What is the origin of the idiom "plenty as blackberries" and when to use it?

The idiom "plenty as blackberries" is derived from the abundance of blackberry bushes and their prolific fruit-bearing. Blackberry bushes tend to produce a large quantity of berries, especially in favorable conditions, making them readily available in many places. This phrase is used to describe a situation or resource that is abundant or readily available. It conveys the idea that there is an ample supply of something, often in a positive or favorable context.