibration is important whether using a granular or a liquid formulation.

Picking products based on price.

Making a purchase decision purely on price does not guarantee good results. Instead, you should select the best pre-emergent for your particular situation.  The best way of ensuring the successful use of pre-emergent herbicides is to work out the application cost and also take into consideration longevity. If the active ingredients are the same between a generic and a branded product, you should also look at the ease of application and handling that the product offers. For example, both Barricade® and Onset 10GR® contain the active prodiamine but one is a liquid and the other a granule.

 

Are Granular or liquid applications best for the successful use of pre-emergent herbicides?

Liquid and granular pre-emergent herbicides are equally effective in controlling susceptible weeds. Whichever you use they must be uniformly applied for control.
 
Granular pre-emergent herbicides have become popular due to their ease of use. They allow you to carry out two operations at the same time with examples being Oxafert®, Echelon® (oxadiazon), and Onset 10GR® (prodiamine). 
 
Particle size and uniformity, and application equipment will determine the evenness of any application. As particle size decreases, the density of particles per unit area increases. Uniform particle sizes are important to prevent ballistic segregation. Onset 10GR® has a 1-2mm particle size and so gives excellent coverage when applied at label rates.
 

 

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

Particle size number/ gramOxadiazonProdiamine
235843
586771
1657274
4657281
1,3107082
3,7286683
10,6066883
LSD (0.05)77

 

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

When a generic turf product is registered in Australia It theoretically performs equivalent to other registered products. This similarity shows in the labels of generic and proprietary products having the same application rates.

One exception to this rule is metolachlor and in the Australian turf market, there are two registered metolachlor formulations. These are Meteor® (Amgrow) and Pennmag® (Syngenta). Although both contain the same amount of active ingredients there is an argument for there not being the same.

 

Isomers.

An isomer is a compound that has the same chemical formula but the molecular composition is made up slightly differently. A good example of this would be your left and right hands. They are the same but one is the mirror image of the other and these are called stereoisomers. During the manufacturing process of metolachlor, all products contain a blend of four isomers.

Pennmag® contains approximately 88% of what is called the S isomer and approximately 12% of R isomers. Meteor® contains approximately equal ratios of S and R isomers. Research has shown that a high content of S-metolachlor means that it works better. This doesn’t mean that the R-metolachlor doesn’t work but it just doesn’t work as well. 1980’s Ciba research showed that it had approximately 50% less activity with  a combination of S and R metolachlor also having less activity. When the S metolachlor-dominated formulation was introduced in the USA the rate was reduced by 30% to account for the enhanced activity of S-metolachlor.

 

Take into account the chemical make up when successfully using pre-emergent herbicides.
The key to successfully using pre-emergent herbicides based on S metalochlor is knowing how they behave once applied.

Use of pre-emergent herbicides – Rotate the chemistry

As with all chemicals a key to the successful use of pre-emergent herbicides is rotating the chemistry. This ensures that you minimize the development of weed resistance. Ideally, you should rotate products with different modes of action every third or fourth year.

Recent work by pre-emergent herbicide research by Bayer has shown that using a PRE/POST tank-mix is more effective than rotating herbicides with different sites of action. It also provides benefits such as:

  • A greater application window allowing for annual variations in winter grass germination and
  • More flexibility due to variable autumn/winter weather including:
    • Heavy rainfall that could leach and degrade herbicides
    • Warm temperatures that could encourage germination throughout the winter.

Turf safety in the successful use of pre-emergent herbicides.

With such a large range of pre-emergent options to choose from these vary in their turf safety and where you are legally allowed to apply them. The table below shows the comparative safety of Australian turf registered pre-emergent herbicides.

 Another factor to consider is the reseeding interval following application. If the chemical is still active in the soil, seeding will be a failure. You should always ensure that seeding is after the reseeding period stated on the label.

 

Turf type

s-metolachlor

dithiopyr

Oryzalin

propyzamide

pendimethalin

prodiamine

indaziflam

oxadiazon

dimethenamid-P (Freehand+)

Couch

OK+

OK

OK

OK

OK

OK

OK

OK++

OK

QLD Blue couch

OK

OK

No

OK

OK

OK

OK

OK

OK

Buffalo

OK

OK

No

OK

OK

OK

OK

OK

OK

Bentgrass

OK+

No

No

No

OK

No

No

No

No

Ryegrass

OK

OK

No

No

OK

No

No

No

No

Tall fescue

OK

OK

No

No

No

No

No

No

No

Zoysia

OK

OK

No

No

OK

OK

OK

OK

OK

Kikuyu

OK

OK

OK

OK

OK

OK

OK

OK

OK

Reseeding period

Established

3-6 months

18 weeks

60 days

30-60 days

6 months

12 months

4-5 months

3 months

Irrigation volume

At least 3mm

Not required

10-15mm

25mm

10-15mm within 1 day

6mm within 7 days

3-6mm within 21 days

Irrigate within 2 days.

10-15mm

Root pruning

Yes

Yes

Yes

Yes

Yes

Yes

Yes

No

Yes

+

Not golf greens

++

Not santa anna

Root pruning

The final factor to consider for the successful use of pre-emergent herbicides, is what effect these have on the existing roots of the turf. Many pre-emergent herbicides will tend to prune or develop club roots. So if your turf is stressed or already has a poor root system using these will only make a bad situation worse.

 

Club-roots on a grass treated with prodiamine.

Image above is courtesy of J. Neal NC State Extension

The degree of root pruning that occurs is a direct result of the chemical used and the turf type your using it on.

2018 work looked at how pre-emergent herbicides affected root architecture on the establishment of hybrid couch grass. The results did throw up a couple of  major surprises:

  • Specticle® failed to achieve a 50% hybrid couchgrass cover by the end of the experiment.
  • Only Ronstar® did not increase the number of days required to reach 50% cover.
  • All herbicide treatments reduced root mass when harvested 6 weeks after treatment (WAT) relative to the nontreated.
  • By 10 WAT, Barricade®, pendimethalin, and Specticle® reduced dry root mass compared with the nontreated.
  • At 4 WAT, all treatments other than Ronstar® reduced root length when compared with the nontreated.
  • 10 WAT, only Dimension®, S-metolachlor alone, and Specticle® reduced root length when compared with the nontreated and finally
  • Results suggest that Specticle®, Dimension®, and S-metolachlor are not safe on newly established hybrid couchgrass and should be avoided during establishment.

Can I aerate after applying a pre-emergent herbicide?

We get asked this a lot. The general perception is that aerating after applying a pre-emergent herbicide will break the “barrier” and prevent it from working properly. However, there is turf research suggesting that this might not be an issue. As a general rule solid tining should be OK but there might be issues with hollow tine aeration due to the volume of soil being removed.