What are those plants?
I can answer this for the Navagrahas at least.Not sure about the Nakshatra plants but the 9 Navagraha plants are well-known.For example see this site.
And the Scripture which mention about these plants is the Yajnavalkya Smriti.
and Kusa(Ketu) ,are,in order,the sacrificial fuels of the Nine
(Yajnavalkya Smriti verse 302)
The next verse then says how to use these plants as oblations in Navagraha Yajnas.
With each class of Samidh,soaked with honey ,clarified butter,curd,or
thickened milk ,oblations,either 108 or 28 in number should be
So,these 9 plants are used as sacrificial materials in Navagraha Yajnas.I don't know if they are themselves worshiped as representatives of the 9 Grahas,probably they are not.