Answer: Nitrate accumulation is minimized by varietal selection, fertilizer management, and timing of harvest. Nitrate concentration in vegetables typically increases with rate of nitrogen fertilization. Generally, it is best to avoid fertilizing with both pre-plant and broadcasting fertilizer, especially those with high amounts of soluble nitrogen. A more stable nitrogen source, such as compost, is recommended.
Washington State University has produced a PowerPoint presentation on this topic, titled "Leafy Greens as a Winter Crop in Washington State." It is available at http://csanr.wsu.edu/BIOAg/symposia/2007/Ott%2007.pdf. The take-away message from this presentation is that the amount of accumulation varies from each variety but, in general, Asian greens tend to have more nitrate accumulation. We suggest looking at the varieties list and grow the ones that demonstrate lower accumulations. For example, Mizuna and Pac Choy had high accumulations, while arugula had very low accumulations.
Before you plant next winter, consider getting a soil test to determine the soil nitrogen level. Also, the longer the greens are in the ground, the lower the accumulation, so you should wait at least 11-12 weeks before harvesting.
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