Also zone 5
1. I've always found it hard to kill yarrow, but I do think you need more root protection for the dug up plants than having the pot exposed on concrete. Can you pile leaves/straw/etc over after the first few frosts to insulate the pots a bit?
2. Shallow watering encourages shallow rooting. And, if your helper is spraying the plants, he could be encouraging disease splashed up from the soil.
3. Start seed or transplant starts into the pots early - if they've had a chance to grow in the pot a few months they do better than one dug up and plopped in the pot then brought inside. Supplemental light is a must. Even with a south window, the winter light is still weak.
4. Bt spray or powder (specific for caterpillars), or diatomaceous earth (will also take out slugs and other bugs) on your plants.
