I am running a game of dungeons and dragons in which there are some orcs living in a realatively infertile cold northern hilly area a couple hundred miles inland, in the foothills of a mountain range to the north. Altitude is negotiable. Some players are going around killing these orcs, but I'm intending for them to figure out that killing people because they have green skin isn't a very nice thing to do. One of the players is a big fan of kale, so I had an idea that orc children need large supplies of kale (or some similar nutritious leafy green) when growing up or else they never become green and are yellow and weaker (they've seen yellow orcs but I haven't explained them). Humans stopped selling them leafy greens and other food about 10 years ago, and since then they've been raiding human lands, which are steadily encroaching on their territory, which is why the army hired the players to fight them. Eventually, the players are hopefully going to get the hints that the orcs aren't as bad as they're made out to be, and I want them to go on a quest to obtain a variety of greens that the orcs can grow in the land they have left to them.
So looking at nutritious dark leafy greens of the kale/spinach/chard/etc variety, are there any that would be particularly good at growing in the orc environment (50+ degrees latitude, hilly inland)? I also need a variety that isn't good at growing in that environment, which is what they used to obtain from the humans. One of the two should be kale, depending on whether or not kale is good at growing in an environment like that. Any suggestions as to what kinds of greens I can use?