Every year, customers ask me which pre-emergent herbicide they should use in the spring, and my answer is always, “Well, it depends.”

There are several options, but it usually comes down to two: dithiopyr (Dimension) and prodiamine. Both are great products depending on when and how they’re used, so let’s talk about them in-depth.

What Do Dithiopyr and Prodiamine Do?

Pre-emergent herbicides don’t actually stop weed seeds from germinating. Instead, as a seed germinates, the baby seedling comes into contact with the pre-emergent chemical layer in the soil and absorbs it. The chemical stops cell division in the plant, either root or shoot, preventing it from reaching the soil surface and sunlight. As a result, it dies.

This is why they are called pre-emergents. They kill the seedling prior to emerging from the soil, but they don’t kill the seed. Over time, pre-emergents are broken down by microbes and environmental factors, so they must be reapplied annually (and sometimes semi-annually to extend control).

A Short History Lesson

Years ago, the first pre-emergent applied to granular fertilizer was pendimethalin. This active ingredient was a huge victory for the lawn care world because it opened the door for many other granular combination products to be created. This made it possible for many new businesses to offer lawn fertilization because they no longer needed liquid application equipment to get the job done. Yet, we all know about the issues with pendimethalin. Many of my customers still have yellow floors, pavement, equipment, and toilet seats from pendimethalin. It stains!

Dithiopyr was brought on the scene by Dow, which is now Corteva. The new chemistry was applied to fertilizer, increased the length of control from weeks to months, and best of all, didn’t stain. It worked a little bit differently than pendimethalin, but accomplished the same goal.

Prodiamine is a more recent chemistry, and it works much like pendimethalin. It has a yellow color, but has much less of a staining effect and a much longer residual. It has, in my opinion, become the dominant pre-emergent product used today. Prodiamine has performed better and is more cost-effective than dithiopyr south of the Mason-Dixon line. Plus, there are many post-patent alternatives now available, which has brought the cost of this chemistry down significantly.

So, Which One Is Better?

Since I sell and monitor the performance of both, I think I can help. If you plan on going out with a pre-emergent first thing in the spring, I recommend prodiamine because it does not leach with heavy rainfall the way dithiopyr does. We can have some pretty wet springs in my geography (Cleveland, OH), and prodiamine generally outperforms dithiopyr when pre-emergent applications are made in March.

Some of my customers opt to apply pre-emergent later in the spring, often because that timing is close to the crabgrass germination window, or they have late sign-ups that they still want to treat. In those cases, there is less risk of pre-emergent leaching, so dithiopyr may be the better choice. Dimension is more soluble, so it will set up in the soil faster than prodiamine. It also has some early post-emergent control if you are lucky enough to hit crabgrass at the right time.

In some cases, I have my customers mix the liquid version (Dimension 2EW) in with their weed control, since they are usually spraying entire properties at that time anyway. I have always felt that the time frame in which my customers start spraying for broadleaf weed control is the perfect time to start applying Dimension.

Let’s Talk Rates

Some of my customers have a strong preference for one product over another, and that’s fine as long as you achieve proper timing and use an appropriate rate.

In my region, I typically recommend a rate of 0.65-0.73 pounds of active ingredient per acre of prodiamine, or 0.25-0.38 pounds of active ingredient per acre of dithiopyr. You should always refer to the label and talk with your local ATS representative first because each region is different.

The research I have read suggests that there is no benefit from split applications of prodiamine, so if you are a one-and-done kind of person, either product can be applied as long as you apply the right amount of either product.

If you’re a Dimension lover, you can split your rate over two applications in the spring, or apply your first application as a granular and follow up with a liquid application mixed with your weed control in mid-to-late spring. Some have even gone with a full rate of Dimension on the first round and reinforced with more on a second round. Personally, I feel the cost of applying more outweighs the benefit because breakthrough can be caused by many factors (more on that in the next paragraph). You are better off, in my opinion, to budget for the cost to treat a percentage of breakthrough each year on your total acreage in your pricing structure, say 3%, as an insurance policy to use when you need it.

Some Parting Thoughts

Back to breakthrough, there are many things that can cause it. If conditions are too dry to disperse the chemistry into the soil, if the ground cracks, if the turf isn’t mowed at the proper height, or if it is too wet in a certain year, all of these things can affect performance and are out of your control. Unfortunately, the customer is not usually as understanding, so make sure that your agronomic program is flexible. My customers do a second pre-emergent application to compensate for problem properties or adverse conditions, and it may be advisable for you, too. So, keep an open mind and be willing to make changes to your program when necessary and you and your customers should be trouble-free.

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