Photo by clayirving (via Flickr)

We're nearly at the height of tomato season. Co-workers and neighbors are starting to bring over loads of extra fruit to share and menus around the city are placing tomatoes front and center, whether in BLT form or as part of other featured dishes, such as the Tomato and Padron Pepper Salad at Province. Adding to the festivities is Chicago Tomato Fest, which is gearing up for its annual potluck (see below for details).

In addition to the round, bright red varieties we're used to seeing piled high in the supermarket, heirloom tomatoes are becoming more and more common. Perhaps you've seen them, but never really knew what they were: Sometimes gnarled and dark purple, sometimes bright green and the size of a golf ball, striated with yellow, sometimes a fleshy pink color and slightly flattened with extra curves and lobes, these are heirloom tomatoes. And as unique as their individual exteriors are, each also has a flavor all its own, with varying sweetness and acidity unlike any year-round hothouse hybrid.

But what is an heirloom tomato?

Surprisingly, the answer is more difficult to nail down than one would think, although the concept is fairly simple to understand. At a dinner given by Tomato Fest at Osteria Via Stato last week, Rink  DaVie from Shooting Star Farm  was asked by host Damien Casten (of Candid Wines) to define what an heirloom tomato is, to which DaVie responded, "Well, I'll take a stab at it."

He continued, "First of all, heirlooms have to be open pollenated, versus a hybrid, because they're seeds that people are saving and often the traditions of an heirloom seed were passed down from generation to generation because a home gardener or a community or whatever tradition decided that that tomato was something that they value even when the seed company decided that perhaps that wasn't something that they were going to produce. Because, you know, seed companies are businesses, and they make decisions based on what sells and what the popular trends are.

You know, some of the tomatoes that we think of as being such poster [children] for heirloom tomatoes, like a Brandywine tomato, really, the story that I heard was that people were saying, 'What happened to that beefsteak tomato that I liked so much?' and they found that somebody was saving that tomato in their home garden, just like any of you could do.

'Heirloom' is a provocative term because it's something that has value to the people who save it and pass it down."

Heirloom tomatoes might not be as disease-resistant as hybrid cultivars that have been bred to survive blight and inclement weather, and they might soften a little earlier too, but they are a flavorful, visually delightful hand-me-down that serves to remind us of simpler times while helping us preserve them for the future.

Although heirlooms can be found nearly everywhere now, including big-box grocers like Jewel, Dominick's and even Wal-Mart, your best bet for getting locally grown, often organically raised 'maters is to head to your nearest farmer's market. The Green City Market in Lincoln Park boasts more than 15 different kinds of heirlooms, with Dana Benigno of Chicago Cooks offering the following primer in the GCM's most recent newsletter:

(This primer is reprinted with the gracious permission of Dana Benigno, owner of Chicago Cooks. To view the full guide, complete with gorgeous pictures of each variety, click here)

