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Organic Gardening

Weed Control With Preen

18 years ago

Does anyone know how Preen works? Does its action prevent germination of vegetable seeds, for instance? and does it interfere with the operation of any soil microbes?

I was asked this in a talk on organic gardening and, being totally organic, I was at a loss for an sensible answer. I said I wouldn't use it, but this was unfair, based - as it was - on total ignorance. Regards, Peter.

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Comments (3)

JAYK
18 years ago
The active ingredient in Preen works by affecting seedling root growth. It doesn't stop germination, but does keep the seedling from establishing. It would prevent germination of vegetable seeds as well as weed seeds.

A search for trifluralin will provide you with information about its properties. A few starting places below.

Extoxnet link:
http://pmep.cce.cornell.edu/profiles/extoxnet/pyrethrins-ziram/trifluralin-ext.html

Here is a link that might be useful: Trifluralin information-Infoventures
Kimmsr
18 years ago
From the Extoxnet link,
"In August, 1979, trifluralin was brought under Special Review by the EPA because of the presence of a N-nitrosamine contaminant which had been shown to cause tumors and to have mutagenic effects in animals. The principle manufacturer of trifluralin had already instituted manufacturing methods to reduce N-nitrosamine contaminant levels. The Special Review was concluded in 1982, with the requirement that N-nitrosamine contaminant levels in trifluralin not exceed 0.5 ppm, a level which EPA believes will have no toxic effects (12, 13)."
There is a requirement that even at the 0.5 ppm level there be on the label a cautionary statement. And that should be reason enough that this is not a good rpoduct for an organic gardener to use.
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You can buy corn gluten meal at feed stores. It's so disappointing that people are still using so many fungicides, herbicides and insecticides in their gardens when there are alternatives that don't impact the health of animals, insects and people. Believing claims that these things are not harmful to the earth is like believing tobacco companies who swore up and down for decades smoking wouldn't hurt you. STOP SPRAYING CHEMICALS IN YOUR YARD!! This may be unpopular, but I think those of us who are able to have a nice garden without killing/harming other living things have to speak up and say it's just not right. I also don't buy into the argument that it's easier for those of us with small yards to manage pests and weeds. I've taken care of an eight hectare property where no spraying was done -- and it was beautiful!! The impact these pollutants have spread far beyond your yard and collectively have devastating consequences.
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In areas where I don't encourage self-seeding of valuable plants, I use mulch to both control weeds and feed the soil. Preen does nothing for soil quality, obviously, has to keep being applied (the cost is not insignificant) and last I checked there was some question about its possible deleterious effect on amphibians (i.e. toads, which I encourage in the garden to keep down pests). As to whether there might be an unknown long-term health effect on humans from using a broad-spectrum herbicide, that's one more thing I don't have to think about by not using Preen.
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Steve, the challenge with this situation is that your pre-emergent needs to reach the soil surface at a certain rate of application. The chemical needs to be watered in properly in order that the solids can liquify, enter the soil, and form a temperary barrier against germinating seeds. Applying the granules over the juniper and hoping that it gets to the soil in a relatively even layer is not the best use of Preen. There are other options, but I would first need to know what kind of weeds you're battling with. For example, I have a dense growth of beautiful Parsoni juniper in our front beds. Few weeds are able to infiltrate but we always end up with a crop of nutgrass and a few broad leaf weeds. We are able to spray a mist of herbicide that kills grassy weeds but we pull the broadleaf weeds. Preen (or similar) is not an option.
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Let's be accurate in this reporting: "EPA considers trifluralin to be a possible human carcinogen (11). This classification is used when there is limited or uncertain information indicating that a chemical may cause cancer in animals receiving high doses of the chemical over the course of their lifetimes. In a 2-year study of rats fed 325 mg/kg/day, the highest dose tested, malignant tumors developed in the kidneys, bladder and thyroid (7, 12). Because there is a possible increase in the risk of cancer to humans, EPA's Lifetime Health Advisory level for trifluralin drinking water includes an additional safety margin " Since humans generally don't eat Preen, and are even less likely to do so in high doses, the chances of this chemical actually causing cancer are rather low. And ALL kinds of substances - not just pesticides - can cause cancer or be overly toxic if ingested or exposed to in high enough doses.
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henry_kuska
18 years ago
Pesticide science is a rapidly changing area. I feel that to quote studies that are 5 or 10 years old as the "last word" can be very risky - health wise. The link below gives a recent discussion of Preen.

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According to the links below, trifluralin is currently banned in Denmark, Norway, and Sweden.

Http://www.pesticideinfo.org/Ref_Reg.jsp?Rec_Id=PC35146&Country=Denmark&Key_CountryChem=DenmarkPC35146

Http://www.pesticideinfo.org/Ref_Reg.jsp?Rec_Id=PC35146&Country=Norway&Key_CountryChem=NorwayPC35146

Http://www.pesticideinfo.org/Ref_Reg.jsp?Rec_Id=PC35146&Country=Sweden&Key_CountryChem=SwedenPC35146

Here is a link that might be useful: link for recent discussion

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