As with all vegetables, there are a few things to look for to ensure that newly purchased leeks are the best quality possible. The overall appearance of the leek should be fresh, vibrant and healthy with dark green leaves on top and a bright white bulb and base. If the leaves are dried, curled or yellowing, or if any part of the surface of the leek is gouged, then it should be avoided. Depending on how leeks are stored in the marketplace, any plants that have a thin slime or wetness around the base should not be purchased, because it means they have either been stored incorrectly or are very old. A good leek should not be overly large for its variety and the time of year, and it should have a light, fresh oniony scent instead of an overpowering sulfur odor.
One of the first things to look at when choosing the best leeks is its color. A healthy, well-treated leek will have a bold pair of contrasting colors. The leaves on the top will be dark green, have some life to them and not be blackened. The bulb at the base of the leek should be very pale or white and healthy looking. Yellow or brown tints and dark spots indicate some problem and the leek should be avoided, if possible.
The texture of leeks also should be look at. The leaves should be robust and feel strong; they should not be thin, wilted, full of holes or drooping. The white portion of the leek should be firm to the touch and not too slimy while also not being so dry that the skin is starting to peel. If the roots on the bottom of the bulb are very brittle or black, then it can be an indication that the leek has been sitting around for a long time or has been mishandled.
During the two times of the year when leeks are in season, they usually are different sizes. Spring leeks tend to be smaller and thinner than those harvested in the fall. When choosing a leek, it generally is preferable to have a medium-size plant. Larger leeks tend to develop very fibrous cores the longer they are allowed to grow. A general rule of thumb is to avoid a leek that measures more than 1.5 inches (about 4 centimeters) wide, because it will have a very tough center.
If more than one leek is being purchased, then it can be beneficial to select leeks that are similar in size. This can help all the pieces in a recipe to cook at the same rate, ensuring that nothing burns or goes uncooked. Similarly, one should be aware that most recipes call for only the white part of the leek, so it might take several stalks to have enough whites to garner the amount of leek required.