Comfrey and nettles are both known as weeds, but they do have one thing of value: it's possible to make liquid fertilizer out of their leaves. I won't keep the fact that it smells bad, but experienced gardeners swears on it, so it's worth a try making it.
Making fertilizer out of comfrey has to be one of the most easiest thing in the garden. Make a hole in the bottom of a bucket and put it on a stand to make it possible to put a can under it. Then pack the bucket with comfrey leaves and leave it for a couple of days. A blackbrownish liquid will start dripping into the can and that's your concentrated natural fertilizer. To use it you dilute it with 10 to 20 parts of water. Rich in nitrogen, phosphorus and potassium.
Nettle water is well known for its rancid smell. You make it by putting nettles in a bucket of water and leave them to soak for some days. Filter the liquid and dilute it to the colour of tea before you water it out. A mild fertilizer with good credentials.
Comfrey and nettles can be dried for future use. In that case you use the recipe for nettle water for comfrey too.
And then another kind of green fertilizer:
Grass klippings are easy to handle and full of nitrogen. You mulch around the needing plant with it and the nutrition is released through nature's processes. Appart from feeding the plant it keeps water in the ground and may even reduce weeds. The back side is that it's a dream environment for slugs and snails, so if you have a slugproblem you should probably look for other fertilizers.
I have yet to try comfrey and nettle water indoors, these are the two fertilizers I'd like to try in my containers. Outdoors I could use all three, but I have to make some extra arrengements for the green mulch; we do have a slug problem in Uppsala right now. What I'd do is to use it in raised beds with a slug barrier.