They are just increasing the probability of mutation with a mutagen and having new (and often uglier) flowers as result. The type of mutagen is not so relevant and gamma x-rays are not the only way to achieve this, some poisons are also used since many years (they play an important role in the first 'jurassic park' plot for instance).
Some mutations are aesthetic, other so-so and other are really unnecessary and ugly IMHO. If you have a tiny cristated opuntia near your PC you probably have an atomic garden yet. Of course any trace of the original radiation is first diluted and then cleaned before to reach the market by the propagation phase of these cacti and flowers. These are not radioactive plants.
If you are brushing your teeth with a toothpaste that contains peppermint oil you're ingesting a Atomically Modified Organism or HMO every day!
I prefer this article on the subject though.