Hazel trees are wind-pollinated and there must be a compatible pollinating variety for effective pollination. In addition, the flowering time of male and female flowers is important because the receptivity of female flowers must overlap with the time of pollen release. Since an American hazelnut (Corylus americana) has male and female flowers that can be cross-pollinated, it doesn't need another variety for pollination. Male growths develop in autumn, persist on the tree during winter and bloom in spring at the same time as female flowers.
The wind carries pollen from male flowers to female flowers and the fertilized flowers become nuts that ripen in autumn. Nuts are good to eat fresh or to add to baked goods. Filbert trees need a pollinator to produce nuts. All Filbert varieties 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 problems that need to be solved in order to match the pollination of hazelnuts well. The time of pollen release to coincide with the time of flowering is vital. The compatibility of alleles between varieties is another very important aspect of pollination.
Since the trees are monoecious, any hazel tree can produce nuts. However, in commercial production, producers generally plant two different varieties at a distance of 50 or 60 feet from each other to facilitate cross-pollination through the wind. Growers choose a variety that is a good pollinator and a second variety that is a good producer of nuts. The pollinator will produce its own crop of nuts, but the harvest will not be as heavy or of the same quality as the grower's nuts.
Members of the genus Corylus, including hazel trees, are monoecious, with flowers of both sexes on a single plant. American hazelnuts grow to 20 feet wide and tall, and can be left as a shrub or pruned into a tree. Although hazelnuts are monoecious (they have male and female flowers on the same tree), they are incompatible with each other, meaning that a tree cannot produce nuts with its own pollen. Because pollination cannot take place if cold temperatures damage flowers, hazelnut cultivation in the United States has been geographically limited to the Pacific Northwest.
Scientists at Rutgers University in New Jersey are working with improved and recently cultivated hazelnut varieties to create even better varieties that can be grown for both commercial and ornamental use in the eastern United States and other similar climate zones. Usually, in hazelnut orchards, three pollinating varieties (those that pollinate at the beginning, middle and end of the season) are placed throughout the orchard, not in solid rows. Hazelnut or hazelnut trees produce nuts after about four years, but they don't reach peak production until they are at least 7 years old. Most plant flowers have an ovary containing ovules with eggs prepared for fertilization, but hazelnut flowers have several pairs of long styles with stigmatic surfaces receptive to receiving pollen and a small portion of tissue at their base called an ovarian meristem.
The maximum pollination of hazelnuts occurs from January to February, depending on weather conditions.