Preheat oven to 325˚F. Grease and flour a 9x5 loaf pan.
In the bowl of a stand mixer fitted with the paddle attachment, beat the butter at medium speed until creamy, about 30 seconds. Add sugar gradually and beat until light and fluffy, about one minute. Add one egg at a time, beating until just blended after each one, then stop the mixer.
Whisk together flour, salt, and baking soda. Turn mixer on low, and to butter mixture, alternating between the flour mixture and sour cream*. Beat at low speed just until blended after each addition.
Using a spatula, fold in zucchini, lemon zest, and poppy seeds (if using).
Add batter to loaf pan and bake about 75-90 minutes, or until a toothpick inserted in the middle comes out clean.
Cool for 10 minutes in the pan on a wire rack. Remove from the pan, and cool completely on a wire rack.
Notes
*To alternate flour and sour cream: Add ⅓ flour mixture, then ½ of the sour cream, another ⅓ flour mixture, the remainder of the sour cream, and finally the remaining ⅓ flour mixture.
**For three 5x3 inch disposable foil loaf pans, fill with about 1⅓ cups batter per greased and floured pan. Bake for 40 to 45 minutes, or until a toothpick inserted in the middle comes out clean. Cool as directed.