- ≡ Lomaria aggregata Colenso, Trans. & Proc. New Zealand Inst. 20: 223 (1888)
- ≡ Austroblechnum ×aggregatum (Colenso) Gasper & V.A.O.Dittrich in Gasper et al., Phytotaxa 275: 203 (2016)
Blechnum ×aggregatum is the hybrid between B. chambersii and B. membranaceum, usually arising wherever the two species occur together, and characterised by its intermediate morphology. It is more easily recognised in the field than in the herbarium, possibly because the differences between the hybrid and its two parents are more obvious in live material. The fronds are of intermediate size and outline, but hybrid plants can sometimes be recognised by having relatively narrow, obovate sterile laminae, similar to those of B. membranaceum, but fertile fronds that are equal to or shorter than the sterile ones, like those in B. chambersii. Spores of both species have a very smooth perine (Large & Braggins 1991), and hybrid plants usually have spores of normal appearance. Only very rarely are malformed spores seen (e.g. WELT P001231, Pukeatua).
Blechnum chambersii and B. membranaceum are morphologically and genetically very similar. They differ by only one base pair (Shepherd et al. 2007). It is quite possible that hybrids have some degree of inter-fertility, and that hybrid swarms may include F1 and back-cross hybrids, along with the parent species. This requires further investigation.