In containers this season, I planted Pennisetum setaceum ‘Rubrum’ and ‘Burgundy Giant’ (fountain grass) as well as Phormium (New Zealand flax), which are treated as annuals. I don’t have a greenhouse, but can they be overwintered indoors? I successfully overwintered cannas last season.
Around late August, move the ornamental grasses and Phormium into a very shady spot outside and remove any other plant material from the container. Doing this will help your plants acclimate to indoor light conditions. Remember, full sun inside is about the same light as full shade outside. After a few weeks in the shade, remove any plumes from the grasses. Check both the plants and their soil for insects, and spray if needed. Find a location inside where the plants will receive as much sunlight as possible (a south or west exposure is best). Keep in mind that Phormium is a tropical plant and enjoys very warm temperatures, so be very careful to keep it away from drafts. Wait a couple of weeks and prune back the grasses to 6 inches above the soil line (not necessary with Phormium). Be sure to cut back on your watering through the low light, winter months. Next spring, after one more trim, set the plants outside in the shade again and gradually move them into the sunlight over a 2-week period.