I've always wanted to try the field-medicine use of plantain. I've always just chopped it up with a knife, but I thought I'd try it the chewing and applying method. It surprisingly sticks well to the hand this way--it does not when chopped with a knife.
It is my understanding that the anti-inflammatory compounds, present in plantain, are not drastically inhibited by freezing. However, water is not the best stabilizer for for these compounds so it will deteriorate over time, hence the note in the medicinal journal to use the poultice within 1 year. If you wanted to use plantain for some of its internal uses you would not want to freeze it.
It always best to use these herbs fresh; however, I live somewhere that snow is on the ground nearly 6 months of the year.
So, I stuck my hand somewhere I shouldn't have and found a wasp nest. Luckily it was late in the evening and they seemed to be uninterested past the initial interaction. After the sting I went found some plantain in the yard and went to work. I was in a bit of a rush so I chewed it and applied it directly to the spot and it started to feel better very quickly.