Hazelnuts are monoecious, meaning they have separate male and female flowers on the same tree. Male and female flowers can bloom at different times. Hazelnuts are self-incompatible, which means that a tree cannot produce nuts with its own pollen. In addition, certain combinations of varieties are cross-incompatible.
That is, pollen in some varieties does not produce nuts in other varieties. Filbert trees need a pollinator to produce nuts. All varieties of filbert are not cross-pollinated. Rebecca McCluskey and others have done a lot of research at Oregon State University on pollination.
There are a number of issues that need to be aligned to make a good combination for pollinating hazelnuts. The time when pollen is released so that it coordinates with the timing of flowering is vital. Variety-compatible alleles are another very important aspect of pollination. If a hazel tree is more than five years old and has not yet produced nuts, the tree is likely to lose its mate.
Hazelnuts require cross-pollination of a different hazelnut cultivar to produce a nut crop. You must grow two hazel trees with strong genetic differences, one as a pollinator and one as a producer to get a nut crop. These trees must be about 65 feet apart from each other for cross-pollination to occur. All hazel and coconut varieties are effectively self-sterile, so you should always plant at least two different varieties.
The table below shows which varieties will pollinate. This publication provides a glossary of botanical terms related to hazel pollination and describes the process of flower and nut development, pollination, and related topics. However, hazel flowers have several pairs of long styles with surfaces that are stigmatic and receptive to pollen, and a small piece of tissue (0.25 mm or less) at the base called the ovarian meristem. Hazel trees are wind pollinated and there must be a compatible pollinator variety for effective pollination.
Some of the flowers in clusters that do not fall from the tree (because at least one nut has developed normally) are also subject to stopped growth. This period of 4 to 5 months between pollination and fertilization is one of the unusual characteristics of hazelnut flower biology. In addition, some varieties are cross-incompatible, making it even more difficult to pollinate hazel trees. I have found several wild trees that grow along the road where I live (west of Boone NC) and have planted 17 hazel trees from the Arbor Hazelnut Initiative.
The potential loss of these “developmental dropouts” amounts to 75— 85% of the total individual flowers produced by the tree. Native hybrid hazelnuts provide a crop that is constantly in short supply, is well known to consumers, and almost self-grown. Hazel pollination peak occurs from January to February, depending on weather conditions.