Jimmy, you pose an interesting post! While I can't yet relate on the farmer's market level (ask me again this time next year, the planning's already started

) I can relate on the Produce Manager's position. Since I have been in the trade I have dealt with a few local farms who are able to supply me consistently (season permitting.) I have always had a department that emphasized local farms and offering connections with the grower to promote local agriculture (and our neighbors!)
There is one farm in particular that most of my dealings are with, she grows about 20 acres of produce... and to date the best watermelons I have ever had in my life... though I need to try one of these heirloom's you speak of
She grows 4 main varieties, a Charleston Gray- which I often offer to my customers as the best "old fashioned" watermelon taste.. it can tend to have a hint of the mealiness without being mealy... if that makes any sense... and is extremely sweet and juicy (extra juicy.) Then she grows a variety of a yellow Seedless melon (not exactly sure the name) that is a "unique" experience, being less strong in the watermelon department and more of a sugar-water taste. Then she grows the classic Sugarbaby - which I don't usually recommend when a Charleston Gray is available because they are similar (to me) and the Charleston is sweeter - though the Sugarbaby is a good choice if you aren't needing a lot of melon (they seem to avg. 8-12 lbs, compared to the 30-40+ pound Charleston's.)
MY personal favorite however, are the Revolution seedless melons. Though I generally prefer the sweetness of a seeded when the local aren't available... these melons are the single best watermelon I have ever had... they are seedless (for those whom it matters to... usually day cares and such) but they are extremely sweet and have a rich watermelon flavor. They range from 25lbs to over 40 lbs depending on the melon, and are very dark green in color - generally somewhat oval shaped.
If I have a customer ask me for the best watermelon flavor I usually ask them if they want the melon to me more crisp or juicy and if seeds are an issue. If they want the crisp- I grab them a good Revolution... and if they want to take a bath while they enjoy the sweet goodness I pick them a good Charleston Gray. The yellows are often a hit with the kids though :lol:
Soo... the point of this long post Jimmy is I guess I just try to categorize them by texture and sweetness level and at times will just keep one of each aside and cut as need be to let them try if I can't talk it in to them :lol: You are absolutely correct though - only the customer can truly decide what they want. Now... I just need to get my hands on a good Ledmon!
