I harvested several large basil plants, some forming woody stems, to make pesto before cooler frost hit last night. I got a mix of purple and green but after you blend it all together you can't really tell, it's just a little darker than normal.
The long part of making pesto is picking the leaves off of the plant. You don't want the woody stem in the pesto since it won't cook down quite right when you add it to a dish. i can't imagine eating pasta with chicken or possibly shrimp and finding a hard woody piece of basil in the mix.
The recipe I used calls for pine nuts, but I used walnuts instead, along with garlic and olive oil. As usual it took a while for the leaves to get chopped up enough to blend well but in the end it made 16 cubes of pesto. One cube is plenty to season a pasta dish for my wife and me and should last most of the year the way we use it. Last year we didn't get quite as much and had to get some pesto from the store.
I tossed the flower heads along with the stems back into the garden to finish reseeding for next year's crop. There were also some smaller basil plants up front that I left. The mild freeze seemed to have mostly gotten to them and weren't many good leaves to use.