It's a casual outdoor event so dress cutely but comfortably. Please don't wear pointy heels unless you want to aerate the lawn.
If it looks like rain is in the forecast, pack your umbrellas. If you're allergic to bees, pack your epipen as this is an outdoor event with lots of fresh flowers and food.
There will be limited street parking available so carpool if possible and golf carts will you to the ceremony location if you park far away.
I would take a Claritin... the field is covered in goldenrod and hay.