Better nutrition from in-season vegetables
Japanese studies find vitamin content higher in many vegetables grown in-season, compared with their off-season counterparts.

A number of Japanese studies have found that many vegetables grown in season contain more vitamin A, C and carotene than their off-season counterparts grown under plastic. Joel Gruver, from the Department of Soil Science at NC State University emailed us these findings, from Kagawa Nutrition University, Laboratory of Bio-organic Chemistry, 3-9-21 Chiyoda, Sakado-shi, Saitama, 350-0288 Japan:

Vitamin C
The content of vitamin C seems to be significantly influenced by the cropping season. In spinach, for example, the vitamin C content was reduced by 1/5th to 1/8th in off-season produce compared to its peak in in-season produce. It was also reduced in tomatoes and broccoli by up to 1/2 in off-season produce.

The content of carotene, a preceding substance of vitamin A, was also affected by the crop season. In off-season broccoli, it was sreduced by 1/4th, and in off-season carrots it was reduced by more than half, compared to peak in-season levels.

Vegetables not affected by off-season growing
The vitamin C and carotene content of some vegetables and fruits, such as sweet peppers, celery and kiwifruit, was fairly stable across seasons.