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What is a pre-emergent herbicide?

A pre-emergent herbicide prevents weeds from gaining a foothold. A post emergent herbicide i.e. sulfonylureas kills weeds when they are already present. The key to using pre-emergents is knowing how they work and behave in the soil. Failing to do this this will lead to poor results.

In the above image, ryegrass is being affected by the pre-emergent diflufenican which is still active in the soil. Our recent turf blog covers split applications of pre-emergents and reasons for pre-emergent failure


How do pre-emergent herbicides work?

Pre-emergents have zero effect on weed seeds. They act by preventing germinated weed seedlings from becoming established by forming a barrier at the soil surface. The majority of pre-emergents act on weed roots. In contrast, oxadiazon is the only shoot active pre-emergent for use in turf and this is the active in Echelon® and Echelon Duo®. 

To work properly you must wash the herbicide into the soil. It must also be present when the weed seeds are germinating so soil moisture is vital to allow these to work. 


Are pre emergents safe?

All turf 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.  Remember, that as you’re the one using them it means that you are liable. Having said that if you follow the instructions these products are safe to use.
The term acute toxicity means the amount of active ingredient required to kill 50% of a test population. This is expressed as LD50 (lethal dose 50). The lower the LD50, the greater its toxicity to humans and animals. whereas 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 emergents.

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

Chemical Group

Active constituent (common trade name)

LD 50 Oral


Inhibition of microtubule assembly


propyzamide (Proforce Checkpoint®*500SC)


Benzoic acids

chlorthal-dimethyl (Chlorthal-dimethyl 900®)


Dinitroanilines: (DNAs)

pendimethalin (Proforce Battalia®)


prodiamine (Barricade® herbicide)



dithiopyr (Dimension®)



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


diflufenican (Warhead Trio®*)



Inhibition of protoporphyrinogen oxidase (PPO inhibitors)


oxadiazon (Echelon®, Oxafert®)



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


dimethenamid (Freehand®*)


S-metalochlor (Pennmag®)



ethofumesate (Tramat®)



Inhibition of cellulose biosynthesis


indaziflam (Specticle®)


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 seeding. If you are going to seed 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, how well it binds to soil particles;
  • Third, the weather conditions when it is applied, and finally
  • The application rate.

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

The following is a summary of these factors. After all, a
lways remember that pre-emergents don’t all behave the same!

Factors affecting the successful use of pre-emergent herbicides:



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. Volatilization is greatest under windy, hot temperatures. The table below shows how likely common pre-emergents are to volatilize with those at the top being more likely to volatilize in contrast to those at the bottom. 
Pre emergents with a high vapour pressure like Pennmag® need to be immediately watered in 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 can 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 often losses will occur before that time.

Table showing the vapour pressure of common pre-emergents.

s-metolachlor (Pennmag)


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

pendimethalin (Battalia)


dimethenamid-P (Freehand)


oxadiazon (Ronstar/Echelon)


terbuthylazine (Numchuk Quad)


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

propyzamide (Rustler)


Prodiamine (Barricade/Onset 10GR)


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 like the soil type, the environmental conditions, and the application rate.   

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

DT50 Value

dimethenamid-P (Freehand)


s-metolachlor (Pennmag)


terbuthylazine (Numchuk Quad)


0 to 30 Non persistent

dithiopyr (Dimension)


Oryzalin (Prolan)


30 to 100 Moderately persistent

propyzamide (Rustler)


pendimethalin (Battalia)


diflufenican (Warhead)


prodiamine (Dimension/Onset 10GR)


 >100 Persistent

indaziflam (Specticle)


oxadiazon (Ronstar/Echelon)


Very persistent

Not Watering in the Product.

Uptake of most pre-emergents is via the roots. As a result, there has to be good soil moisture present as if the soil is too dry there will be at best limited uptake and product will fail. Watering in also causes weed seeds to germinate which is important in making sure that these chemicals work.
The table below shows the solubility of turf pre-emergents. 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 whilst others are difficult to dissolve but have a high solubility.
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 pre-emergent turf herbicides.

diflufenican (Warhead)


Prodiamine (Barricade/Onset 10GR)


pendimethalin (Battalia)


oxadiazon (Ronstar/Echelon)


Oryzalin (Prolan)


Dithiopyr (Dimension)


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

Indaziflam (Specticle)


terbuthylazine (Numchuk Quad)


propyzamide (Rustler)


s-metolachlor (Pennmag)


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

dimethenamid-P (Freehand)


Soil properties.

Organic matter and soil texture affect how these behave in the soil. Clay and high organic matter soils will “tie up” more chemical than for example sandy soils. Higher rates are then needed to give the same level of control as these higher rates take into account that less is available for weeds to take up. 
Reasons for increases in movement out of a rootzone:
  • High sand content soils have little ability to hold onto any chemicals;
  • In low organic matter soils less herbicide is likely to become “tied up”;
  • Physical movement occurs on slopes and areas of thin turf cover where water movement occurs.

The sorption coefficient (Koc) is a measure of how much a chemical binds to soils. Chemicals that have a high Koc tend to bind tightly to soil and organic matter i.e. pendimethalin. 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)


Koc > 4000

Likely to bind tightly to soil and organic matter

Prodiamine (Barricade/Onset 10GR)



oxadiazon (Ronstar/Echelon)


diflufenican (Warhead)


indaziflam (Specticle)


Koc 500 to 4000 slightly mobile

oryzalin (Prolan)


propyzamide (Checkpoint)


dithiopyr (Dimension)


terbuthylazine (Numchuk Quad)


Koc 75 to 500 moderately mobile

dimethenamid-P (Freehand)


s-metolachlor (Pennmag)


Koc 15 to 75 Mobile

Chemical Breakdown of pre-emergent herbicides.

