The scientific classification of Onion and Petunia includes the entire classification chart that includes kingdom, family, clade, tribe etc. A kingdom is the second highest taxonomic rank. A taxonomic rank is the level that a living organisms are placed in hierarchical level of arrangement of life forms. All the living organisms are divided into kingdoms. Plant kingdom is a main classification of living organisms that include all plants. There are six kingdoms which are: Plantae, Animalia, Fungi, Protista, Aschaea/Archaeabacteria and Bacteria Eubacteria. Onion and Petunia kingdom and all other plants kingdom is Plantaea.
Plant family is the group of plants which have something in common. Onion and Petunia family is a major factor of Onion and Petunia Scientific Classification. Plant family helps you narrow the search of your plants. It also gives you idea about, how the plant looks, how the seed will be like etc. These two plant families can be different or same. Onion belongs to the family Liliaceae and Petunia belongs to the family Solanaceae.
While comparing scientific classification, Onion and Petunia genus and other classification are also important. Also, when you compare Onion and Petunia, other factors should also be taken into considerations like order, subfamilies, tribe, clade etc. First plant's genus is Allium and other plant's genus is Petunia. Onion tribe is Allieae and Petunia tribe is Not Available. Onion clade is Angiosperms and Monocots and order is Asparagales whereas Petunia clade is Angiosperms, Asterids and Eudicots and order is Solanales. Every plant have subfamilies. Onion subfamilies are, Allioideae and Petunia subfamilies are Petunioideae. In this way you can compare scientific classification of Onion and Petunia and other products also.