Q.Ferns
My ferns turn gray/brown and die down in late July or early Aug. Until that time they fill in nicely and grow to about 3-4 ft tall. We water pretty consistently and this still happens. They seem to kill out companion planting after a couple of years. They have spread and taken over the hosta plants that were planted in front of them in the bed.
This could be a watering issue as some ferns need heavy watering.
Certain cultivators are quite invasive and can be difficult to control as you mention. Digging up plants and removing is likely the only way to reduce the invasion into other garden areas.
I would treat the plants with Neem OIl as it may be a fungus issue you are experiencing. Late summer can bring on these issues.
Here are some links for you.
http://extension.uga.edu/publications/detail.html?number=B737
https://www.gardeningknowhow.com/ornamental/foliage/ferns/taking-care-of-outdoor-ferns.htm
https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm
