Preheat your oven to 375°F (190°C) and line a muffin tin with paper liners.
In a large bowl, mix the all-purpose and whole wheat flour, sugar, baking powder, and salt.
In another bowl, whisk together the buttermilk, vegetable oil, and egg until well combined.
Pour the wet ingredients into the dry ingredients and stir until just combined.
Gently fold in the chopped rhubarb, being careful not to overmix.
Divide the batter evenly among the muffin cups, filling each about two-thirds full.
Bake in the preheated oven for 20-25 minutes, or until a toothpick comes out clean.
Remove from the oven and let the muffins cool in the pan for 5 minutes.
Transfer the muffins to a wire rack to cool completely.
Enjoy your fresh rhubarb muffins warm or at room temperature!