Should You Mix Pre Emergent with Post Emergent?
It is definitely possible and sometimes even desirable, to mix together pre emergent and post emergent herbicides for the best weed control. Many home gardeners encounter this issue when trying to eliminate weeds from around their homes or in their gardens, and it’s a valid question to consider. Each one of them has its specific uses, and sometimes, when combined, they can achieve the best possible results.
The big difference between the two types of weed killers is that they kill off weeds in different stages of their life cycles. In this article, we’ll discuss exactly what parts of those life cycles each herbicide is most effective at. First, we’ll need to describe what each one does and why it should be used. Then, we can discuss situations where you might want to combine them for maximum effectiveness.
What is pre-emergent herbicide?
According to Gardening Vibe, The best time to apply pre-emergent besides is right before any seeds have had the chance to germinate, so as to prevent those weeds from maturing into full-fledged plants.
Pre-emergent herbicides are used to control summer annual and winter annual weeds. However, they have very little effect on weeds that are perennial. To keep control of summer annual weeds like crabgrass, you would have to apply pre-emergent herbicide to the ground around two weeks prior to the time that seeds begin germination. Usually this occurs early in the springtime, when the soil has a chance to reach at least 55 degrees for a stretch of several days.
To control weeds that are winter annuals such as henbit, chickweed, and poa annua, the best time to apply an herbicide is late summer or early fall. This should be about the time when the soil temperature has dropped to the neighborhood of 70 degrees. The way a pre-emergent herbicide works is by instilling a chemical barrier right in the soil itself.
Then, when the weeds attempt to germinate, all developing shoots and roots will come into direct contact with the herbicide. The chemical associated with the herbicide inhibits cell division and all enzyme function, causing the seedling to die before it has a chance to emerge through the topsoil. This is also where the herbicide gets its name from.
What is post-emergent herbicide?
Post-emergent herbicides are used to control weeds which have already developed and are showing above ground. Depending on the particular herbicides you choose, you’ll be able to kill off grassy weeds, annual broadleaf weeds, and perennial weeds as well. There are many different types of post-emergent mixtures you can use, including contact herbicides and systemic herbicides.
Herbicides classified as contact herbicides will begin killing weeds as soon as they are sprayed on them. However, they don’t actually kill off the root system, and this means the weeds will have a chance to regrow. That will require you to reapply your herbicide at some point when the weeds grow back. Systemic herbicides operate in a slightly different way, by killing off weeds from the inside.
Foliage on the weeds absorbs the herbicide and actually transports it all throughout the plant, right down to the root system. Because systemic herbicides are capable of killing off the weeds’ roots, they can completely prevent the weed from re-growing. In some cases, with very stubborn weeds, it may be necessary to repeat an application, but a systemic herbicide can definitely kill off a weed and its entire root system eventually.
Post-emergent herbicides may have one drawback, which is the fact that they are unable to prevent weed seeds from germinating and then developing into full-blown plants.
Apply both herbicides at the same time
You can save yourself a lot of time and effort by applying both pre emergent and post emergent herbicides at the same time. As long as you apply the pre-emergent at the appropriate time of year, it will be very effective at killing off most types of weeds. Mixing in post-emergent in the same container will allow you to kill off all weeds which have begun growing above the ground in your yard.
As one example, when you apply your pre-emergent early in the springtime to prevent those summer annual weeds, you can include a post-emergent that will also kill off any winter annual weeds which might be growing in the treated section of your lawn. That will prevent them from spreading their seeds, so that you control them all in one fell swoop.
In order to control certain varieties of weeds, it may be necessary to reapply a pre-emergent several months after the initial application. At this time, including a post-emergent will also kill off any weeds that have begun to sprout and develop. There is one catch to all this, however. In most cases, the best time to apply a pre-emergent is usually not the best time to use a post-emergent herbicide with it.
Depending in the section of the country that you live in, there might also be some restrictions on how frequently you can use some specific post-emergents. Therefore, if you plan to use an herbicide in early spring, before summer annual weeds emerge from the ground, you may be limited in the applications you can use later in the year.
When you do apply your pre-emergent herbicide, it has to be activated by water, so the chemical barrier will form throughout the layers of your topsoil. However, this can be a little tricky because watering will have the effect of diluting your post-emergent herbicide, and it will run right off the weeds.
Since post-emergent requires a certain amount of time to work, it’s very possible that your post-emergent herbicide will be totally ineffective, because it has run off the plant before it has a chance to work. This being the case, it’s generally a good idea to wait at least 24 hours before watering a pre-emergent and post-emergent mix. Check the label of your post-emergent weed killer to make sure it doesn’t become rainfast very quickly, because this might cause you to alter your strategy.
You’ll also have the opportunity to purchase an herbicide that is both pre-emergent and post-emergent, and this will save you the trouble of having to create your own mixture. However, you’ll have to be careful about this as well, since not all mixed herbicides are safe for all types of turf grass.
According to Green Care Turf Management, the label on the mixed herbicide will tell you whether or not it’s safe for use on your type of lawn. When you do apply a mixed herbicide, you should wait a few days after mowing before actually applying your mixture.
Weeds in the ground will absorb post-emergent through their leaves. That means the more surface area that’s available, the better results you’ll achieve. Also, it would be a good idea to wait several days after spraying before you mow the lawn again, so that your systemic weed killers will have an opportunity to do their work.
Frequently asked questions
Q: How are pre-emergent herbicides applied?
A: The best way to apply a pre-emergent is with a sprayer, but you should wait until your weather forecast calls for at least one dry day and maybe more, so your herbicide doesn’t get washed away. In some cases, your pre-emergent will be sold pre-mixed, and in other cases, you’ll have to add water before spraying your lawn.
Q: How should post-emergents be applied?
A: A post-emergent herbicide should be applied when you have at least eight hours of dry weather, so it can dry on the weeds, and be absorbed. Make sure to apply when the air temperature is between 55 and 80 degrees Fahrenheit, and then water a couple of days later. Avoid applying your post-emergent on windy days, because it could spread to desirable plants, or it could be inhaled and cause respiratory issues.
Q: How much will it cost if I have a professional apply my weed killer?
A: On average, it will cost you somewhere between $50 and $100 to have a professional apply the herbicide for you, whereas if you do the work yourself, your only cost will be the herbicide itself. However, having a pro manage the application for you will eliminate the need for any mixing or any guesswork on using the right herbicide for your lawn.
Q: When is the best time to apply a mixed herbicide?
A: A mixed herbicide should be applied early in springtime, when you would normally apply a pre-emergent. It will still be necessary to apply your herbicide at a time before germination begins, and that requires an early springtime application. If you apply at any other time, you will probably only be getting the benefit of the post-emergent herbicide in the mixture.
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