Pre-Emergent (Prodiamine 65WDG) Not Working

Question: Hello,

I put down Prodiamine 65WDG at a rate of 1.5 lb/acre before Spring (2/23 this year), and it seems every year, I still have a lot of weeds (attached are the ones that are particularly aggressive this year - deadnettle I think)  which I then have to deal with. Can you suggest reasons why it may not be working? Or am I just expecting to much from the pre-emergent? How soon do you have to put down preemergent before a weed or grass germinate so I can't grow? If a seed has already started germinating, and I put down the pre-emergent, will the pre-emergent stop it from continuing to germinate?

Sorry for all the questions. I guess I'm really just trying to fully understand how pre-emergent works. Do you have any good resources you can point me to?

Thanks!
Eddie

responder: Answered by the Home and Garden Information Center (HGIC), University of Maryland Extension. For timely gardening information and tips, subscribe to our Maryland Grows blog and quarterly newsletter. Follow us on Facebook, Instagram, and YouTube. Replied April 04, 2023, 1:42 PM EDT
response: Hi Eddie,  Pre-emergent herbicides work by preventing germination or by killing newly-germinated seeds. They have no impact on seedlings already beyond a certain stage of development (fairly soon after germination in most cases) unless the herbicide product is formulated to contain an additional post-emergent herbicide ingredient. The label for Prodiamine 65WDG indicates it is a pre-emergent: "controls susceptible weeds by preventing growth and development of newly germinated weed seeds." Our Herbicide Options web page goes into the basics of different herbicide categories. We are in the process of generating more web content about pesticide selection and use, though the focus will primarily be on insecticides and fungicides since they are more widely used with regularity. Our Weeds collection of web pages can also help you to choose what type of chemical approach may be best when dealing with specific weeds.  Weeds that are up and blooming now are in many cases "winter annuals" which means they germinate in autumn, overwinter, and mature and bloom (and produce their own seed) in spring before the summer weather kills them. Therefore, treatment needs to be made before germination takes place, which means application(s) are made the autumn prior. How long an application lasts (called a "residual") before it degrades and becomes ineffective will vary from product to product and can also vary with environmental conditions (temperature, moisture). Even though winter annual weeds aren't always evident in fall or winter, and even though a few scattered individuals might not sprout when expected and would germinate late, the targeted time to prevent establishment of winter annual weeds is still autumn. (Early, such as around August-September as a generalization.)  Annuals complete their life cycle in one year, and will die after going to seed regardless of whether you attempt treatment or not. Summer annuals germinate in the warming temperatures of spring and die over the winter. Winter annuals germinate in the cooling temperatures of fall and die the following summer. Perennials are plants that live for more than one year, and unless you have an issue with perennial weed seeds, a pre-emergent typically won't be effective against perennial weeds since they are largely just re-emerging from dormant established root systems each spring. Biennials are an in-between category that live about two years before dying since they only produce blooms and seeds in their second (and final) year of life, but we don't have many weeds that are biennial compared to plain annuals or perennials.  The rate you applied appears to be the upper limit for an annual dosage based on label details, but you can look closely at the product instructions to be sure. When a range is given, the dosage will depend on the type of lawn it's applied to plus potentially also what weeds it's intended to control. Given the time of year you put this down, we don't expect it will be able to prevent much in the way of Deadnettle, Henbit, Veronica, Bittercress, or similar winter-annual weed growth. The weed in your photo is indeed Purple Deadnettle, a close relative of Henbit. Henbit is on the Prodiamine 65WDG label, but Deadnettle is not. This does not necessarily mean it hasn't been tested on that particular weed, but such label lists are not always exhaustive since the manufacturer likely just chose a representative group of commonly-encountered species. As you may notice on the label for this product, the subscript instructions under Henbit mention that applications need to be made in late summer or fall. Even though it also lists winter in the application timing note, this is probably too late (after the seeds have germinated) based on the weather conditions in our region.  One final note about pre-emergent products: they form this chemical barrier to germination at the soil surface, which is naturally where the weed seeds also need to be in order to sprout. Any disturbance to this layer, such as from roto-tilling a garden bed or core-aerating a lawn, moves some of the chemical down deeper into the soil where it won't be effective. Therefore, once a pre-emergent application is made you should not disturb the soil surface. Many such chemicals also need to be watered-in (via rain or irrigation) by a certain amount of moisture in order to become "activated", so take note of any such instructions in the product label directions in case rainfall isn't forecast and you'll need to irrigate the lawn where applications were made.  Miri

responder: The Question Asker Replied April 04, 2023, 5:28 PM EDT
response: Hi Miri! Thank you for another detailed explanation! Just what I was looking for. And the link was very informative as well.  Out of curiosity, how does the "chemical barrier" work? Do the chemicals inhibit the seed from germinating? Or when it germinates, it soaks in the chemicals and immediately kills the plant?  Can a plant germinate under the chemical barrier and grow through it?  Thanks!

responder: Answered by the Home and Garden Information Center (HGIC), University of Maryland Extension. For timely gardening information and tips, subscribe to our Maryland Grows blog and quarterly newsletter. Follow us on Facebook, Instagram, and YouTube. Replied April 05, 2023, 2:43 PM EDT
response: As a generalization (since the exact mode of action of a herbicide can vary from one ingredient to another), the barrier forms as a layer of chemical-infused soil on the surface of the ground. As the seed root emerges and passes through this layer, or as the seed coat splits and absorbs moisture (such as chemical-infused rainwater), the exposure to the chemical affects normal growth processes and the plant dies, either quickly or gradually. Some herbicides interfere with chlorophyll production, alter the production of plant growth hormones (causing excessive and deformed growth which depletes the plant's energy reserves, or which cause stunting and poorly-formed stems/foliage), affect the transport of sugars or other compounds between the foliage and roots, and so on. You can explore Oklahoma State University's page about herbicide modes of action on the linked web page, though the technical details of this complex activity aren't important in this case...it's just a handy example of how many different groups of herbicide impacts there are. Weed that develop genetic resistance to one category of herbicide may be resistant to every chemical within that mode of action category, because they're all impacting the plant in the same way, even if the active ingredients themselves are chemically different. Some garden and agricultural weeds have developed resistance to certain herbicides (especially if over-used over long periods), but we don't have a compiled list of which those are, and it likely varies regionally anyway.  If the seedling weed matures too much before it encounters the pre-emergent barrier, whether it starts growth below or above the application zone, then yes, presumably it won't be affected as much if at all. This is why soil disturbance after application is not recommended (and would be stated on the product's directions label) or else there is a risk the application will be poorly effective or entirely ineffective since it jumbles the zone of chemical germination inhibition and allows some weed growth to escape exposure. As with any pesticide application, over time the ingredients degrade (such as from exposure to sunlight, moisture, soil microbes, or simple breakdown over time regardless of environmental conditions) to the point where they become inactive. While this means future weed seeds colonizing the former treatment area are now free to germinate, it's a good thing the residual of the application is finite so that there is less risk of environmental pollution and negative impacts on other organisms as soil moves around in wind and rain events.  Miri

responder: The Question Asker Replied April 06, 2023, 8:20 PM EDT
response: That's exactly the info I was looking for. Thanks Miri! You're the best!

responder: Answered by the Home and Garden Information Center (HGIC), University of Maryland Extension. For timely gardening information and tips, subscribe to our Maryland Grows blog and quarterly newsletter. Follow us on Facebook, Instagram, and YouTube. Replied April 07, 2023, 8:58 AM EDT
response: You're welcome!