One of the things that Patrick and I have wondered and talked about is if the medicinal plants we grow in our beds are as medicinal as those collect from the wild or in our native dirt. This is a true concern as it would be wrong to cultivate a plant and then dispense it to a person if it did not contain the proper chemical properties.
I have just put some Lobelia seed (Lobelia inflata) into a seed flat. While I was researching what they would need and what I should do for them I came across this article on line.

Two things of note in the article:
1) they found that the concentration of lobeline in the plants did not seem to change for the location or media that the plants were growing in.
2) the young plants had higher concentrations of lobeline in them but the larger plants had more total lobeline. A really important note is that the FLOWERS had the absolute highest concentration.
I am still trying to wrap my head around what would be the best time to harvest is. My initial thoughts are that the flowers would be best with the young plant next best. There would be more volume with the plant material vs the flowers though. You might want to use the more mature plant if you were going to do extractions as there would be more total target chemical available. It would take a LOT of material processed though.
Obviously if you were wildcrafting you would take what was available and deal with it appropriately.