The fertile fronds in ostrich fern are long-lived. Similarly to those of sensitive fern, they are going to overwinter and disperse spores when there is no foliage around. That's why ostrich fern does not need to produce tall fertile fronds. (In cinnamon fern fertile fronds have to be taller than sterile. They last for only about a month and disperse spores while the green sterile fronds obstruct dissemination.)