I like radishes. I know what Jerry meant. First, he didn't care much for them but more importantly they do grow like weeds. I actually use them as planted weeds. Let me explain.
Carrots take a week to germinate in summer and up to three weeks in cool spring soil.
Dry soil can delay germination further. When the seedlings do emerge they are very small and slender, difficult to see and easily overwhelmed by broader leafed seedlings. I like to mix radish seed with the carrot seed when I plant carrots. The radish seedlings pop out of the ground in just three or four days. Their baby leaves are broad and it is easy to see them which marks the row for me.