You have a couple of options i would say.Start in spring indoors if possible.Then transplant outdoors the last week in may or first week in june.You would be able to use quite a few differents strains if you go that route.You are correct about aug.20 th up here.Depending on strain you will begin to flower closer to aug 7th trough 14 though.If you can not start indoors then just simply get strans that flower in around 45 to 55 days.I know you will be cutting it close with temps in the high 30s at night in sept.But i think you will be fine. Just keep eye on low temps as they are finishing up.Minny is agreat place to grow .You get the right strian and you will be fine.I always take a drive along 61 along the shore every fall.It is some of the prettiest senery the state has to offer you are lucky to be where you are.