Why do all my leaves turn yellow then brown starting from the bottom up? Happens every year. The flowers are beautiful but the leaves are so ugly. I have been picking them off but not sure if I should.

I think this is normal sunflower behavior.

Here is my reasoning. We started growing sunflowers because we were "vandalized". Three years ago my neighbor, and sometimes partner in gardening adventures, snuck into my yard and creatively vandalized a couple of empty spots by planting a dozen or so sunflowers that had come up as volunteers in his yard. I didn't pay any attention to them until they started to flower, and then I noticed that the lower leaves were dying and falling off, even as the tops of the plants were growing ever upward and putting on more new leaves.

I have just assumed this was normal sunflower behavior. We grow a few for the seeds and they are in a row by themselves. I snip off the ugly leaves, but purely for aesthetic reasons.

The rest of the sunflowers grow in the jumble of flowers in the kids gardens. In among the cosmos, batchelors buttons, sweet alyssum, mixed poppies, sweet william, columbine, etc, the lower stems of the sunflowers are hidden by the other plants so I don't really notice the dead leaves.

I have a cousin who is in the cut flower business and plants a couple acres of sunflowers for cut flowers each year. Iv'e noticed that his stalks are always void of leaves at the bottoms. He grows many different kinds so its not inherent to any particular species.