Tarragon got to be invasive and I had to dig it out (twice). That was one of those dig and hole and plop it in. Nothing fancy! No feeding, no amending. It was between sage and chives and overran both of them. As it turned out we didn't use that much tarragon anyway. Don't know why you might have a problem.

Lavender had been planted in two spots. One was with the chives, tarragon, sage and mint (don't ever plant mint, smells good but goes everywhere). That did ok. Other lavender was next to the house and ended up not getting enough sun due to the growing apple tree.

All got moved this spring to one newly cultivated spot and they are doing great.

Are your plants getting at least 6 hours of sun? More is better, like 7 am - 7 pm.

We are in east central Illinois.

Jim

Lavender does well in St Louis. Needs fully blazing south sun all day to be at its best. Do not plant in a wet area; needs to be dryish in winter.

To look its best, cut it back to about 6" each spring. It will have a gorgeous shape and lots of flowers on all that new growth.

For varieties, check out High Country Gardens. The one I have is 'Royal Velvet' from HCG.