Grow a Garden Volcano weather event and Molten mutation explained
What causes the Molten mutation and how to get it

The Volcano weather event and Molten Mutation were added to Grow a Garden as part of the Working Bees update.
Alongside a set of two other mutations, the Molten mutation is one of the major ones that many players are wondering about since the launch of Grow a Garden's latest Working Bees update.
To run through all you need to know about this new mutation, below we've ran through how to get the Molten mutation from the Volcano weather event.
How to get the Molten mutation in Grow a Garden
As it stands, the Molten mutation is a very rare mutation to obtain, and seems to primarily come from the Volcano weather event.
This event doesn't seem to function like many of the other events in the game, and so rather than being random like rain or thunderstorms, it instead is triggered by admins of the games.

The first time this event debuted was in the leadup to the release of the Working Bees update, whereby a Volcano event occurred, causing the Molten mutation to be applied to various plants. At the time of writing, this appears to have been the only time it has currently been ran, and it may continue to be an admin-triggered event.
As you can see in the image below, it appears there may have been a bug during the rollout of this event albeit one that works in your favour - if you were in-game during this event, you might have had your entire garden set to Molten.

Unfortunately, I didn't seem to be affected by this and didn't seem to have any plants struck by the Molten mutation, but you'll want to be looking out for plants that are glowing a orange-y yellow, comparable to the visual effects caused by the Shocked mutation.
Once your plants are affected by it, you'll be able to sell those plants with a 25x multiplier, boosting their original value pretty significantly.

If you'd like to see an in-game representation of how it looks when your plants have been successfully affected by this weather event, ItzVexo has a video on YouTube showcasing it.
All the best with your Molten mutation hunt!