I planted this grass about 14 years ago. It took two or three years to really take off, then every year increased in volume and height (3-4m). Then last year, it grew about half the previous volume of shoots, and they were not as tall. This year, as of June 10, I can only see about 5-6 shoots, and am worried that it is dying. Over the years, there has been no change in the amount of sun/moisture it receives or other plants nearby. I will admit to not fertilizing it in recent years and don’t remember whether I bothered much with fertilizer in previous years, because it appeared to be doing so well. Location of plant is at a vacation property near Orangeville. Please note: About 2 or 3 years ago, I started cutting the canes off in September near ground level (taking care not to pull up the roots). Previous to that, I had let the canes dry out over the winter, but found they were difficult to cut in the spring. Thanks so much! (Photo is from Google Images. Feel free to edit for publishing.)
It is not uncommon for clumping grasses such as Miscanthus floridulus to turn brown and die out in the centre over time but your situation sounds a little more drastic. In cases like this where a long established plant starts to decline I always look for what changed. I don’t think a lack of fertilizer is the issue as grasses do not have high fertility requirements. However, I note that you started cutting down the canes in the fall several years ago and the plant has declined more each year. I found a description of this plant and it’s care on the Missouri Botanical Garden site (link below). They advise leaving the clump standing during the winter for visual interest but also to provide protection to the crown. Your plant may be suffering from winter damage to the crown.
Dividing the clump should rejuvenate the plant but it will take some time to develop to it’s former size. Dividing warm season grasses is best done in the early spring but I don’t think it’s too late to do this now as long as you’re willing to water frequently over the summer. I’ve copied a link below with detailed instructions on dividing miscanthus. This will not be a simple task given the size and age of your grass. As the instructions note, an ax may be necessary to slice through the bigger roots.