Planted mid summer 2020, my two painted ferns have struggled to grow, and are less than a foot in diameter now, and about 6 in. high. Admittedly I have not fertilized them to this point, but earlier this summer they received some general flowering plant food that went to their astilbe neighbours. They too are struggling a bit.
Thank you for contacting Toronto Master Gardeners to inquire about your Japanese Painted Fern.
Japanese painted ferns are a native of the woodlands of Asia. In general these ferns prefer moist, well-drained soil rich in compost. This perennial griws best in partial to full shade. It is not drought tolerant and needs regular watering to keep the soil moist. Your fern will also benefit from adding 2 to 3 inches of compost to the soil each year.
You do not mention the site location for your ferns. Do you have the fern located in full sun(6+ hrs) part sun (4hrs), part shade or full shade? Do you water your ferns? We have less than average rainfall for the month of June and a lot of the plants require additional moisture. Are theferns sited in an area that is water-logged?
Here is an excellent article on Japanese Painted Ferns