Preheat the oven to 350 degrees Fahrenheit. In a medium-sized bowl, mix together the flour, salt, and baking soda until well blended. Set aside. Mix the sugar, butter, yogurt, vanilla, honey, and lavender together in a large bowl at medium speed until smooth. Add the eggs, one at a time, mixing well between each addition. Alternate between adding the flour mixture and the buttermilk to the egg mixture until the batter is just blended. Gently stir in the berries and plum pieces. Pour the batter into a well-greased 9 x 13-inch baking pan and bake for 30-40 minutes, or until the cake is golden brown on top and set. Remove from oven and allow to cool.