Any clue what kinds of plants were in the grass?? There's not a whole lot that grows in lawn-type places that's poisonous in small amounts. If it was a grown-up, weedy place (what they call 'waste ground') then I might be a little more worried.
You don't have to be real specific, but what part of the country and about what latitude are you at??
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"Alas, our dried voices when we whisper together are quiet and meaningless, as wind in dry grass, or rats' feet over broken glass in our dry cellar." --TS Eliot, "The Hollow Men"