'Can I just harvest the leaves? Cut the flowers? And save the seeds for next year? '

All of the above will work. But if you cut off the flowers you won't have any seed. If you do cut them you will be able to harvest leaves a bit longer.

Thank you! How long do I have to wait to get the seeds?

I can't tell you as I live in a different climate. But just wait until the flowers are dead and gone and you can see the seed has started to dry and darken. You could put a paper bag over the flower heads just before they dry completely to catch them as they ripen and fall.

When the nights (and maybe days) starting getting under 60 degrees F, that is when basil stops growing. Seeds may still mature in those cooler times but the plant really stops growing much at all.

FataMorgana

what good is purple basil? or what I have growing is a look a like? I planted 9along with severalother varities of basil)it in my former home and it got as big as small bushes... Its very invasive here at this place....

100 or so places its growing here... I let it grow wild in my raised beds. I hoped it would keep away bugs...Heck no that is the only thing untouched growing in my garden. some plants are 4 to 5 ft tall.********blankety stuff

heck only good I can se is it helped support some volunteer tomato plants

I'm surprised that purple basil self sows in zone 6. I suspect that what you have possibly isn't purple basil, jeanwedding. If it is p.b. you use it just as you use any other basil. If you post a picture maybe we can id the plant you have.

Ditto on the self-seeding comment. If it is purple basil, it dries pretty well for basil and is pretty in dried herb blends. Gardeners supply co website also used to have a great recipe for a purple basil pesto with sun dried tomatoes. Yummy!