It 's definitely nice to be able to eat locally and seasonally, but if you live in the Northern US or Canada, there are going to be slim pickings 6 months out of the year. My idea of "in season" is not strictly local. I live in the midwest but I also buy crops from California, Washington State, Idaho, Georgia, which have definite "seasons" in the US.
Onions and garlic, I wouldn't stress about. You need them all year round, anyway, and the "local" season is a few weeks at best, no matter where you live.
Most of the garlic you buy these days in the supermarket or fruit market is coming from China. The onions you buy could very well be US grown--they're grown in over 20 states and store well, (but also in Mexico.) How "local" they are to you will depend on the time of year, where you live and the cost to the seller.
Actually, the "local" season for just about any fruit or vegetable is only a few weeks. Outside of that, you're doing the best you can to expand your definition of "in season" to include other parts of the country. Unless you live in California, you'd be produce-starved if you depended only on what is in season locally all year round.
Right now, citrus is in season, but unless you live in Florida or California, it's not exactly local. Still, now is the time to buy and eat citrus in the US. Not in the middle of summer, when it is coming from farther afield. That said, I buy lemons and limes year round because I need them and in the out of season months, I'm not picky about where they are coming from as long as they look good.
Apples and pears are probably still from this fall's crop but have been in storage. Same with potatoes, hard squashes and other root vegetables. Apples and pears in the spring and summer are usually old or from further afield.
Broccoli, cabbage, carrots, green beans, zucchini, and lettuces are grown in California year round, so they are domestic crops, and probably about as fresh as you're going to get most of the year--except for the few weeks you will find them fresher at your farmer's market.
Florida starts producing tomatoes and strawberries in January --so, nominally "seasonal" but because of how they are grown and harvested there, they are not worth eating. In my opinion, the only tomatoes worth eating come from the farmer's market from late July-August through the first frost. (I do buy the little grape tomatoes during the other months of the year. They always seem to have decent flavor. Often they come from Mexico.) I'll admit to happily eating California strawberries when they show up later in the spring. The season in the Midwest, where I am, is June/July but I start buying strawberries from California much earlier. The local berries, though, are far superior.
Asparagus is a spring and early summer vegetable in the US. Asparagus right now in the stores is coming from S, America.
Most berries and stone fruits have short seasons during the summer months in the US. You will find them locally at farmer's markets for a few weeks but if you are not being a strict localvore there will be a couple of weeks both before and after your local season where you can find US grown fruits and berries from California, the Midwest and the South. In the winter, they are coming from S. America.
Michigan and Washington state grow most of the world's cherries and apples--and I don't hesitate to buy either when they show up in my local fruit market, but you may find cherries locally for a couple of weeks in the summer at your farmer's market. This time of year they are coming from S. America.
California grapes are available from May-January. Sometimes, though, I see grapes from Chile in my local market. I tend to only buy grapes when good berries and stone fruits are no longer available and when they are relatively inexpensive.
Melons are a late summer/early fall crop in most of the US. Outside of that window, any melon you see is probably coming from S. America or Mexico.
That said, I bought a melon last week --totally out of season for where I live, but it was on sale for $.99 and smelled good--and it was wonderful. Not sure where it came from. The lesson there is "trust your nose."
A lot of the peppers I see year round are grown in Mexico. I'm sure there are some grown in California, too, but there are strong ties between Chicago and Mexico--and my local fruit market is Mexican, so I see a lot of produce from there. (and buy it, too, when it looks good. Which it usually does.)
If you have a fruit market --not a grocery store, but a fruit market--in your area, you will get a good idea of what is in season by paying attention to what is on sale for the best prices and what smells and looks good. Read the tiny writing on the POS look-up stickers--it will tell you where something was grown. But, above all, trust your eyes and your nose.
Edited by ChicagoTerry - 1/18/13 at 4:56pm