How do I cut back spiderwart plants and when?

It's best to cut it back when cooler weather arrives and growth slows and flowering stops. Either late fall or and early winter. Cut the stems back about 8 to 12 inches from the ground.

If you want them to bloom again, Julia, cut them to 8 inches after they have flowered. When fall approaches, you can cut it back for the winter.

Hello, Julia. I'm not sure where you'r located and that can dictate the 'when' part of your question. I have found this great source of information on gardening which included tips for spiderwort. You can designate your location and the tips will apply to your area:

https://www.gardeningknowhow.com/ornamental/flowers/spiderwort/growing-spiderworts.htm

Good luck!