About the example of grass that you talk about, I think that amounts to a criticism not of empiricism but direct hypothesis checking. We have built up a model of human body and grass which tells us that it would be harmful. Now this background model has been subject to empirical checks at lots of times and has evolved in presence of new empirical discoveries. (Though of course, the model has not been derived axiomatically from empirical data).

Almost nothing in science is justified by direct checks. Think of any clinical trial or say, the LHC experiments. There are so many implicitly assumed background theories about how the instruments respond to their stimuli. Some of these instruments are based on principles which were discovered only in this century. So scientists from previous times wouldn't accept the way the results are derived unless they are shown the results derived from previous experiments justifying the way the instruments work. For example, the atomic hypothesis was controversial till 1900 or so. How would one even interpret LHC electron beams if one one doesn't believe in the atomic hypothesis? A useful analogy here, is minesweeper - a new opened square, whose information, via a very long chain can tell us if there is a mine on a far away square.