I haven't grown much Thai basil in the open ground as I usually plant it in pots on my porch. Whatever I did plant in the garden never re-seeded. In fact, even though I have been growing at least 10 kinds of basil every summer for over 20 years in the 3 states where I've had herb gardens, I only ever had 1 basil re-seed itself and that was a Holy basil [Tulsi) when I lived in zone 6, Lancaster County, PA. I had planted it in a small patch in the corner of a fence near a stone wall. I figured that the heat from the sun reflecting off the stones had created a micro-climate [warmer spot] for the basil that fall so it ripened its seed before frost and the stones made the area warmer in winter. Later on I read that the Thai Holy basil will sometimes re-seed itself where it doesn't get too frigid, like Lanc Co or NJ, so you probably had the Thai HOLY basil. I never grew it again after that because I didn't like its smell so I didn't use it in my cooking...
This summer I am growing over 20 kinds of basil, including 44 kinds of Thai basil, but not the Holy.

Thanks for the reply! I never used in in cooking, it was a pretty ornamental. I had already planted so much Italian basil before someone gave me these plants last year that I couldn't use it all. You could be right about the microclimate. It was planted next to the house (like 18 inches) in a bed that had red stone mulch. I hate that stone, but it came with the house and it does help with drainage. It could be Holy basil I suppose, the flowers looked more like a google image search for the Thai. Dark purple stems and flowers. Do you have a photo of what you think I have?

As an aside: what do you do with all that basil? Do you preserve it?

Chuckling--I just re-read my previous message and found I must make a correction. I am only growing a total of 20 kinds of basil this season, including 4 kinds of Thai basil--not 44!
At home, I am growing a lot of single plants in pots on our big old-fashioned front porch and I also planted single plants of about 10 kinds in 2 large fruit baskets--my "basil baskets", which are filling in and looking nice. Then in my community garden plot, I planted 3 kinds of Italian basils (about 22 feet in all) and 3 feet of purple basil. My 4'row of lime basil never germinated, but I will soon replace it with seedlings of 'Mrs. Burns', my favorite lemon basil, that I grew on the porch in a window box along with extra 'Geneovese' and some 'Purple Ruffles' basils from seed. We also planted 20 varieties (about 25 plants in all) of mostly heirloom tomatoes in our garden plot, (and 5 kinds of hybrids in our little home veggie garden), so we plan to sell the extra tomatoes beyond what 2 people can use, and most of the basil, to benefit the living history farm where we both volunteer. I imagine that most people who buy it will make pesto with it. Wanna buy some fresh basil?? Also, I'm sorry, but I'm a bad photographer (wish that I was better) so I have no basil photos. However, if you visit Mr. Google, and search for 'Thai Holy' basil, I'm sure that you can come up with a picture...Maggie