Q.Coleus plants
I have several potted coleus plants and the stalks are hardening and turning brown. What is causing this to happen?

Coleus will eventually produce woody stems. Gardeners often turn these plants into standards.
You can pinch out the side shoots and support the stem with a stake.
When the plant reaches the desired height you can pinch out the top growth and continue pinching back any side growth.
https://www.gardeningknowhow.com/ornamental/foliage/coleus/coleus-plant-care.htm
https://www.gardeningknowhow.com/ornamental/foliage/coleus/coleus-plant-blooms.htm
https://www.gardeningknowhow.com/ornamental/foliage/coleus/growing-coleus-in-a-pot.htm