Mugwort Plant

ALL of my Richters plants continue to thrive very well. It's amazing - all I've used is soil, water, and sunlight and everything is growing like crazy. For example, my angelica and Valerian plants are about 5 feet tall. I don't know why people use fertilizer or pesticides. Maybe I'm missing something.

Soils vary greatly in their fertility, depending greatly on composition and previous use. For example when I first started growing vegetables on my sandy soil that had been fallow for many years, everything grew like "topsy" the first year without any fertilizer. But every year the crop growth became more subdued until it was downright stunted. After that I had to incorporate a neighbors well-rotted manure to get anything close to that first year harvest. In clay soil the "honeymoon" will last longer, because nutrients don't leach out as easily.

I have 3 mugwort plants - one new to this year, one that's about 3 years old, and another that's about 1 year old. The leaves on each plant are similar but different in a few minor ways. Is this typical of the plant's growth cycle? Or are there slight differences or mutations that occur with different sets of seeds?

Plants all vary a bit especially the species. Leaf shape is only one way in which they vary and sexual reproduction was so successful as a survival strategy, because it mixed up all these accumulated mutations and hopefully produced some individuals that would survive better during a change in the environment- and our environment changes all the time.
But the other factor, the age of the mugwort plants is a factor too. Leaf morphology changes in many plants as they age and may be especially drastic as such plants reach flowering size. This is the case with mugwort. The leaves get smaller near the flowers, probably to not impede pollination.