How to use pre-emergent herbicides. 11

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?

When not to use pre-emergents.

Factors affecting the successful use of pre-emergents:

Volatilization.

The persistence of pre-emergents.

Water.

Soil properties.

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

Chemical Breakdown of pre-emergents.

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.

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

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. 

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?

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

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..

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.

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.