In this blog we cover: What is a pre-emergent herbicide, How do pre-emergents work, How to use pre-emergent herbicides, When to apply pre-emergent herbicides and What is the best pre emergent herbicide.

What is a pre-emergent herbicide?

A pre-emergent herbicide prevents weeds from becoming established. As their name implies, the best time to use pre-emergent herbicides is before weeds appear.

A post-emergent herbicide like the sulfonylurea, Recondo herbicide, 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 will give poor results.

In the above image, ryegrass is affected by the pre-emergent diflufenican whilst still active in the soil. This is an example of when not to apply a pre-emergent herbicide

Our recent turf blogs cover factors effecting herbicide performance,  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. Instead, they prevent germinated weed seedlings from establishing by forming a barrier at the soil surface. Hence, you need to apply pre-emergents before weeds germinate. Most pre-emergents act on weed roots except for oxadiazon, which is the only shoot active pre-emergent for use in turf. Oxadiazon is in Echelon® and Echelon Duo®. 

To work, you must wash the herbicide into the soil. It must also be present when the weed seeds are germinating, as you need soil moisture to allow these to work. 

Once inside plants, movement varies. Some pre-emergent herbicides move in the xylem, and some through the phloem. Others are contacts that affect the entry point. How well a pre-emergent herbicide works, is effected by the formulation, rainfall, temperature, timing, and soil type.

Pendimethalin.

The pre-emergent herbicide pendimethalin works on root tips and is a “root pruner”. It needs to be watered into the rootzone to work well.

Prodiamine.

As a pre-emergent herbicide, prodiamine works when you water it into the root zone. It is a “root pruner”, and little moves within the plant.

Oxadiazon.

Oxadiazon, works in a unique way as a pre-emergent herbicide. It acts on shoots as they grow through the treated soil.

Dithiopyr.

Dithiopyr is a mitotic inhibitor, and even though it is a pre-emergent herbicide, it also works as an early post-emergent herbicide.

Metolachlor

Metolachlor, works as a pre-emergent herbicide, mainly for annual grass and broadleaf weeds. It enters through leaves and shoots meaning you can apply it in lower water volumes than many other pre-emergent herbicides.

Dimethenamid-p

Dimethenamid-p is a pre-emergent herbicide, controls a number of broadleaf and grass weeds for 6-8 weeks. How this pre-emergent herbicide works is by inhibiting Photosystem II, part of the photosynthesis pathway and controls annual grasses, annual sedges, spurge, oxalis, doveweed, willowherb and many others. It also suppreses yellow nutgrass. It does not control purple nutsedge, evening primrose, morning glory, or pearlwort.

 

Are pre emergents safe?

Turf chemicals can be dangerous to humans, animals and the environment if you don’t follow the label. Therefore, when you use turf chemicals, follow the manufacturer’s directions. Remember, as you’re the one using them, it means you are liable. Having said that, if you follow the label, these products are safe to use.

The term acute toxicity is the amount of active required to kill 50% of a population. Acute toxicity is expressed, as LD50 (lethal dose 50). The lower the LD50, the greater its toxicity to humans and animals. Chemicals with a high LD50 are the least toxic. For example, caffeine has an LD50 of 200mg/kg. In contrast, the LD50 of prodiamine, the active in Barricade herbicide, is >5000mg/kg. The table below shows the LD50 of common pre-emergents.

 
 
 
 
 

When to apply pre-emergent herbicides.

Most of the time, the best time to apply pre-emergents is before weeds germinate. Early spring and autumn are the best times to apply pre-emergent herbicides. As a rule, if you can see weeds your too late, so its worth closely checking the turf canopy. Often weeds are present but are hidden due to close mowing. The aim with these, is to create a barrier to weeds before they germinate but there are exceptions.

The best example of this is Dimension herbicide. Dimension herbicide gives great pre and post emergent weed control of a range of grasses and broadleaf weeds in turf.

 
 
 

Pre emergent turf herbicide mode of action table

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-emergents.

The turf condition is the main factor that impacts when to apply pre-emergent herbicides. Do not use pre-emergents on thin turf under stress, with a weak root system or if you are going to seed. If you are plan to seed or renovate, then think carefully before 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 you use it, and finally
  • The application rate.

It is also important to know the weed history of a site as this helps in your choice of 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, always remember that pre-emergents don’t all behave the same!

 

Factors affecting the successful use of pre-emergents:

 

Volatilization.

Some pre-emergents are prone to volatilization. This means that if they remain on the soil surface, they turn into a gas and evaporate. Evaporation is greatest under windy, hot temperatures. The table below shows how likely common pre-emergents are to volatilize. Those at the top are more likely to volatilize in comparison to those at the bottom. 

You have to water in pre-emergents with high vapour pressure like Pennmag® as soon as you apply them, because as soon as the spray dries losses begin. On the other hand, products with a low vapour pressure like Specticle® do not need immediate watering and can be left for up to 21 days.

In brief, labels with a water-in period are the time after which losses may start to become too high. However, losses will occur before that time.

The best pre-emergent herbicide to use if water is an issue: Envu Specticle.

 

Vapour pressure for common pre-emergent 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

The persistence of pre-emergents.

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

DT50* values for 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

Water.

The uptake of most pre-emergents is via the roots which means there has to be good soil moisture present. If the soil is too dry, there will be limited uptake, and the product will fail. Watering also causes weed seeds to germinate which is important to ensure that pre-emergents work.

The table below shows the solubility of turf pre-emergents. Pre-emergent solubility does not mean how easily the product dissolves. Instead it is the highest concentration that can dissolve in a volume of water. For instance, some chemicals easily dissolve but have low solubility. Others are difficult to dissolve but have a high solubility.

