When we talk about fast-growing plants, we usually use this term to signify that the plant is rapidly developing and growing. However, in order to avoid repeating the same word over and over again, we may want to look for synonyms for "fast-growing" to add variety to our language. Some synonyms for "fast-growing" could be: steadily developing, quickly expanding, rapidly flourishing, swiftly evolving, or speedily spreading. All of these words convey the sense that something is advancing quickly, but they offer some different connotations and shades of meaning that can enrich our writing.