The phrase "are fast growing" is correct and usable in written English.
It can be used to describe something that is increasing in size, number, or importance at a rapid pace.
Example: "The tech startups in the region are fast growing, attracting significant investment and talent."
Alternatives: "are rapidly expanding" or "are quickly increasing".
These sub-tropical plants are fast growing, prolific and need plenty of food, water and, above all, heat.
For all that, they are as real as you or me – and they are fast growing in number.
He says the voices calling for immediate change are fast growing in strength and volume, with radical groups like Julius Malema's Economic Freedom Fighters surging in popularity, as more and more grow disaffected with the ruling ANC.
Photovoltaics, or solar cells, are fast growing both with regards to industrialization and research.
These trees are fast growing and can potentially produce high-quality timber for appearance, structural and engineered wood products.
Sanjana Hattotuwa, editor of a citizen media initiative, groundviews.org, showed me some of the anti-Muslim web pages that are fast growing in number.
But America is as slow growing a market for Scotch as China, Japan and Korea are fast-growing ones.
Their pitch is that Mr. Obama's appeal has grown in traditionally Republican states like Arizona, where there are fast-growing Hispanic populations, and that Republicans have alienated independent voters with "extreme" positions on popular programs like Medicare.
The large phytoplankton represent taxa that are fast-growing under nutrient replete conditions, including diatoms, and the small phytoplankton can grow at relatively low levels of nutrient availability and light, and represent picophytoplankton [19].
They are fast-growing — Prince William's population has quadrupled over the past 40 years and Loudoun's has grown tenfold — affluent and diversifying with a mix of Latinos, blacks and Asians.
Eucalyptus spp. are fast-growing with desirable wood qualities and are, therefore, a preferred species in plantations; they are widely grown in the tropics and subtropics, especially in Brazil, India, and China [10].
