One way to meet these limits is to harvest the salad crops late in the day when tissue nitrate concentrations are at their lowest. This recommendation is based on evidence that has been gained from crops grown in controlled environments.
In the present experiments, 'Cracoviensis' lettuce and 'Giant Winter' spinach were grown in open fields in Washington State (46 degsN) and Alaska (64 degsN). Both crops were seeded directly into the soil on 13 May in Washington, but in Alaska, they did not go into the open field until 9 June.
The available nitrogen in the soil was similar at the two locations. Samples were taken from the crops at two-hour intervals throughout three separate days.
These showed that the total tissue nitrate concentration usually did fall during day-time and was lowest at the end of the day, but the fall was not large at either location.
Lettuce tended to have a higher tissue nitrate concentration than spinach. Overall, the nitrate concentration in these field-grown crops was usually below the limits set for edible plant tissues grown in the summer.
Diurnal Fluctuation in Tissue Nitrate Concentration of Field-Grown Leafy Greens at Two Latitudes by Neely, Koenig, Miles, Koenig, and Karlsson (2010). HortScience 45 (12): 1815-1818. ISHS members can view HortScience from the website www.ishs.org.