• The Aunt Ruby's German Green - Large green beefsteak tomatoes that ripen to pale green with a tint of yellow and pink blush underneath that extends inside the flesh. The flavor is sweet, yet spicy, and quite delicious and the tomato has the smoothest shape of any large heirloom tomato.
• Djena Lee's Golden Girl - A large golden orange fruit with a semi-thick skin. The tomato's intense orange color is very striking. It has an excellent balance of acid to sugar giving it a sweet yet tangy flavor. Flavor was so highly prized that these tomatoes won first place at the Chicago Fair 10 years in a row.  It is on Slow Food's Ark of taste as a tomato that is in need of preservation.
• German Pink Tomato - This is a favorite heirloom variety. The deep pink beefsteaks are sweet and flavorful with meaty flesh that contains few seeds. German Pink tomatoes weigh about 1 pound and are produced in abundance, making it a good choice for canning.
• Orange Oxheart Tomato - The Orange Oxheart tomato is a family heirloom from the Virginias region. This fruit has a deep orange skin, which hides a dense, orange flesh. The Oxheart is appropriately heart-shaped and can grow to a large size, often weighing in at one pound or more. The meat of the tomato is dense with a rich aroma. This heirloom tomato has superior flavor and its meatiness makes it excellent for salsas and canning.
• Big Zebra - A stunning tomato that looks much like a giant version of the popular "Green Zebra," this 8-10 oz. beauty has vibrant green and deep gold striped skin, with delicious red-streaked, green flesh. This tomato is superb tomato for growing at home and for farmer's markets and a must for all who love the beautiful and unique.
• Black Prince - Black tomatoes are an heirloom variety from Russia. They have a delicious blend of sugar and acid and a distinctive, complex flavor that is to be savored. Some say this variety is one of the best-tasting black tomatoes and prefer it also for its medium size.
• Copia - These very beautiful tomatoes are a stunning combination of fine-lined golden yellow and red stripes. While visually exciting, the real treat comes when you cut them open. Their gold flesh is streaked with red and the fruit is very juicy, flavorful and sweet. A stabilized cross between Green Zebra and Marvel Stripe, these tomatoes weigh about one pound each, They were named in honor of Copia, the American Center for Food, Wine and the Arts, in Napa California
• Green Zebra - Developed in 1985 by tomato breeder Tom Wagner, this is an unusual and exquisite green tomato.  The 2-inch round fruit ripens to a yellow-gold with dark-green zebra-like stripes. The flesh is lime-emerald in color that has an invigorating lemon-lime flavor. This tomato's acidity is great for brightening up salads and other tomato dishes.
• Dr. Wyche's yellow tomato - This tomato is  a beefsteak heirloom that produces slightly flattened smooth, blemish-free, golden-yellow fruit with a meaty interior and few seeds. It's rich flavor and larger size sets this variety apart from other yellow heirlooms.
• The Chalk's Early Jewel - About 3-4 inches in size, this heirloom produces early fruit in about 60 plus days. Created by James Chalk of Norristown, Pennsylvania,  it was introduced in1904. Its exterior is deep red in color with a meaty and tender interior. When eaten raw it is juicy, with a great acidity and is spicy without being sour.
• Cherokee Purple - An heirloom from Tennessee cultivated by Native American Cherokee tribe.  Very productive plants producing loads of dusky rose to purple colored, Sizes are 12 oz.-1 lb with deep red colors to the interior flesh and dark shoulders. Cherokee purples are a very popular farmer's market variety because of their rich, complex and sweet flavors. 
• Black Krim - Originally from the Isle of Krim on the Black Sea in the former Soviet Union. This rare, and outstanding tomato yields 3-4" slightly flattened dark-red (mahogany-colored) slightly maroon, beefsteak tomatoes with deep green shoulders. The flavor is described as intense, slightly salty taste (which is great for those not wanting to add salt to their tomatoes).
• Moskvich - This variety is very flavorful with rich tomato taste. An heirloom variety from Russia its flavor is described as mild, smooth but very juicy.
• Costoluto - Italian heirloom tomato with large, deep-red, juicy tomatoes are deeply ribbed but fully flavored and absolutely delicious. This variety is hearty and does well in hot weather, but continues to produce even when the weather turns cool.

Sign up for the Green City Market newsletter (located on the lower left of the homepage) to receive updates on what's available each week, as well as special events and great information like the above heirloom tomato primer.

Celebrate the end of tomato season with Chicago Tomato Fest's second annual potluck on September 10th. Sponsored by the Chicago Honey Co-op, Candid Wines and Slow Food Chicago, the tomato-themed dinner is a great way to make use of the last of your tomato harvest while meeting other tomato enthusiasts. If you've already put up the rest of your crop or you are still trying to figure out the best way to cultivate a garden in your no-balcony-having apartment, feel free to bring a non tomato-centric dish.

The $15 cost ($10 for Slow Food Chicago members) directly benefits Slow Food Chicago and includes admission to the potluck as well as tastings of organic wine from Candid wines and craft beer and soda from Goose Island. The potluck will be held at the Chicago Honey Co-op, located at 3740 W. Fillmore. You can purchase tickets here.