Hi Nancy, thanks for bringing this to the discussion board. We totally welcome this curiosity and critical thinking.
You’re right that generally, it is best to use dried herbs for your infused oils. However, there are some exceptions which include mullein flowers, St.John’s Wort, and for some, calendula. Further, if seeking the anti-viral benefits of lemon balm, fresh is also best. As a loose rule, if you’re really seeking to extract the essential oils of herbs and you’re making small amounts of infused oils, using fresh material can be ideal. I’ll provide some specific information for the herbs I mentioned.
Mullein flowers: usually if making an infused oil with these, you’re making a small amount as the oil will have a very specific use – earaches. With only making a small amount, it is less likely it will go off and further, less risk that you will lose a lot of oil if it does. The rationale behind using them fresh comes from the fact that you won’t often be able to harvest a large amount at a single time and so, you can keep adding fresh flowers to your oil as you gather them. The flowers are so small and delicate, they don’t dry very well as compared to other blossoms and mullein leaves. There may be some other reasoning for using mullein fresh and I will add to my answer when I learn more!
St.John’s Wort: It has been noted that the active constituents in St.John’s wort are largely lost in the drying process and, when exposed to heat. If the herb is heated or dried, it’s unlikely to have your oil turn red, which indicates the presence of hypericin, one of its most prominent constituents. Tinctures with this herb are usually made with fresh plant material.
Lemon balm: This herb’s medicinal uses are largely due to its essential oils, which degrade once the herb is dried. Therefore, if making a cold sore oil or salve, it’s best to use the fresh herb (wilted) with low heat added.
Calendula:The benefits of calendula are not degraded by drying and so, in order to reduce the risk of the oil going off, they can be dried first. However, some herbalists prefer to capture the fresh, living energy of the herb in oil. Compared to the others, the choice to use fresh or dried seems more of a preference.
So, there’s no simple answer I have to give to you here but I love this question. I’d be keen to hear if anyone else viewing this board has anything to add to this. I hope this helps!