"The wedding will be in the early evening in one of the fields behind the house," usmagazine.com quoted a friend as saying.
The 34-year-old is expected to wear a Monique Lhuillier design when she exchanges vows with Toth, 40.
"She's spending most of her time on that dress," usmagazine.com quoted a source of the dress designer as saying.
The actress is sweating it out in 75-minute yoga classes to look her best on her D-day.
Witherspoon was earlier married to actor Ryan Phillippe, but separated in 2006. She has two children - daughter Ava Elizabeth and son Deacon Reese - with Phillippe.