A collective noun for dry weather is spell. eg. A spell of dry weather
A Spell of Dry Weather
There is no collective noun for "dried plants."
A collective noun is used to refer to a group of people or things. The opposite of a collective noun is a countable noun. If I say there is a flock of birds, you really don't know how many I mean...
Yes, collective nouns would always be common nouns. For example: flock, group, series, collection, herd, shoal. Those are all common nouns.