<p>A thick growth of sunflowers stood ten feet tall, and their brown heads drooping over the fence with the weight of their seeds. </p>
<p>Why is the "and" unnecessary"?</p>
<p>Well, and is a conjunction, which means that the two things between it must be complete sentences, and " their brown heads drooping over the fence with the weight of their seeds. " is not a complete sentence</p>
<p>Yes. The second part of the sentence is a subordinate/dependent clause so the “and” is not needed since it would imply that it is a sentence when it is not one.</p>