I guess it depends where is the wedding taking place, if it's Canada, September is definitely Fall here… if it's somewhere down south in the States where the Sun is always your best friend, you'll probably have no problem pulling a summer wedding at all.
Having a dash of purple (depending on what tone of purple it is), it doesn't necessarily create a summer feeling. Purple in general usually gives a very romantic feeling to people.
I personally would go for the green, ivory, brown & a dash of purple, as you'd probably want green to be your main color throughout the wedding.