The Irradiated Berry has an abusive growth issue tied to picking it up and saving and quitting your run. You can pick it up. Save and Exit. Load your run back up. You will still have it in your inventory and a new one will have grown. You can abuse this for a bunch of endo mutations before even leaving the first town area. From the screenshot you can see that I have used it to get increased range x3, vampiric hits, and waterproof without loading into W1-1.
I wanted to see how much further i could take it. This is the extent that i could abuse it. Irradiated berry only gives positive mutations.