Q: I harvested one lone filbert nut on my tree. When will the tree mature and give me more?
A: Congratulations! You have accomplished the typical yield from a single filbert tree. The reason is that filberts (and hazelnuts) have male and female flowers on the same plant but the pollen is self-incompatible. Filberts require another tree nearby to provide appropriate pollen.
If you plant another tree, let me know how your yield increases.