When is best to use pre-emergent and post-emergent chemcials, and can you mix the two chemicals together?
First, yes you definitely can mix pre- and post-emergent herbicides for a single application of both, and this is commonly recommended and even addressed on many herbicide product labels. The best example, perhaps, is Roundup, which has label directions for tank mixing with a wide variety of other herbicides. There may also be directions on some product labels that could prohibit mixing with other products, so it is very important always to read the full label before using any pesticides. They are not all going to say the same things, and if you miss some fine print on a label you could be using that product improperly, leading either to problems with your control program or even a citation by a regulatory inspector. This is not lecturing you specifically, but just general blabbing on my part.
When to use either a pre or a post emergent product will depend a lot on the weed you are dealing with, the time of year, and even the product. Many herbicides offer BOTH pre and post activity, and adding another herbicide to the mix would be redundant. A look at the Label, again, will tell you what activity that active ingredient gives against weeds. Some herbicides give only pre-emergent activity, and this may be fine if the ground you are treating is already bare and without any weed growth, but if weed foliage is already showing above the soil that strict pre-emergent may not have any effect on the existing weeds, and now the addition of a post-emergent is called for. Vice versa, if a product offers only post-emergent activity and you know there are abundant weed seeds in the soil, treating just the existing weeds would not stop new ones from sprouting immediately after.
A couple of examples. Roundup is 100% post-emergent. It has no activity in the soil, and degrades rapidly in the soil, so it is highly effective at killing any weeds that already exist in that place but will not affect seeds that have not yet germinated. For annual weeds along a roadside this may be adequate. You can wait until you know the majority of the seeds have sprouted and then apply Roundup, and you will get bare ground for quite awhile after those weeds die. On the other hand, Surflan is 100% pre-emergent, and this is excellent for applying to landscape beds or turf prior to the germination of weed seeds. As they begin to germinate the active ingredient kills the sprouting weed and prevents any growth from showing above ground. But, if weeds are already in place they will continue to grow. Combining the Roundup and Surflan then offers the best of both worlds.
Know your weeds, and our extensive Weed ID program on PestWeb should be helpful in identifying the weeds you deal with and offering hints on their seasonality and biology. A perennial weed could have foliage that dies in the winter but roots that survive, and for this you need a systemic post emergent that will kill those roots. It would have to be applied during the growing season to allow the plant to take up the active ingredient and move it to the roots. A pre-emergent used in the winter may not affect the hardy roots. Bermuda grass is a good example.
Your decision on what to use is also determined by the setting, and it really pays to know the capabilities of the herbicides. If you want absolutely bare ground you could use a product like diuron, which is capable of entering the roots of nearby trees and shrubs and killing them along with the weeds. Great for a roadside or railroad track, bad for a landscape. In the landscape you may prefer products like Ronstar, Surflan, or Pendulum, which are much easier on existing plants and can be used around desirable perennials. This is a pretty extensive topic, but hopefully this brief answer is helpful. I encourage you to take a couple of our Master Technician courses - numbers 603 and 604 cover in detail the weeds and the herbicides, and will offer a lot more information.
Mr. Pest Control
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Please note, Mr. Pest Control is answering questions supplied by PMP customers across North America. His answers are generated from industry and manufacturer-provided information. The answer may not be specific to the laws and regulations for your State, Province, Territory or Country. In addition, products mentioned may not be registered and or available in all areas. Always check with your local Univar office for specific information to your area. Always read and follow label directions.