Liquid manure

Not sure about Rhubarb but nettles do make excellent liquid feed. Put leaves in a bucket and add water. cover it and leave it for several weeks. Warning - it will smell! Then drain off the liquid and dilute it down by about 8:1 with water before use. Comfrey is also excellent.

Also comfrey leaves can be used this way,there are two kinds one can be invasive but ok if contained.
Butterflies lay their eggs on nettles if you like pretty butterflies in your garden don't cut them till the caterpillars have finnished with them.