If I spray some grass with a poison like Round-up or Kills All, how long before that spray is not dangerous to plants?
So for example, if after spraying, I walk through the sprayed area and then onto my lawn, how long until I am not going to be leaving "foot prints of death"?
Although the label might say it's ok to plant again after 2-3 days, do note that different products have different time frames and you should read the label for your product carefully.
Even then, you shouldn't trust manufacturer's claims. This article cites a 1993 EPA study showing that glyphosate (Roundup) half-life can persist in agricultural soil for over four months. (Scroll down to figure 6 and the section titled "Persistence and Movement in Soil".) And that's just the half-life! Glyphosate does not break down quickly.
Note that Monsanto has been in trouble in both New York and France for false advertising regarding Roundup biodegrading in soil and exaggerated safety claims.
Glyphosate breaks down in normal soil within a few days, or even quicker.
It can be rapidly bound to soil particles and be inactivated, unbound glyphosate can be degraded by bacteria.
So if your soil is very poor and does not have a good level of active bacteria then the glyphosate may remain active for a lot longer. (Many agricultural soils have few active bacteria as they have not seen any real compost for the last 30 years!)
Also, newly spouted seeds are a lot more sensitive than established plants. So you can kill off the weeds with glyphosate and safely plant out pre-grown plants once the weeds start to die. But if you try to sow seeds and hit the weeks with glyphosate just before the seed germinates you may get more issues.
I expect that a lot of the time when people claim issues from repeated glyphosate usage, it is more down to having “dead” soil that has not seen any compost for a long time with little plants left growing it in, therefore few of the bacteria and fungus that plants need.
Other weed killers are designed to remain active for many months.
Often more than one weed killer will be mixed when sold under a brand name.
