In a large bowl or in a stand mixer, add the sourdough starter, whole milk, whole wheat flour, honey. Stir well to combine. If you are not confident your starters ability to "lift" your loaf (perhaps it is still young and hasn't fully developed), sprinkle the yeast over this mixture to hydrate. Let this mixture sit for approximately 20 minutes, after which you should see some bubbles forming.
Remove 2 tablespoons of sunflowers from the called for amount and set aside. Coarsely chop the remaining seeds.
Add softened butter, salt, calendula flowers, and sunflower seeds to the starter mixture. Mix and add bread flour in batches until the dough is cohesive and slightly sticky.
Turn dough out onto a floured surface and knead, adding flour, until the dough forms a smooth elastic ball -- about five minutes. The dough should be very soft, but not sticky.
Place dough in a buttered or oiled bowl and cover with plastic film or a towel. Let rise for about 1 1/2 hours or until doubled in volume.
Turn dough out onto a floured surface and pat into a 12"x6" rectangle, approximately. Cut dough lengthwise into three logs. Separate the dough logs slightly and select a center point. From the center of the dough logs, braid dough from the center to the end, repeating on on the other side.
Transfer braided dough loaf, to a parchment lined or buttered baking pan, tucking the ends under, I prefer a ceramic baking dish with sides, just slightly bigger than the dough braid itself to discourage spread in the next steps. Cover with plastic film or a towel and let rise until doubled in size, about an hour.
Remove plastic film or towel and brush loaf with an egg wash created from one egg beaten with a small bit of water. Sprinkle the top of the loaf with the reserved sunflower seeds. Bake in a preheated 375 degree F oven for 25-32 minutes, until the top is evenly golden and the loaf sounds hollow if tapped (I suggest rotating at least once while cooking).
After baking, remove from oven and cool completely before slicing.