Which pre-emergent Herbicide and at what rate to use on stiltgrass seed? - Ask Extension
I’d like to use a pre-emergent Herbicide to kill the Japanese Stiltgrass seed Before they have a chance to germinate, but at a strength that’s unl...
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Which pre-emergent Herbicide and at what rate to use on stiltgrass seed? #919185
Asked October 03, 2025, 2:54 PM EDT
I’d like to use a pre-emergent Herbicide to kill the Japanese Stiltgrass seed Before they have a chance to germinate, but at a strength that’s unlikely to prevent the germination of any of my native seeds, including the perennial graminoids, like my Virginia Rye.
Can you please advise on which one I should go with and at which strength? Thank you.
Montgomery County Maryland
Expert Response
We don't have information about dosage as it pertains to not affecting native grass seed germination. Japanese Stiltgrass germinates in spring (on average, about 2 weeks prior to Crabgrass germination), so a pre-emergent would need to be applied just prior to that expected time period. Montgomery County, however, prohibits the use of synthetic herbicides on lawns, unless you live in the sole area that has opted out of the law, Laytonsville.
The organic product corn gluten can function as a pre-emergent herbicide, but it's non-selective, meaning it will impact any seed trying to germinate in the treated area. When corn gluten breaks down, it becomes a fertilizer (nitrogen source).
Some products whose active ingredient is a form of iron can be herbicides, but studies are still ongoing as to how effective they are on Stiltgrass in particular. Early trials suggest it does not work very well, at least at the dosages the current product labeling allows. Iron in this form is somewhat selective, but mainly for broadleaf weeds versus grasses, not one type of grass among other grasses.
For native grasses whose seed germinates in the cooler season of early spring, chemically controlling Stiltgrass germination may be challenging, or not possible (organically or otherwise). If any of the native grasses germinate later, once soil temperatures have become much warmer, then you might be able to use one of the organic herbicides permitted by the county to eliminate those seedlings while sparing the native species. However, some individuals of Stiltgrass could germinate later (as can Crabgrass), which is often why synthetic herbicide pre-emergents can recommend making a second application about 6-8 weeks after the first, which may coincide with when desirable species are germinating.
A pesticide label's usage information is Federal law, so it must be followed as written by the manufacturer. If a product label lists different dosage options for different situations (such as targeting different weed species, or for use in different types of turfgrass when weeds are growing in a lawn), then you can adjust how much is applied based on those label parameters, but we can't recommend a dosage rate that is not specified by the label.
Miri
The organic product corn gluten can function as a pre-emergent herbicide, but it's non-selective, meaning it will impact any seed trying to germinate in the treated area. When corn gluten breaks down, it becomes a fertilizer (nitrogen source).
Some products whose active ingredient is a form of iron can be herbicides, but studies are still ongoing as to how effective they are on Stiltgrass in particular. Early trials suggest it does not work very well, at least at the dosages the current product labeling allows. Iron in this form is somewhat selective, but mainly for broadleaf weeds versus grasses, not one type of grass among other grasses.
For native grasses whose seed germinates in the cooler season of early spring, chemically controlling Stiltgrass germination may be challenging, or not possible (organically or otherwise). If any of the native grasses germinate later, once soil temperatures have become much warmer, then you might be able to use one of the organic herbicides permitted by the county to eliminate those seedlings while sparing the native species. However, some individuals of Stiltgrass could germinate later (as can Crabgrass), which is often why synthetic herbicide pre-emergents can recommend making a second application about 6-8 weeks after the first, which may coincide with when desirable species are germinating.
A pesticide label's usage information is Federal law, so it must be followed as written by the manufacturer. If a product label lists different dosage options for different situations (such as targeting different weed species, or for use in different types of turfgrass when weeds are growing in a lawn), then you can adjust how much is applied based on those label parameters, but we can't recommend a dosage rate that is not specified by the label.
Miri
