The climate in the far north of Europe made cultivation of cereals even more difficult, and the wheat so popular in the south was a luxury in northern Europe. Wheat could cost at least twice as much as the most common grains, barley and rye. Barley was the most widely grown, but a great percentage of the harvest went into the extensive beer production, while rye was the principal bread grain. Like in the rest of Europe, oats was considered only fit as animal fodder and was only eaten when no other choice was available, and then usually as porridge or gruel. Though grains were in the highest regard among medieval Scandinavians and were among the most frequently mentioned foods, other vegetable foods were an indispensable part of the diet: peas, turnips, beans, carrots, onions, leeks and various greens and herbs all provided essential nutrients and vitamins. Kale, a form of cabbage, was especially important in Denmark and Sweden and was a valuable source of fresh food in the winter months, since it kept well and could be harvested (and even tasted better) after the first frost had set in.