|I have lots of perennial outdoor ferns. But they have turned brown and have these hard brown centers that look like 4|
|I think what you are seeing are fertile fronds, they are hard and brown and short and emerge in late summer. They are normal and there is nothing that needs to be done about them.
If the older fronds are also turning brown in late summer, it might be the result of stress due to lack of humidity or lack of soil moisture or too much sun eventually taking its toll on the plant over the course of the summer.
Many ferns will show some stress by late summer. You can try to reduce the effects by watering as needed to keep the soil slightly moist, never dried out, and mulching with an organic mulch such as chopped leaves. Planting in soil that is organic and humusy is also important.
Too much hot sun can cause browning also. Most ferns prefer a shady location from full shade to a spot with dappled light or perhaps some gentle morning sun. Noontime and afternoon sun are too harsh and can cause them to brown in summer. Some ferns are more tolerant of sun than others.
I hope this helps you trouble shoot.