I was recently asked whether it was possible to ferment fresh herbs and the answer is a resounding yes. The following is just a general guide of how to ferment herbs. This could be one herb or a variety. If you want a specific recipe, this is my fermented chimichurri. I plan to add other recipes, like a pesto, in the future.
For soft herbs (e.g. cilantro, parsley, tarragon, etc.), simply include all the leaves and stems. For hard herbs with a woodier stem (e.g. rosemary, oregano, thyme, etc.), you may wish to include only the leaves. If your aim is to blend a sauce, for example, it would be best to remove the stems at the outset of fermentation.
Pack your mason jar tightly with the herbs; smaller masons (e.g. 8 or 16 oz.) may be better here as the rate of decomposition or softening will be faster once the jar is opened, so you could have several smaller jars at once. (If it’s kept in a fridge post-fermentation however, it should stay good for several months, though you’ll notice a slow softening over time, especially once opened.)
A fermentation weight is ideal as the herbs must be fully submerged, but most 8 oz masons can’t fit the weights (the weights are sized for widemouth). It’s possible to find 8 oz widemouths but if you are not in the market for these, use the smallest jars you have. 16 oz. is a great choice. Pack them with the herbs and then cover with a brine at the ratio of 1 tsp non-iodized salt to 1 cup of water.
You need to submerge the herbs completely and so you’ll either need a fermentation weight after the brine is poured, or a small ziploc filled with more brine can be used instead. Just put it on the top and then see where your water line is. You don’t want the water right up at the top so you may need to gently remove a bit of your brine until you see you have at least 1/2″ of headspace.
If you have trouble completely submerging herbs, you should consider a completely anaerobic fermenting vessel such as the Probiotic Jar system.
In the absence of an airlock or other fermenting lid, just use the 2-piece lid which comes with the jar and burp it every day (don’t open the lid, just give it a good turn and then secure back). The ferment will normally last 5-10 days. You can “stop” (slow a lot) the ferment by putting in fridge once active bubbling stops. Alternately, you can burp every other day for another week and then just leave it in a pantry or the like (if it’s cool temp). Until it is opened, it will be fine there for 2-3 months.