To mushroom is to grow or multipy very fast.
It has its origins in the fact that mushrooms are well known for growing very quickly.
Examples of use:
1. The number of holiday homes in Cornwall has mushroomed in the last few years.
2. We only planned to redecorate our bedroom, but the project mushroomed into a whole house renovation!
3. The number of English language blogs has mushroomed.
4. Our business started small but it is mushrooming. We expect to turn over £250,000 in our first year.
Image © Michael King