In research at Colorado State University, researchers measured the seasonal changes in the nutrition of leafy vegetables. The specific class of nutrients evaluated were the phenolics in vegetables such as arugula, amaranth, and Pac Choi.
First Study Results
Other than the amaranth, the levels of phenolics were higher in mid-season and late-season crops than the earlier crops.
Leafy green vegetables that were planted later and harvested later had more phenolics (as much as seven times higher) than earlier crops.
In the second and separate study at Kansas State University, researchers measured concentrations of similar phytochemicals between vegetables grown outdoors versus those grown in grow–tunnels.
Yields were higher indoors but nutrient levels were lower indoors than outside. Researchers thought lower light levels were the culprit here as higher light was one of the few variables that differed.
The second part of this study evaluated the effect of fertilizer on the phenolic levels and there was a negative correlation between chemical fertilizers and phenolic levels. The highest concentration of phenolic levels in the conventionally grown plots was with the crops that were not fertilized. In the organic growing sections, there was no negative effect of organic fertilizers on phenolic levels.
Bottom line: those mid-to-late season vegetables are both healthier and easier to grow than the very early crops. And, it would seem organic production produces a plant with higher nutritional value.