Contents

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?

First and foremost you must realise that all chemicals can be dangerous to humans, animals and the environment if you do not follow the label instructions. Therefore, use turf chemicals as recommended by the manufacturer as being the applicator, you are legally liable. Having said that if you follow the instructions these products are safe to use.
 
 
Acute toxicity is the amount of active ingredient required to kill 50% percent of a test population. This is expressed as LD50 (lethal dose 50). The lower the LD50 of a chemical, the greater its toxicity to humans and animals. Turf chemicals with a high LD50 are the least toxic to humans. For example caffeine has an LD50 of a 200mg/kg. On the other hand the LD50 of prodiamine the active in Barricade herbicide is >5000mg/kg. The table below shows the LD50 of common pre emergent herbicides.
 
 
 
 
 

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

Chemical Group

Active constituent (common trade name)

LD 50 Oral

3

Inhibition of microtubule assembly

Benzamides

propyzamide (Proforce Checkpoint®*500SC)

5,620

Benzoic acids

chlorthal-dimethyl (Chlorthal-dimethyl 900®)

>10,000

Dinitroanilines: (DNAs)

pendimethalin (Proforce Battalia®)

1,250

prodiamine (Barricade® herbicide)

>5,000

Pyridines

dithiopyr (Dimension®)

>5,000

12

Inhibition of carotenoid biosynthesis at the phytoene desaturase step (PDS inhibitors)

Phenyl-ethers

diflufenican (Warhead Trio®*)

>2,000

                14                 

Inhibition of protoporphyrinogen oxidase (PPO inhibitors)

N-Phenyl-oxadiazolones

oxadiazon (Echelon®, Oxafert®)

>5,000

15

Inhibition of very long chain fatty acid synthesis (VLCFA inhibitors)

α-Chloroacetamides

dimethenamid (Freehand®*)

849

S-metalochlor (Pennmag®)

2,780

Benzofurans

ethofumesate (Tramat®)

>20,000

29

Inhibition of cellulose biosynthesis

Alkylazines

indaziflam (Specticle®)

>2,000

When not to use pre-emergent herbicides.

Do not use pre emergents on thin, stressed turf with a weak root system or if you are overseeding. If you are intending to overseed or renovate then you should think carefully about using these. There are several factors to consider as to how well a pre-emergent performs:
  • Firstly, its solubility;
  • Second, its ability to bind to soil particles;
  • Third, the climatic conditions when it is applied, and finally
  • The rate of application.

It is also important to know the weed history of a site as this helps in choosing the right pre-emergent. There is little point in applying a pre-emergent if it doesn’t work against the target weed.

In short the following is an overview of these factors. After all, a
lways remember that pre-emergent herbicides do not all behave the same!

Factors affecting the successful use of pre-emergent herbicides:

 

Volatilization of pre-emergents.

Some pre-emergents are prone to volatilization which means if they are left on the soil surface, they turn into a gas and and evaporate into the atmosphere. Volatilization is greatest under windy, hot temperature conditions. The table below gives the tendency for common pre-emergents to volatilize with those at the top have a higher tendency to volatilize in contrast to those at the bottom. 
 
Pre emergent herbicides with a high vapour pressure like Pennmag® need watering in immediately because as soon as the spray dries losses begin. On the other hand products with a low vapour pressure such as Specticle® do not need immediate watering and watering can in fact be left for up to 21 days.
 
 
 
In brief, labels with a watering in period are the time after which losses may start to become too high although frequently, losses will be occuring before that time.

Table showing the vapour pressure of common pre-emergents.

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

The persistence of pre emergent herbicides.

The DT50 value is the number of days for 50% of a herbicide in the soil to break down. Its length of soil activity depends upon several factors including the soil type, the environmental conditions, and the breakdown and application rate.   
 

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

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.

Uptake of most pre-emergent herbicides is by the roots. Consequently, there has to be good soil moisture present as if the soil is too dry there will be at best limited uptake and product failure will occur. Watering in also causes weed seeds to germinate which is important to ensuring that these chemicals work.
 
The table below shows the solubility of turf pre-emergent herbicides. Pre-emergent solubility does not mean how easily the product dissolves but is instead the highest concentration that can dissolve in a volume of water. For instance, some chemicals easily dissolve but have low solubility, whereas others are difficult to dissolve but once dissolved have a high solubility.
 
In other words, herbicides with low solubility need larger water volumes to wash them into the soil. They also need good levels of soil moisture for the length of control. This is the opposite to those with high solubility which are easier to water in. 

Table showing the 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 affect how pre-emergent herbicides behaviour. Clay and high organic matter soils will “tie up” more herbicide than for example sandy soils. As a result, higher herbicide rates are then needed to give equivalent control as these higher rates take into account that less is available for weed uptake. 
 
Increased pre-emergent herbicide movement out of a soil profile occurs:
  • 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 (Koc) 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 Koc 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.

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 (Checkpoint)

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 of pre-emergent herbicides.

Soil breakdown of herbicides is a result of microbial decay and chemical hydrolysis. Several soil factors impact the speed at which this occurs:
  • Firstly, soil temperatures. The higher the soil temperature the faster the breakdown;
  • Secondly, soil moisture. In moist soils chemical breakdown is quicker than in dry soils;
  • Thirdly, adequate soil oxygen and ample nutrients will speed up this decay, and finally
  • A neutral soil pH.

