There are a number of ways to tell if a fruit or vegetable is in season, and there are many reasons to seek out those which are in season. One of the best ways to tell is to use a fruit and vegetable chart, which will tell you specifically what produce is in season in your part of the world. In addition, the appearance of fruits and vegetables in season tends to be very good, they are usually easily available, and the price is often lower.
Many produce items taste best when they are eaten in season. They tend to be transported across shorter distances, because they do not need to be grown in parts of the world where they are in season. It is also more environmentally friendly to buy when a fruit or vegetable is in season, because they do not need to be grown in climate controlled greenhouses.
One of the best ways to tell is to use a fruit and vegetable chart which is relevant to your part of the world. Some grocery stores hand out such charts, and you can often obtain a chart through a Community Supported Agriculture (CSA) association. There are also a number of websites which provide such charts. These charts can also provide information about wildcrafted foods, like fungi.
Another clue is the cost and appearance of produce. When a fruit or vegetable is in season, it tends to be abundant at the grocery store, and significantly less expensive than it is while out of season. It may also look particularly good, suggesting that it has been well grown and picked at its peak. Berries, for example, will be plump and full with no signs of discoloration while in season, while out of season berries may appear wizened, moldy, or mushy. When a vegetable is in season, it will tend to be smaller, with a more tender texture and intense flavor, rather than large, woody, and tasteless.
As a general rule, spring is a good time for tender new fruits and vegetables. New potatoes, radishes, salad greens, and asparagus are all in season in the early spring. Later in the spring, you will start to see peas, onions, rhubarb, and some berries such as strawberries. In the summer, a profusion of summer squash is in season, along with raspberries, melons, corn, tomatoes, and okra.
The fall is the time when dark green leafy vegetables like kale and chard are in season, along with persimmons, grapes, apples, broccoli, and of course pumpkins. In the winter, you can find potatoes, beets, dark leafy greens, citrus, and cabbages in season. Many winter vegetables can be eaten year-round, with the assistance of a root cellar for storage.
The determination of whether or not a vegetable is in season is also dependent on where, exactly, you live, especially if you want to eat locally. As you approach the equator, more things become available all year, but you also miss out on things which require a frost to mature properly. In more northern or extreme southern climates, fewer foods are available year-round, and things like dark leafy greens and root vegetables are more abundant and diverse.