Thyme is low growing, fragrant, and has small purple-pink flowers.
Did you spot the error? The three items in the series aren’t parallel. We can test this by reading the sentence with each item alone.
Thyme is | low growing
Thyme is | fragrant
Thyme is | has small purple-pink flowers
This is a common error, and it most often appears in this form: the first two items share a verb (in this example, is), and the third item has a verb of its own (has). We can fix the error by eliminating the series and making a compound sentence.
Thyme is low growing and fragrant, and has small purple-pink flowers.
Another common parallelism error is mixing verb forms.
Jill likes gardening, cooking, and to hike with her dog.
The third item isn’t parallel, but breaking this sentence down item by item doesn’t reveal the error: “Jill likes to hike with her dog” is grammatical. The -ing verb form (acting as nouns here) is mixed with the infinitive form (to walk). This error is easily fixed by making all the forms the same.
Jill likes gardening, cooking, and hiking with her dog.