ctrl = rpart.control(maxdepth=6)
dt_model <- rpart(formula, data, method='class',control=ctrl)

# this doesn't work
fit <- rpart(class ~ ., data = df, method = "class", control = ("maxdepth = 1"))

# changing maxdepth here effects your tree depth.
rpart(class ~ ., data = df, method = "class", control = list(maxdepth = 1))