There is much more to properly maintaining a lawn than just mowing it. One of the greatest threats to a lawn’s health is the growth of weeds.

Weed control, like a medical condition, consists of two parts: prevention and cure. As is often the case with medical conditions, weed control frequently requires both because circumstances change and a single approach rarely suffices.

Prevention and Cure

In lawn care, we talk about pre-emergent and post-emergent treatments. Pre-emergent herbicides are the prevention; when applied correctly and at the right time, they keep weed seeds from taking hold and sprouting. Post-emergents are more like the cure because their use is for after weeds have begun to grow and spread.

Most likely, a lawn is going to need both types of treatment (we’ll talk about why in the next section). This article is going to focus on pre-emergent herbicides — what they are, what they do, and how to effectively use them.

What Pre-Emergent Herbicides Do

Pre-emergent herbicides are applied to the soil and are effective at preventing common weeds such as crabgrass and foxtail (and several others). They work before the weeds they target ever appear. If weeds are already present, that calls for a post-emergent treatment. It does not, however, mean that a pre-emergent treatment is useless because that treatment can still stop other weeds from taking hold.

There are some weeds, such as dandelions and clover, that pre-emergent herbicides cannot prevent. The effectiveness of pre-emergents can also vary according to timing and soil temperatures (more on timing and temperatures in the next section).

Pre-emergent herbicides can come in granular or liquid form. Either way, it is key to their effectiveness that they are applied and watered into the soil right away. The weed-fighting chemicals are inside the granules, and water activates them, so if they are not watered into the soil quickly, weeds can grow in the meantime and the treatment is ineffective. Likewise, liquid applications need to be watered in right away because grasses and other plants can otherwise trap the herbicide and keep it from ever reaching and penetrating the soil. So again, the treatment could prove to be completely ineffective.

While it’s tempting to compare pre-emergent pesticides to a vaccine, they don’t work quite the same way. Whereas a vaccine immunizes the recipient for several years or for a lifetime, pre-emergents require application multiple times per year every year. As already mentioned, their effectiveness can vary based on the conditions. However, applied properly, pre-emergents do reliably provide a great deal of prevention from outbreaks of troublesome weeds such as crabgrass.

Timing and Temperature Are Key

Although pre-emergent herbicides can work at any time of the year, they are most effective in early spring and fall. This is because those are the times of the year that many weeds like to germinate and because of the soil temperatures at those times.

Of course, calendar spring and fall can be very different from meteorological spring and fall. The growing season here in the Sioux City area is hardly the same as it is in the Florida panhandle. This is why understanding the prime soil-temperature windows is so important and why it is so important to go with a lawn-care provider who knows the local climate well and can accurately measure soil temperatures.

That window of ideal soil temperature is about 55-70 degrees (F). Crabgrass, for instance, thrives in that range, as do many other plants, and for that reason, it is essential to treat during those periods. Application at times when the soil is frozen or otherwise still too cold means the treatment will not penetrate the soil and thus will have no effect. When summer is fully on and the soil warms to well above 70, there is less growth taking place, and so a treatment is not doing as much.

That does not mean re-treatment should not take place at other times. Treatments can last from around 6 weeks to a few months depending on the ingredients and conditions, so re-application makes sense in all but the shortest growing seasons. However, if treatment does not begin at the right time, later attempts are largely playing catch-up and may not be effective at all, calling for post-emergent treatments instead.

Knowing the duration of the treatment is also critical because you do not want to reseed a lawn, for example, while the treatment is active.

Thus, we strive to treat in early spring when the soil begins to warm into its prime window, and then again in fall when the soil cools back down to 70 and lower and weeds once again experience their ideal growing conditions.

The Active Ingredients

The two most common ingredients in pre-emergent treatments are Dithiopyr and Prodiamine, which are available on their own and are also the active chemicals in products such as Barricade and Dimension, respectively. Both are effective at inhibiting the growth of crabgrass and other weeds during their germination windows, and both come in granular and liquid form.

Which one is better? Well, both are excellent products, and the answer really depends on timing, soil and grass type, and climate. It is therefore important to consult with a qualified lawn-care expert to determine which is the better choice for a lawn.

Summary

Pre-emergent herbicides are proven to be effective at preventing the growth of crabgrass and some other weeds when applied correctly during peak germination periods. They do not stop all weeds, and they do not work once weeds are already present, and so almost every lawn is thus going to require post-emergent treatments as well. Still, a pre-emergent treatment can spare a lot of ugliness and headaches, and Sharp Lawn Care is ready to help beautify and protect your lawn.

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