How old is the usage of 蔬菜 to mean vegetables? Did it used to be only 蔬？Or only 菜？ I can't find any other usage of 蔬 (other than things like 果蔬， where 蔬 seems to be just a stand in for 蔬菜）, whereas 菜 is a much more productive and free morpheme. What meaning can 蔬 be said to have by itself?
3 Answers
It seems that 蔬 comes from 疏. 疏食 means "coarse food" (unpolished cereals and vegetables). Later, 艹 was added to 疏 to refer to vegetables specifically. I guess that' why the characer 蔬 does not form many words.
Looks like the earliest use of 疏菜 can be traced back to "疏 - 周礼太宰注 疏菜也". https://baike.baidu.com/item/%E8%94%AC/6708649
In short,蔬 means wide variety and 菜 means it's eatable.