Breakdown in soils 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 this is quicker than in dry soils;
  • Thirdly, adequate soil oxygen and nutrients will speed up this decay, and finally
  • A neutral soil pH.


Herbicide Soil Texture and Organic Matter Soil pH Soil 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 factor Soluble and may leach in sandy soils after 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 factor Soluble, but less soluble than Dimethenamid-p and so 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 product 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.

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

Weed  Temperature °C
 winter grass/ annual bluegrass 21
Crowsfoot 16-18
Summergrass 12-15
Knotweed 2-10


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 this around 2 weeks after you see summergrass.

There are some 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 get good control. Another example is Specticle® herbicide which has pre and post-emergent activity against winter grass although work needs to be done to discover the effects of plant size on the results. The image below is of a Specticle® application giving great 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


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 you do not water them in after application.

Getting the best results:

The following are some general tips to give you a better chance of getting great results:

Make sure your equipment is properly calibrated.

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

Choosing pre-emergents on price.

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

  • Firstly what weeds are you trying to control? Different products work better on different weeds and the cheapest option might not even work on your target weed;
  • Secondly what is the true cost of application? You should consider factors such as rate, length of control and bag size;
  • Thirdly what is your turf type? Not all of these are safe on all turf types. There isn’t much point in buying the cheapest product on the market if its going to kill your turf;
  • Next when you make a purchase are you buying it from a supplier who can give you good agronomic advice on how to get the best results?
  • Lastly how long do you want it to last? Longer control isn’t always the best if your example you want to seed later etc.

If the actives 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. 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. Granules 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 granules 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 uniformity is effected by both the granule uniformity and size. The 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/ gram Oxadiazon Prodiamine
23 58 43
58 67 71
165 72 74
465 72 81
1,310 70 82
3,728 66 83
10,606 68 83
LSD (0.05) 7 7


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 having the same amount of active there is evidence that they don’t perform the same.



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

Timing their use for spring applications

Microbial activity is at its lowest in the spring due to low cool soil temperatures that exist. This means that ideally applying pre-emergent herbicides earlier than you would generally think will still give excellent results. The reason for this is due to the low level of microbial activity that breaks down these herbicides.

As discussed previously summergrass germinates when soil temperatures at a depth of 100mm are between 12-15°C whilst crowsfoot germinates later when soil temperatures are between 16-18°C. This means that in regions such as Canberra, the Southern Highlands and Melbourne that these can be applied in the autumn at the high rate and will provide season long control of summergrass the next year. This is especially the case if this is followed by a split-application in the spring. 

Mixing pre and post emergent herbicides to delay resistance

A mix of both a pre and a post emergent works better than simply rotating chemistry for slowing down the development of 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;
  • Increases application flexibility and allows for weather extremes such as :
    • Heavy rainfall that can leach and breakdown herbicides;



Delay weed resistance by changing the chemistry

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

Mixing pre and post emergent herbicides to delay resistance

A mix of both a pre and a post emergent works better than simply rotating chemistry for slowing down the development of 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;
  • Increases application flexibility and allows for weather extremes such as :
    • Heavy rainfall that can leach and breakdown herbicides;

Using pre-emergents safely.

Turf safety and seeding interval vary dependant on which pre emergent you use. If the chemical is still active in the soil, any seeding work will fail, so make sure that you carry out any seeding work after the time period listed 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









dimethenamid-P (Freehand+)


Safe to use+

OK to use these


Fine to use

QLD Blue couch




OK to use these





OK to use these


Safe to use+

Not safe to use


Not safe to use







Not safe to use

Tall fescue





Not safe to use






OK to uses these


OK to use these

Reseeding period


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 within 1 day

6mm within 7 days

3-6mm within 21 days

Irrigate within 2 days.


Root pruning

Yes these all prune roots



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 your turf is under stressor it already has a poor root system avoid the use of these as they will only send your 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.


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 research1. Branham, B. E. and P. E. Rieke. 1986. Effects of turf cultivation practices on the efficacy of preemergence grass herbicides. Agronomy Journal 78:1089- 1091. 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.
Although pre-emergent herbicide labels don’t recommend aerating, research has not shown any reduction in summer grass control when this is carried out.
  • 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 grass2Monroe, J. H., W. M. Lewis. J. M. DiPaola. 1990. Aerification effects on preemergence herbicide activity. Weed Science Society of America Abstracts 30:27.
As a general rule though if the site requires coring then you should carry it out.


  • 1
    . Branham, B. E. and P. E. Rieke. 1986. Effects of turf cultivation practices on the efficacy of preemergence grass herbicides. Agronomy Journal 78:1089- 1091.
  • 2
    Monroe, J. H., W. M. Lewis. J. M. DiPaola. 1990. Aerification effects on preemergence herbicide activity. Weed Science Society of America Abstracts 30:27
Senior Turf Agronomist at Gilba Solutions Pty Ltd | Website | + posts

After Graduating from Newcastle University with an Hons Degree in Soil Science in 1988, Jerry then worked for the Sports Turf Research Institute (STRI) before emigrating to Australia in 1993.

He followed this by gaining a Grad Dip in Business Managment from UTS and has worked in a number of managment roles for companies as diverse as Samsung Australia, Arthur Yates and Paton Fertilizers.

He has always had a strong affinity with the Australian sports turf industry and as a result he established Gilba Solutions in 1993. Jerry has written over 100 articles and two books on a wide range of topics such as Turf Pesticides and Nutrition which have been published in Australia and overseas.