Filberts are varieties and hybrids of Corylus Avellana and of C. maxima, both natives of the Old World. In Europe, the term Filbert is used for those varieties with tubular husks much longer than the nut, which is usually oblong; Cob Nuts are roundish and angular with the husks about the length of the nuts. This distinction is not maintained in North America, where all varieties of C. Avellana are Filberts and the native species of Corylus are Hazelnuts.
Filberts are not grown much in the eastern United States, but a commercial Filbert industry is developing rapidly in Oregon and Washington where climatic conditions are favorable. Filbert blight, low winter temperatures, and the early blooming habit which causes the flowers to be subjected to severe temperatures, have limited Filbert culture east of the Rocky Mountains. In favored regions, particularly in the northern part of Peach-growing areas, Filberts have been grown with some degree of success.
Site and Soil. A northern slope is preferable as it delays blooming, which should not occur, if possible, until temperatures lower than 15 degrees above zero are no longer to be expected. Warm, sheltered spots near buildings and southern slopes should be avoided. Shelter from prevailing winds is also desirable. In the East the native Hazel, C. americana, serves as a host to the Filbert blight fungus, to which C. Avellana varieties are very susceptible. Native Hazels should be avoided or destroyed if C. Avellana varieties are to be grown nearby.
Filberts have no special requirements as to soil except that it be well drained and moderately fertile. Soils suitable for fruit trees are suitable for Filberts.
Planting and Care of the Planting. Planting, soil management and fertilization are similar to that required by the Apple. In New York the trees are spaced 18 ft. apart each way, but in Oregon a spacing of 25 ft. or more is recommended.
Pruning. The tree at planting time is pruned as are the fruit trees; it is headed at two feet above the ground with 4 to 6 scaffold branches well distributed to avoid the formation of weak crotches. Until bearing age, pruning should be light and corrective only.
The Filbert bears its nuts laterally and terminally on wood of the previous season's growth; and pruning, after the tree has come into bearing, should be sufficient to stimulate a moderate amount of new growth each year. A moderate thinning without heading back should accomplish this. In general, Filberts should receive about the same type of pruning as Peaches, but less severe. Severe pruning should be avoided, as winter killing may result. Pruning should be done at the close of the blooming period, or early April in central New York, at which time the catkins have shed their pollen.
Some varieties of Filberts sucker profusely, and unless these growths are removed the plant will become a multistemmed bush instead of a standard tree with a single stem. The standard tree is easier to manage than the bush; hence the suckers should be subdued as they appear. As the tree becomes older fewer suckers are produced.
Propagation. Filberts are commonly propagated by layering, and are consequently on their own roots, a distinct advantage in cold regions where the tree might occasionally die back to. the ground.
If only a few plants are wanted, soil may be mounded up around the suckers in the spring to a depth of several inches. By the following spring, roots will have developed at the base of the sucker which is then taken up and grown for a year in the nursery before setting in a permanent location.
Another method of layering is to peg a sucker down to the bottom of a trench several inches deep in the spring, just as growth is starting. When the shoots from the buds have reached a height of several inches, soil is carefully worked around them to a depth of two or three inches. This depth is increased at intervals through the season. If the shoots are well rooted by fall, they may be dug then, or in the spring, and grown for a year in the nursery.
Filberts may be raised from seeds, but the varieties do not come true. Seeds require after-ripening for germination which may be done by stratifying them in sand over winter, protected from rodents by wire netting. In the spring the seeds are planted in the nursery for two years.
Pollination. The filbert is monoecious; that is, the staminate and pistillate flowers are borne separately on the same plant as in the case of Corn, and the pollen is transferred by the wind. All varieties require cross-pollination in order to produce nuts, and every planting should, therefore, have two or more varieties.
Filberts begin blooming about the middle of March at Geneva, N.Y., and continue until mid-April. In Oregon, blooming is from early January to late March.
Varieties. Barcelona is the principal variety in Oregon with Daviana and Du Chilly being grown as pollenizers. A few others are grown to a limited extent.
These varieties are not so hardy or so productive as others in New York where Cosford, Medium Long and Italian Red have been the best of over 100 varieties. Purple Aveline is grown for its deep-red foliage in the spring which becomes greener and less conspicuous as the season advances.
Rush and Winkler are selections of the native Hazel, C. americana, that are much hardier, but with much smaller nuts than the C. Avellana varieties.
Hybrids between Rush and various C. Avellana varieties have been produced and some of these promise to be of real value in the East when sufficiently tested and planting stock is generally available. Among them are Bixby, Buchanan, Reed and Potomac. Mixed hybrid seedlings are also being sold as Jones hybrids.
Harvesting and Storing. The nuts are harvested by picking them off the ground. After drying, they should be stored in an unheated building where outside temperatures and humidities prevail. They soon become rancid if stored at room temperatures.