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Raised Bed Over Spot Where Weed Killer Sprayed Safe?

I am building my first raised bed. I am going 4’x12’ and 11” high. The area I decided on was spot sprayed with Round Up for Lawns about 2 weeks ago. It is the version that doesn’t kill grass, more like a weed and feed. The cust service rep on my first chat session at Scotts told me that I need to wait 1 year for edibles. When I called to double check that person said I was okay now. I removed the grass under my bed and want to do a mix of tomatoes, carrots, and lettuce to start. Some of the articles I read said that by law weed killer has to dissipate by a couple of days. Should I be concerned? Anything I should do on the bottom? Or should I go higher? Thanks in advance!

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ID jit
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Re: Raised Bed After Weed Killer

My bet is you will find a metric butt ton of contradicting information, ranging from "Yeah, go ahead and plant on top of it right now" to "you are going to burn in hell for using herbicide".

From what I understand, the active ingredient(s) in homeowner herbicides HAVE to be absorbed by the plants or become benign/dissipate in a very short period. I could be completely wrong here too.

Anyhow, me being a clueless idjit and a bit on the practical and reckless side... I would remove the sod and 4" - 6" of soil under the raised bed, then fill with clean, fresh mix and see what happens.

If you are hyper concerned about it, line the raised bed the with heavy duty plastic or a pond liner. use some 4" perforated drain pipe and landscape fabric and make the "raised bed" a sunken SIP.

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Re: Raised Bed After Weed Killer

Read the label. The label is the law. The new weed and feed is probably not glyphosate, where you can plant within a few days.
It actually says on the label not to compost grass clippings or use in vegetable gardens. It contains 4 different weed killers. You would have to know the persistence of those. The reason you can plant after using glyphosate is because it is absorbed by the plant leaves but it is tightly bound to soil and inactivated. Eventually glyphosate will be broken down by soil oganisms over a period of 6 months or so.

There is no law that I know of that says a weedkiller has to dissipate within a couple of days. Most are safe to for reentry when it has dried about 2 hours, but persistence depends on how it is bound and how fast it breaks down. There are different factors like pH, soil type, temperature, microbial activity, exposure to light, leaching that will change the rate that things break down. Also some herbicides have things added so they persist longer.

I would err on the side of waiting on filling that bed or moving it to a spot where it was not sprayed. Even though you removed the grass, the chemicals may still be bound in the soil. Eventually all chemicals have to be broken down by the soil organisms, we just don't know what their timetable is.

For future reference. It is easy to build over a lawn. All you would have to do was hand weed the nastiest ones and cover the area with thick cardboard and build your garden over that. if it is at least 8 inches deep most weed seeds will stay dormant.
While covering the ground with weed block or plastic sounds like a good idea, it can cause drainage problems if you build a garden over that and if you have deep rooted plants like tomatoes, they actually will grown down below the raised bed into the native soil if it can. ... ll_CFL.pdf ... es-in-soil

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Re: Raised Bed After Weed Killer

There IS NO law that says herbicides have to dissipate after a few days. In general the US compared to other countries has very lax laws about poisons and hardly any enforcement.

Weed n' feed is probably NOT RoundUp/glyphosate. It is a combination of a synthetic fertilizer, 2,4-D, and mecoprop-p.

Here's a thread where I collected a bunch of information about commonly used garden poisons: viewtopic.php?f=11&t=57653

Here's part of what I found about 2,4-D:
Consumption of plants treated with 2,4-D has killed horses and cattle. It can be toxic to fish and aquatic organisms if it leaches in to rivers. 2,4-D is slightly toxic to wildfowl (mallards, pheasants, quail and pigeons) and moderately toxic to other birds. In birds it has been shown to cause birth defects and reduce hatching success. It also indirectly affects birds by destroying habitat and food sources. Moderate doses of 2,4-D severely impaired honeybees brood production, reducing offspring numbers. It kills predatory (beneficial) beetles and ladybug larvae, resulting in an increase in aphid infestations. 2,4 dichlorophenol, a breakdown product of 2,4-D is extremely toxic to earthworms, 15 times more toxic than 2,4-D itself.

Fate in the Environment: 2,4-D is not very persistent in the soil, with a half-life in the soil of 16 days or less. 2,4-D has been included on the EPA list of compounds that are likely to leach from soil. Despite its short half-life in soil and in aquatic environments, the compound has been detected in groundwater supplies in at least five States and in Canada. It has also been detected in surface waters throughout the United States at very low concentrations.
Note that half life of 16 days means that in that time period the amount of 2,4-D has been reduced by half. In another 16 days it will have reduced by half again, so 1/4 of the product will be left. AND it doesn't mean it is gone. It means it is no longer identifiable as 2,4-D, because it has broken down into component parts. But as noted, sometimes the breakdown products can be worse than the original poison. 2,4-D has been banned in Australia, Canada, Europe and other places...

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