Preheat oven to 425°F. Grease a muffin tin, or line with paper liners.
Combine flours, baking soda, cinnamon, nutmeg, and ginger. Add rhubarb to toss in flour mixture.
In a separate bowl, mix together sourdough starter, oil, sugars, eggs, vanilla, and salt. Fold in flour and rhubarb mixture until just combined. Do not overmix.
Divide muffin batter evenly between 18 muffins (about ⅔ full) using a large cookie scoop. Bake for 20 minutes. Cool for 5 minutes in muffin tin. Carefully remove and cool on a cooling rack or serve warm.