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How Pre-Emergents Work

Discussion in 'Pesticide & Herbicide Application' started by JimLewis, Apr 5, 2006.

  1. JimLewis

    JimLewis LawnSite Fanatic
    Posts: 6,839

    I was talking to a guy in the landscape / horticulture field the other day. And we were discussing Pre-emergents. He was dead set against them (in favor of organic gardening, and no chemicals) and I was for them, having had very favorable experience with Pre-emergents like Cassaron and Snapshot. (Yes, I understand that Cassaron is a post-emergent too! Get off my back!)

    So as we were talking he said, matter of factly, "Pre-emergents are really a waste anyway. Because as soon as anyone sets one foot into the area where you've applied your pre-emergent, they've totally negated it's effectiveness."

    I'm not sure that's true. It seems to me that Pre-emergents work by changing something in the ground so that seeds won't germinate when they land in that area. Seems to me that once the ground is "sterilized" like this (for lack of a better term) then it should stay as such, regardless of whether someone walks there or not.

    But does anyone know for sure? Whose right? Him or me?
  2. olive123

    olive123 LawnSite Senior Member
    Posts: 500

    the labels state not to disturb the treated areas i think they need watering in the soil layer to be truly effective. usually people who give answers without explanation especially ones who think they are above others intelligence (the way some types in the hort field are) are telling half truths or fishing with their answers.
  3. Grassmechanic

    Grassmechanic LawnSite Silver Member
    Posts: 2,697

    You are right, Jim. The problem with most pro organic folks is that they base their beliefs on emotion rather than sound, scientific study. I've walked in beds several times over a course of a season that I've applied pre-emergents to and I've never had weed issues. The only ways a seed can germinate is if the barrier is physically destroyed, such as digging or cultivating or if the barrier is buried by mulch or topsoil. Simply walking will not destroy the barrier.
  4. walker-talker

    walker-talker LawnSite Platinum Member
    from Midwest
    Posts: 4,771

    I have personally not read or seen the recent studies, but I know they have mentioned here a lot, stating that even aeration does not break the pre-emergent barrier.
  5. GreenUtah

    GreenUtah LawnSite Senior Member
    from SLC, UT
    Posts: 866

    since pre-emergents form a "barrier" if you will and do not keep seeds from germinating, but merely kill tender seedlings that try to push through the barrier, it makes sense that disruption can lessen the effects of the pre-emergent. However, that would normally entail adding soil above the barrier, which would be through things like turning a bed in. This would give seeds a place above the barrier to root. In turf, I have mixed feelings about pre-emergent being a blanket use in every program. On well established, thick lawns that are properly maintained, it's a waste of an expensive chem and an addition of an unnecessary herbicide into the environment you are trying to create for optimum growth. Finding the balance in your programs to accomodate the properly maintained, established turfs with the rest of the usual is the tough part.
  6. qps

    qps LawnSite Bronze Member
    from Indy
    Posts: 1,484

    I have attended some CCH classes and professors have data that backs up the aeration has no effect on crabgrass control...would I want to poke holes in my crabgrass barrier....no....but they say it doesn't have an effect....also some pre-emergents can be applied in fall...for next season control...data backs that up also...but again...do I want to do pre-m treatments in Oct...No......
  7. ThreeWide

    ThreeWide LawnSite Bronze Member
    Posts: 1,116

    This is due to the fact that preemergent herbicides will persist in the soil when there is no microbial activity.

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