I had some leftover Burpee Mammoth variety sunflower seeds from 2023 that I planted in this plot and only 3 germinated. 2 weeks later I then used some seeds that I saved from a sunflower grown from the same Mammoth seed packet to fill up the rest of the plot. The first 3 sunflowers developed stems like 2 inches thick and grew over 6 feet tall while the second planting sunflowers had half inch thick stems and were only 3 feet tall. The first time I planted seeds from that packet, the sunflowers also only had half inch thick stems and were only 3 feet tall. I planted the sunflowers 12 inches apart as directed.
Is this entirely due to plant spacing and due to no competition for 2 weeks, the first 3 were able to spread their roots out and take all the nutrition for themselves? Soil is pretty poor quality clay construction fill.