Though you can find kale in supermarkets year-round these days, this leafy vegetable really comes into season in late summer. And, as anyone who's gone to make salad or a smoothie only to realize their kale has wilted and gone limp will know, kale has a fairly short shelf-life even though it is the considered the hardiest of the leafy greens. So how do you prevent that sad, soggy fate? By learning how to store raw kale properly, because that's how you make your kale last as long as possible.
The good news is that kale has been bred to withstand cold fairly well. It's native to the Mediterranean, writes Harold McGee in On Food and Cooking: The Science and Lore of the Kitchen, with "thick, succulent, waxy leaves and stalks that help make these plants so hardy." So you can, and should, store your fresh kale in the refrigerator. The first step is to separate the leaves from each other; you shouldn't put the entire bunch of kale that you picked up from the store in the fridge.
Unlike other salad greens that wilt quickly after the first exposure to excess moisture, kale's sturdiness means it can be washed before being stored. In her book Salad for President: A Cookbook Inspired by Artists, Julia Sherman recommends washing greens in a salad spinner full of ice water. "Drain and spin," she writes, then "To store the greens, wrap them in a loose bundle with a damp paper towel or a thin cotton kitchen towel, place them in a large freezer bag, seal and refrigerate them in the crisper drawer."
And that's all you have to do to keep fresh kale fresh for as long as possible! Most experiments and experts seems to recommend using these greens within a week, but if you know that's going to be impossible, consider freezing your kale. The folks at Epicurious explain how to freeze kale, and it starts with blanching the kale—or putting the leaves of kale into a pot of boiling water for a couple of minutes then dunking them into a bowl of ice water to cool. Dry the blanched kale, and then "freeze the dry leaves flat on a rimmed baking sheet until frozen solid." You can then pop them in a freezer-safe bag and keep them frozen for up to 8 months. They're the perfect addition to green smoothies, no thawing required.