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Cut the plant down to 4 to 6 inches in early spring before new growth starts. When the center starts to die - you'll notice a lack of foliage in the middle, then dig up the entire plant, split it and replant. Some authors advise waiting to cut it down until after it is divided. It sometimes help to have something to grasp as dividing can be difficult. Some grasses have sharp edges so wear good gloves if your pennisetum is one of these.

Pennisetum Rubrum is one sterile cultivar of purple fountain grass. Propagation is through division of the clump.

Cutting the plums now would likely cause the plant to go into dormancy. Your plant should grow and look nice well into late summer and early Autumn and actually look great as a winter accent.

https://www.gardeningknowhow.com/ornamental/foliage/purple-fountain-grass/grow-purple-fountain-grass.htm

Ornamental plants, including flowers, shrubs and trees, may be sensitive to the herbicides in weed and feed products. For example, 2,4-D and MCPP can cause leaf cupping, stunted growth and may kill your plants.

To keep the product away from desired flowers, trees and fruit and vegetable plants, avoid applying when it's windy or if rain is predicted within 24 hours.

There is a lot of speculation on the actual hardiness of this grass. It appears that some cultivars can handle 20's with no problem, while others will die if it dips into the 30's at all. It could be possible that you had one of the not so hardy varieties. It can be hard to tell after it is already dead. I would make sure that you go for a cold hardy variety. This will ensure that even a mild winter will not harm them.

This article will help you.

https://www.gardeningknowhow.com/ornamental/foliage/ornamental-grass/propagating-ornamental-grass.htm

Many ornamental grasses are grown to not be self seeding, so germination rates may be very low. You may want to seek out seed from a grower.