Nature's Landscape Services

When to apply pre-emergent herbicide to prevent crabgrass and other lawn weeds is a question often asked.    Proper timing is indeed critical, as applying pre-emergent either too early or too late is likely to result in a weed problem.

A common practice is to apply pre-emergent with a single application prior to germination of crabgrass and other lawn weeds.  This is an effective means of applying pre-emergent, but minimizes margin of error in regard to the timing itself.  For if application is made too early, the efficacy of the pre-emergent may diminish while weed pressure still exist.  On the other hand, if application is made too late, there will be nothing in place to prevent weeds. 

Though a single application isn’t what I recommend, if I were to apply pre-emergent with a single application in the Atlanta area, I would target March 1 – March 15 as the window of time for it to be made.

There is a superior means of achieving weed control, in my opinion.  To lengthen duration of crabgrass control with pre-emergent herbicide application, and to ensure that treatment is made on time, the pre-emergent dosage can be “split”.  In splitting the application, half of the dosage is applied well before risk of germination, and the other half is scheduled to occur when the efficacy of the first is expected to be weakened.  The result is both insurance against early germination, and an extended window of control.

The timing of the first application should be February – early March, with the second occurring 6-8 weeks later.

Remember, the objective isn’t to put out more herbicide overall, but rather to split the intended dosage in half to gain a longer window of control with the same amount of product.

There are environmental benefits to using the split application method, as a lengthened window of control may result in less use of “post-emergent” herbicides.

Make sure to read the entire label of any pre-emergent used, and use common sense.

America is experiencing a rapid paradigm shift on sustainability and the environment.  “Going Green” is one of the most popular trends in our society today.  Most people love the idea of “Going Green”, but are unsure of how they can contribute.  Many “Green” initiatives involve a cash expense above and beyond normal.  For instance, a recycling bin is an additional cost above a regular trash can.  A hybrid automobile cost more than a traditional one, and “Organic” groceries are typically more expensive than non-organic.   So where does that leave us?  Well, as it turns out, many of you lawn enthusiast are “Going Green” and you didn’t even know it.  According to The Lawn Institute, “If one compares a well managed lawn to a poorly managed lawn or unmanaged grasslands, the net carbon intake of a well managed lawn is five to seven times higher than the carbon output of mowing.”  What does this mean?  It means that managed “Green” spaces are good!  Even though your lawn mower may produce carbon while mowing, your lawn intakes up to 5 times that amount, resulting in a net reduction of carbon.  So whenever “Going Green” comes up in a discussion, be sure to inform your friends of your environmental stewardship in your own back yard!  Learn more about the benefits of turf grass and managed “Green” spaces at www.lawninstitute.com

Fortunately, the drought is over, and though we are blessed to have flowing streams and full reservoirs again, the abundant rain has given rise to other concerns. 

One concern stemming from the abundant rain is that pre-emergent herbicides, which exist as a thin layer on the surface of the soil and prevent emergence of lawn weeds, have been deteriorated.  Deterioration of the pre-emergent layer will result in increased likelihood of lawn weeds, and may be most common in areas that do not drain well and in areas that experience a “flow” from channeled rain water. 

Areas where soil is exposed, either due to “thinning” of lawn or due to low cut height, are also prone to a deteriorated pre-emergent layer.  Reason being, there is no lawn canopy in place to cushion the impact of rain (or anything else that may disturb soil surface), and therefore the pre-emergent herbicide layer is deteriorated.  Also, soil is exposed to sunlight thereby stimulating lawn weed germination.  The above is one reason that maintaining a cut height of two inches, during winter months, should be practiced.

The lawn weed expected to be most problematic, is Annual Bluegrass (Poa annua), as it is a weed that has demonstrated resistance to pre-emergent herbicides even when weed control conditions are ideal.  The scientific community is spending a great deal of time and money in efforts to develop products that will reliably control Annual Bluegrass, but still have a ways to go.

 Read more about Annual Bluegrass (Poa annua) at https://aggie-horticulture.tamu.edu/plantanswers/turf/publications/weed16.html