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I recently purchased a home and would like to plant some trees in the backyard (lemon, oranges, etc). However, I have read that certain weed killers which people put on their grass cause cancer. I was wondering, how can I check if any of those weed killers have been put on the grass in my backyard previously (where I plan to plant the trees)?

Thanks, Mike

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    I believe you're hunting a snark here. The main "weedkiller linkage to cancer" (non-Hodgkins lymphoma) is from direct exposure to glyphosate (which kills everything that's not genetically engineered to resist it, and as far as I'm aware GMO lawn grass isn't a thing yet.) Even if that was used on the yard in the past, it would not make your fruit trees cancer-causing in the future. So I'd ask what your sources for this information are. There's a lot of incorrect information running around the web.
    – Ecnerwal
    May 15, 2022 at 15:01
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    Of ,if there is residual weed killer in the soil ,your trees won't grow. My son got some good looking soil from raised beds that were being removed from an assisted living apartments. He put it in his beds and had trouble growing some vegetables. He found out the maintenance people flooded the soil with weed killer since they were not used. After a year , vegetables were growing well. In the meantime ,he ate what did grow and did not die.
    – blacksmith37
    May 15, 2022 at 16:08
  • @blacksmith37 - Sounds like there was some leftover pre-emergent in that soil (probably Preen). That would leach out over a year or two.
    – Jurp
    May 15, 2022 at 22:23
  • @Ecnerwal You should make this an answer, since it's totally accurate. I do remember, though, that some brainiac was working on Roundup Ready Kentucky Bluegrass, which would be an environmental disaster if it were ever released. BTW, glyphosate does NOT kill mature common violets or mature Campanula rapunculoides (creeping bellflower) - I know, I've tried! It's also pretty ineffective against Queen Anne's Lace (probably due to the heavily dissected leaves) and common mullein (probably because of the wooly leaves). It'll set every one of them back, but won't kill them.
    – Jurp
    May 15, 2022 at 22:25

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When I was working for a hydroponics place their test was to take a sample of the soil and see if lettuce seeds would germinate. A good sample would include multiple locations and depth. You don't need to grow it to the stage when you can crop it. If they will germinate you are good to go because they are highly sensitive to contaminants. References http://ei.cornell.edu/toxicology/bioassays/lettuce/

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    I bought lettuce seeds and am going to try this. Thank you.
    – Dancing Quark
    May 23, 2022 at 0:23
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One guideline would be to look at what's required for organic certification which is in part 36 months with no prohibited substances (In the USA, anyway.) You don't need to find out if you just let it age away (non-organic lawn, not toxic waste dump.)

Assuming you don't have an excess of residual chemicals that would kill your trees, you could go ahead and plant them when ready, but "err on the side of caution" and not eat any fruit (indeed, just pick the flowers once the bees are done with them, or the fruit as soon as it forms and compost it, so the tree spends more energy on being a strong tree) until after the 36 month point (rather than wait 3 years to even plant them.)

Depending on the state of the soils, you might want to take a year of those three and plant various cover crops to till in, or make other improvements to the ground (hauling in organic materials or other soil amendments) before planting your trees.

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