 

HerbicideSoil Texture and Organic MatterSoil pHSoil moisture and Temperature
Dimethenamid-p (Freehand®)
  • In sandy soils this is more likely to leach. 
  • Loamy soils with less than 3% OM also pose a risk for leaching.
Not a factorSoluble and may leach in sandy soils following intense rainfall
S-metalochlor (Pennmag®)
  • In sandy soils this is more likely to leach. 
  • Loamy soils with less than 3% OM also pose a risk for leaching.
Not a factorSoluble, but less soluble than Dimethenamid-p and therefore has a lower activity

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.

Weed Temperature °C
Poa annua / winter grass/ annual bluegrass21
Crowsfoot (Eleusine indica)16-18
Summergrass12-15
Knotweed2-10

 

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.

However, there are a few products that have wider application windows such as Dimension® herbicide. This controls summergrass both before it germinates and in the early post-emergent stage. This means you can apply later and still knockout a significant amount of the summergrass. Another example is Specticle® herbicide which has pre and post-emergent activity against winter grass although work needs to be done to discover how plant size impacts the results. The image below is of a delayed Specticle® application which is giving great post emergent control against winter grass in couch fairways.

Dimension pre-emergent herbicide offers a larger window for application thus optimizing your pre-emergent herbicide application.
Specticle pre-emergent herbicide pre and post treated and untreated

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?

If you apply liquid and granular pre-emergent herbicides correctly, they are equally good at preventing weeds. Granular are popular as they are easier to use as all you need is a fertilizer spreader and there is no need for a spray tank. In several cases they even allow you to carry out two operations at the same time. For instance, Echelon® Duo contains a pre-emergent together with an insecticide and a fertilizer. The public perception of using granules is also better as there is no need to wear personal protective equipment.
 
 
The negatives of granular pre emergents are:
  • First, for larger areas liquids are easier to apply;
  • Second liquids are less likely to wash away in a storm or heavy rainfall and
  • Third, liquids move into the soil a lot quicker.
 
With granules the application uniformity is effected by both the granule uniformity and size. The actual equipment you use also can have a big impact on the result you get. As the granule size decreases, the number of granules per unit area increases. For example, Onset 10GR® has a 1-2mm granule so it gives good coverage even at low rates.
 

 

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

Granule 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?

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.

However, metolachlor is a possible exception to this rule. There are two registered products and these are Meteor® (Amgrow) and Pennmag® (Syngenta). Despite both containing the same amount of active there is evidence that they don’t perform the same.

 

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. 

 

Take into account the chemical make up when successfully using pre-emergent herbicides.
Pennmag is based on S metalochlor

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.

Turf type

s-metolachlor

dithiopyr

Oryzalin

propyzamide

pendimethalin

prodiamine

indaziflam

oxadiazon

dimethenamid-P (Freehand+)

Couch

Safe to use+

OK to use these

OK++

Fine to use

QLD Blue couch

OK

OK

No

OK to use these

Buffalo

OK

OK

No

OK to use these

Bentgrass

Safe to use+

Not safe to use

OK

Not safe to use

Ryegrass

OK

OK

No

No

OK

Not safe to use

Tall fescue

OK

OK

No

No

Not safe to use

Zoysia

OK

OK

No

No

OK to uses these

Kikuyu

OK to use these

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 these all prune roots

No

Yes

safe to use+

Not golf greens

++

Not santa anna

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.

 

Some pre emergent herbicides can cause root clubbing

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.

FAQ

What pre emergent herbicide kills bindi?

Before you go putting down a pre emergent herbicide realise that the best way of stopping any weed is to have healthy dense turf. If you have this it will choke out competing weeds and so you don’t have to spray any chemicals. However in some cases you might need to apply a pre emergent herbicide.

My two go to pre emergents for bindii are an oxadiazon based product such as Echelon® or Echelon Duo® or for longer control products based on prodiamine such as Onset 10GR® if I want to use a granular option or Barricade® herbicide if I want to use a liquid. Both of these active ingredients give excellent control against this troublesome weed.

How does pre emergent herbicide work?

Withouit going into too much detail pre emergent herbicdies form a barrier a the surface and prevent weeds from establishing. They have no effect at all on weed seeds.

Does pre emergent herbicide kill grass?

If you follow the label the answer is a simply no.

Where to buy pre emergent herbicide near me?

Gilba solutions supplies into NSW, VIC, QLD and the ACT. For areas outside of these feel free to contact us and we are only too happy to point you in the right direction as to where to get product at the best price.

Are there any new pre emergent herbicides?

The newest pre emergent active ingredient is indaziflam sold as Specticle® which was introduced several years ago. However, the newest products on the market are both granular products and produced by Indigo Specialty. These are called Echelon Duo® and Onset 10GR® These are based on oxadiazon and prodiamine respectively.

Can I core after applying a pre-emergent herbicide?

We get asked this a lot. The general view is that aerating afterward is not a good idea. The thinking is that this will breakthrough the “barrier” and prevent it from working. However, there is turf research showing that this might not be an issue. Solid tining should be OK but there might be issues with hollow tine aeration due to the removal of soil.
 
Pre emergent herbicide labels don’t recommend aerating, but research has not shown any reduction in summer grass control.
 
  • After an applying oxadiazon, core aeration did not affect crowsfoot control.
  • Monroe et. al., also showed that aeration did not affect the control of summer grass.
 
As a general rule though if the site requires coring then you should carry it out.