If you don't divide Apache dahlias, the plant will become overcrowded and may not bloom as well as it should. The size of the plant will increase, but its roots will become congested, reducing its ability to absorb the necessary nutrients and water it needs to thrive. Over time, the center of the plant will become woody and die, causing the plant to become less productive.
This can also lead to disease problems, as stagnant water can collect in the center of the clump, promoting the growth of fungal pathogens. So, it is recommended to divide Apache dahlias every year to maintain their vigor and encourage healthy growth and blooming.