Hazelnuts need another hazelnut for pollination. Unfortunately many hazelnuts are incompatible with each other. Essentially each hazelnut has two genes in its female flower, for example Barcelona has S1 & S2 in its female flower. One or both of these genes are also in the male flower, using Barcelona again its male flower has only the S1 allele. Hazelnuts with the same genes can’t pollinate each other, so any hazelnut that has the S1 gene in its female flower cannot be pollinated by Barcelona. That’s it in a nutshell. The chart below gives an easy list of the different varieties and their pollinators.
This doesn’t take into account early, mid and late season blooming times. That is another chart for another day. It is highly recommended that you have at least 3 different pollinizers in an orchard plot to cover the different timing of pollen release. Flowers are open and receptive for a longer period of time than the pollen is released from each variety. The different pollinizers need to cover early, mid and late release, set to the timing of the bloom from your main crop variety. For example, Jefferson is a late bloomer. It needs pollinizers that shed pollen later and much later than an early bloomer like Yamhill.