Annual weeds produce thousands of seeds. They are carried in the wind, by people and animals and each seed develops into more weeds. To break this cycle, don't ever allow annual weeds to go to seed. Get rid of them when they first begin sprouting.
Asters range in appearance and come in all colors, including blue.
Most have yellow centers. Asters range in height from one to three feet tall. My favorite is the China Aster. This variety is more difficult to grow but they are great as both cutflowers and in beds. Sow seeds indoors 6 weeks before the last frost.