Tree Fern Advice
I too have a tree fern in my garden that has produced smaller leaves this year. I think all the rain has washed all the nutrients out of the soil. Dicksonia Antarctica are surface feeders and will benefit from adding a deep mulching of leaf mulch and bark chips around the base of the plant. They have extensive root systems in the soil. If a Dicksonia is to thrive and produce good sized fronds (8 - 10 ft. long), it must be encouraged to develop a good basal root system. Over several years, Dicksonias can form a big mat of roots and this should be encouraged as it is not enough for the plant to survive on the moisture and feed absorbed by the 'trunk roots' alone.
Is there any reason why you have your tree ferns in pots? The plant will grow with far more vigour and prove to be hardier if it is planted in a good, free-draining, but moist, humus enriched soil. Large pots are OK as a temporary measure whilst you are deliberating a final planting site, but they are not ideal in the long term and even with the relatively shorter leaves produced, are likely to topple over in high wind.
The fronds should be left on the plant unless they have died off and then should be cut back. The green fronds continue to produce food for the plant. Removing them before they have died off reduces the amount of food produced resulting in shorter and fewer leaves the following season. A tree fern's performance is related to the previous year's growing conditions and these determine the amount of carbohydrates it manages to store in the leaf bases and central core. The more food manufactured and stored, the greater the quantity and quality of fronds produced during the ensuing growing season.
You may be interested in these Fleece Plant Jackets they are available in different sizes and are great for protecting tree ferns against frost. I use them and tie the fronds up before fitting the jacket over. I am going to use Strulch around the base of my tree ferns this winter too this will keep the moisture and nutrients in.
I hope this has helped and that you will consider moving your tree ferns into the ground, I think this is the way forward for you.