Herbicides with low solubility need larger water volumes to wash them into the soil and good soil moisture for the length of control. This is in contrast to high solubility pre-emergents that are easier to water in.

 

Solubility of common pre-emergent 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 these behave in the soil. Clay and high organic matter soils will “tie up” more chemicals than sandy soils. You then need higher rates to give the same level of control. These higher rates account for the fact 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) measures how much a chemical binds to soils. Chemicals that have a high Koc, tend to bind tightly to soil and organic matter, for example, pendimethalin. Low Koc herbicides, for example, metolachlor, are less likely to bind to clay or high organic matter soils and are likely to be mobile.  

Ranking of pre-emergent binding to organic matter.

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

The best pre-emergent herbicides to use in high organic matter soils: Pennmag herbicide or Meteor herbicide (metalochlor)

 

Average adsorption coefficients for common pre-emergent 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-emergents.

Breakdown in soils is a result of microbes 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 heavy 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 big impact on any results, and in our opinion, this is the main reason for product failures. So, let’s clear something up. If you can see weeds, most of the time, it will be a waste of money to apply 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

 

Crowsfoot germinates at a higher temperature than summer grass and tends to germinate 2-8 weeks later than summergrass in the spring. Knotweed germinates even earlier. So if you plan to make a pre-emergent crowsfoot application, a smart move is to make this around 2- weeks after you first see signs of summer grass.

However, some products have a wider application window, such as Dimension® herbicide. Dimension controls summer grass both before it germinates and in the early post-emergent stage. So this means you can apply later and still get control. Another example is Specticle® herbicide, which has pre and post-emergent activity against winter grass, although work needs to discover the effects of plant size on the results. The image below, shows the effects of Specticle® against winter grass in couch fairways.

Best pre-emergent herbicide offering the most flexible timing window: Dimension herbicide

Dimension pre-emergent herbicide offers a larger window for application thus optimizing your pre-emergent herbicide application.
Specticle pre and post effect on winter grass

Photodegradation

Photodegradation is the breakdown of chemicals due to the action of sunlight on 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:

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

Make sure you calibrate your equipment.

Correct calibration is vital if you use a granular or liquid pre-emergent herbicide. Fail to do this, and it means you will make an uneven application and you’ll get poor results.

Choosing pre-emergents on price.

Buying something on price alone 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. The low cost option might not even work on your target weed;
  • Secondly, what is the true cost of the application? 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 if you buy the cheapest product on the market and it’s going to kill your turf;
  • Next, when you make a purchase, did you buy 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 for example, you want to seed later.

If the actives are the same, look at how easy it is 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, we will discuss the pros and cons of both of these.

 

Use of pre-emergents; Generic vs branded?

A generic product performs as well as a branded product. If you don’t believe us, check out the labels for generic and branded products.

However, metolachlor is an exception to this rule. There are two registered products. These are Meteor® (Amgrow) and Pennmag® (Syngenta). Even though they have the same amount of active ingredients, 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 metolachlor, it creates a blend of four isomers. These all have the same chemical formula but have different makeup.

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 content 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, 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 metolachlor products. 

 

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

Timing use for spring applications

​Microbes are least active in the spring due to cool soil temperatures. So you can apply pre-emergent herbicides earlier than you might think and still get great results. The reason for this is there is a low level of microbial activity that breaks down these herbicides in the spring. This means that they last longer.

Summer grass 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. So, in regions like Canberra, the Southern Highlands and Melbourne, you can use pre-emergents in the autumn at a high rate and get season-long control of summer grass, into the next year. This can be followed by a split application in the spring.

 

 

​Delay weed resistance and change the chemistry

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

Mix pre and post-emergent herbicides to delay resistance

A mix of pre and post-emergent works better than simply rotating chemistry for slowing down the development of resistance. This approach also:

  • Gives 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 rainfall that can leach and breakdown herbicides;

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 simultaneously. For instance, Echelon® Duo contains a pre-emergent, 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. This is a consideration on when to apply any pre-emergent herbicide and
  • Third, liquids move into the soil a lot quicker.

 With granules, uniformity is affected by both the granule size and uniformity. The equipment you use also has an 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 you get good coverage even at low rates.

Best granular pre-emergent herbicide for coverage: Onset 10GR

 

 

Influence of granule size on summer grass 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

 

Use pre-emergents safely.

Knowing how pre-emergent herbicides work is important for turf safety. Turf safety and when you can seed will vary and depend on the pre-emergent you use. Factors like when you seed are a factor to consider when you can apply pre-emergent herbicides. If the chemical is still active, any seeding work will fail. So ensure that you carry out any seeding after the time period listed on the label. Plan ahead and be aware of any future use as this effects when you can apply pre-emergents..

 

Pre-emergent herbicide turf safety

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 your turf is under stress or it has a poor root system avoid the use of these. They will only send your turf backwards.

 

Some pre emergent herbicides can cause root clubbing

Root pruning occurs as a result of the chemical and the turf type.

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

  • Specticle® did not give a 50% couch cover by the end of the trial;
  • Only Ronstar® didn’t 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 aren’t safe on new couchgrass, and you should avoid their use 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 afterwards is not a good idea. The thinking is that this will break through 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 soil removal.

Although pre-emergent herbicide labels don’t recommend aerating, research has not shown any reduction in summer grass control.

  • After 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, however, if the site requires coring, then carry it out.

References

  • 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
jerry spencer senior turf agronomist
Senior Turf Agronomist at Gilba Solutions Pty Ltd | 0499975819 | Website

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

He followed this by gaining a Grad Dip in Business Management from UTS. He has worked in a number of management 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 as an independent sports turf consultancy 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.