Like all other types of bark, cinnamon contains anti-fungal substances and cinnamon on seedlings (never tried it with iris seedlings) that are prone to damping-off is a time-tested method and I have used it with success.
Cinnamon is also believed to have other beneficial effects on plants:
https://www.gardeningknowhow.c...
http://www.sun-gazing.com/spri...
Cinnamon and other spices on seedlings:
http://theadventurebite.com/da...
