Is there benefit to applying a granular pre emergent to landscaping beds? If...

Asked September 3, 2013, 10:41 AM EDT

Is there benefit to applying a granular pre emergent to landscaping beds? If so, when would be the best time to apply?

Dane County Wisconsin

3 Responses

Pre Emergents are usually mixed with fertilizers and spring time applications are oftentimes recommended as a routine task in lawn care. I am assuming that your question is referring to lawns - as opposed to gardens.

Here is a lengthy list of common weeds and recommended chemical formulations and application times for each.

I hope this answers your question. Thanks for contacting AaE - please do so again if you have further questions.

Thank you very much for the response. However, we are looking for suggestions of how to control winter annuals and biannuals in landscape beds; mulch beds, perennial beds, shrub beds, rock beds and tree rings. The weed pressures this year have been a little too much for our nerves and we would like to get in front of the issues for 2014.

We are wondering if there is benefit to applying pre emergents in fall without fertilizers to help us out?

If it would be of benefit what would the ideal timing be?

We are looking into the following products;
1) Oxadiazon
2) Flumioxazin
3) Prodiamine with Isoxaben
4) Dimethenamid with Pendimethalin

As good earth stewards we do not want to apply anything that will be of limited to no benefit to the environment.

Thank you again for your assistance.

Oh, I'm sorry for mis-interpreting your question!

Some of the same rules apply, however. To be really effective, pre emergents must be applied Before the plant starts growing. I suspect that many of the plants you are hoping to control have already done so, but you could still do a trial run, and consider 2013 your test year.
Here are some general guidelines that were written for the state of Oklahoma. Furthermore the author is referring mainly to lawns, but I'm sure you can extrapolate the pertinent information:

Here are the basics to a successful pre-emergent program:--Timing. Most pre-emergence herbicides will not control weeds that have germinated prior to application. Therefore, try to apply these herbicides several weeks before germination. If they are applied too soon before germination, the herbicide may lose its effectiveness. For the fall a good rule of thumb is to apply a pre-emergen the third week of August to mid-September. Specific dates for applications of pre-emergence herbicides are difficult to give due to varying environmental conditions for each location and year.To ensure the pre-emergence herbicide of getting into the soil where weed seed is located, remove excessive layers (thicker than 0.5 inch) of thatch, and also remove debris such as leaves and grass clipping before you apply the herbicide.--Amount. Always read the label and apply the recommended amount on your lawn. Check the label to see that the herbicide is safe for use on your lawn.--Coverage. Achieve a complete, uniform coverage by dividing the recommended amount of granular herbicide into two equal portions and spreading each in opposite directions. For adequate coverage, make spray applications at approximately three quarts per 1,000 feet.--Activation. Water the pre-emergence herbicide area if 0.5 inch of rain does not occur within 24 to 48 hours following application. All pre-emergence herbicides are soil applied and must be "washed" into the soil where weed seeds are located.--Second application. A second application may be required for season-long control. This will depend on the particular herbicide and environmental conditions, but pre-emergence herbicides generally remain effective for 60 to 110 days.
If you are especially concerned about environmental impact you could use corn meal. Here is a quote from a University of Minnesota publication: Corn gluten meal may be used safely in gardens around established perennial plants, to keep weed seeds from sprouting. It can also be used after transplants have been in the soil long enough to have "taken hold." Follow label recommendations for application rates.Wait until seedlings are up and growing well before applying corn gluten meal to flower or vegetable gardens where seeds were sown directly into the soil. CGM will stop most seeds from germinating. Potential problems with corn gluten meal stem from the fact that extensive moisture and microbial soil activity can reduce its effectiveness. You can control how much you water right after applying CGM, but you can't control rainfall. Sometimes seeds that had been prevented from forming roots can "outgrow" the problem.Another potential drawback is price and availability. While it is becoming more readily available, corn gluten meal is still significantly more costly compared to other pre-emergence herbicides. Check places that sell garden supplies, farm stores or county co-ops that sell seed and grain for hobby farms, and some stores that specialize in selling food for wild birds.

To respond to your questions about particular chemicals, it is beyond the scope of AaE to provide this level of detail. I will refer you to an excellent publication from Penn State extension which discusses most, if not all of the commercially available herbicide formulations used in landscape care (Please note the links in this article).
I also recommend that you go to the manufacturer's web site for each of the herbicide formulations that you are considering. You will find information that may go beyond what the label on the product states.