Growing and Using Milk Thistle

Is it possible for the layman to grow and harvest for his own use the active ingredient in this herb? What is the proper method for this harvesting? How is the strength for administration determined? Also what is the shelf life of this herb?

Yes, it is possible to grow milk thistle for one's own use. The seeds are the part most commonly used, although John Lust in his book, "The Herb Book" (available from Richters), says the leaves are used also. You need more than one plant to get enough seeds, but fortunately it is easy to grow by scattering seeds in the garden in spring.
The seeds contain the main active constituent, silymarin, which is known to protect and help heal the liver. The only way to determine the strength of silymarin is by laboratory analysis, and that is not practical for home growers. Fortunately, the range of silybarin concentrations do not vary significantly.
According to Lust, an infusion can be made by steeping 1 teaspoonful of seeds in 1/2 cup water. Take 1 to 1-1/2 cups a day, a mouthful at a time, 4-5 times a day. Or, take a tincture made with seeds: 15-25 drops, 4-5 times a day.
The seeds are somewhat difficult to harvest; for home gardeners, the seeds need to be extracted from the dried seedheads by hand.
We don't know how long milk thistle lasts on the shelf. We suspect that it will last at least two years, but that is a guess.