(cob-nut, filbert). The genus Corylus (hazel) includes species native to temperate regions across the northern hemisphere. Of particular importance is Corylus avellana, the cob-nut, distributed throughout Eurasia; cultivated selections are called cob-nuts. Equally important is Corylus maxima, native to southeast Europe, which has given rise to the filbert, including the valuable ornamental purple-leaved nut. The filbert has a husk that is longer than the nut, often enclosing it completely; that of the cob-nut only partially covers the nut. In some filberts, the husk is markedly frilled and twisted; this group is known as frizzled filberts. In the US, the name filbert is commonly applied to both true filberts and hazelnuts.

Evidence of the widespread use of Corylus avellana stretches back to prehistoric times with many sites yielding samples of pollen, wood and fruit - for example, at prehistoric lake dwellings in Switzerland and at Neolithic, Bronze Age, Iron Age and Roman sites in Britain. The hardened shoots formed into faggots have in the past been used in roadmaking; other uses embraced basketmaking, hand implements, bird traps and many more. Records from Theophrastus and Pliny show that selected forms of cobs and filberts were widely cultivated by the Greeks and Romans as distinct from those collected from bushes in the wild. A particularly fine selection was introduced into Spain by the Romans and became known as the Barcelona nut. The nut is large and deeper red-brown in colour than the cob-nut, which it resembles, but the bush is not hardy and does not normally succeed in cool-temperate climates. ‘Barcelona’ accounts for 85% of filbert production in the Pacific Northwest. More recently, many interspecific crosses have been made, their parentage including the small-fruited Corylus americana.

By the 16th century, references show that the distinction between cob-nut and filbert had been recognised and selected forms were in cultivation. Named cultivars resulting from breeding programmes were introduced from the early 19th century as part of the surge of interest in fruit production at that time. Two of these remain famous to this day, the twin-shelled ‘Cosford Cob’ and ‘Lambert’s Filbert’. The latter is more commonly known as ‘Kentish Cob’, descriptively erroneous since it is a fine and true example of a filbert. Another excellent cultivar from the same era is the cob-nut ‘Pearson’s Prolific’. Other recommended cultivars include ‘Butler’ (widely used as a pollinator), ‘Cassima’, ‘Cosford Cob’, ‘Daviana’ (with long nuts), ‘Du Chilly’, ‘Ennis’, ‘Hall’s Giant’ (with large round nuts), ‘Royal’ (large oval nuts) and the red-leaved ‘Fortin’.

During the 19th century cob-nuts and filberts were planted in Kent for the London market; later in the season, Barcelona nuts were imported from Spain. These plantations continued into the 20th century but with soaring labour costs the acreage gradually declined. Today only one or two small areas remain and most samples are imported. The main centres of production are in Italy and Spain; manu nuts are grown in northern Turkey and a few in the northern US and in Australia and New Zealand.

Cob-nuts and filberts are only occasionally grown in gardens in Britain. They need considerable space and the possible depredation of the crop by squirrels can easily nullify their usefulness, although the growths make excellent pea sticks and bean poles. However, given freedom from this major pest they can yield excellent crops of good-quality nuts.

In the garden cob-nuts and filberts prefer a lightly shaded position (they are a natural woodland tree) which will give them some protection from frost and wind. The male flowers are the familiar catkins common in hedgerows in Europe in january and February, the small red female flowers are inconspicuous; both occur on the same bush. pollination is effected by wind but in cooler climates severe frosts can damage the catkins (though seldom the female flowers) and adversely affect the ‘set’ of the crop. Frost hollows should be avoided. Another hazard casued by weather conditions is the failure of pollen-bearing catkins and receptive female flowers to coincide; this can be overcome by planting more than one cultivar, but this can only really be contemplated for larger gardens.

Most garden soils are capable of giving reasonable results; those least suitable are heavy wet soils, hungary sands and very rich soils, which give rise to rank growth and poor cropping. The poorer soils will quickly dry out and this, in combination with lack of nutrients, may give small nuts of poor flavour. A pH of 6.0-7.0 is preferable.

Young bushes should be planted in winter about 4.5m apart. They are grown on their own roots, no rootstock being necessary. For ease of management, an open centre bush should be developed on a short stem of some 45cm with 8-12 main branches. Correct pruning should restrict bush height to 2m. If a one-year-old specimen is planted, the leading shoot should be pruned at 45cm to induce the development of branch leaders from the top 15cm. Any below this level should be removed. In the following winters, branch leaders are shortened by about one-third. Side-shoots crowding the centre of the tree are removed, as are any branch leaders surplus to requirements. In three or four years a shapely bush should have developed. Any basal shoots should be removed; suckers should be cut out with a spade (they can be used as new plants if necessary), not pruned off, which only causes them to proliferate. Once the desired height has been reached, branch leaders should where possible be cut back to a weak side-shoot. Any strong upright shoots should be removed, lateral growth being more conducive to cropping.

On the established bush, summer pruning plays and important role by the use of a technique called ‘brutting’. This consists of breaking by hand stronger side shoots at about half their length in August and allowing the broken part to hang by the rind, the shoot not being completely severed. This reduces the vigour of shoots and encourages the formation of a greater proportion of female flowers, thus increasing future crop potential. It also allows greater light and air penetration to improve the quality of the maturing crop. In winter the brutted shoots are cut back to 3-4 buds. In time some side-shoots become too thick and should be shortened to 3-5cm either with a pruning saw or loppers. Weaker, fruitful, new side-shoots should then develop from the stub. Other needs are the removal of growths that are over-crowded, particularly in the tree centre, and the regulation of height by cutting back to a weak lateral as already described. Winter pruning is best timed to coincide with flowering, the disturbance of shoots assisting pollination. On old bushes the occasional worn-out branch can be sawn out in favour of a strong new growth to replace it. Occasional applications of of a balanced compound fertiliser at 100g/m² are advisable in early spring, and the ground around the bush is best kept weed free. On poorer soils a mulch of compost, even a little rotted manure, is advantageous.

The nuts are best harvested as the husks yellow. Picked too soon they do not keep, while delay means dropped nuts being spoilt by mice (squirrels are often a major pest against which shooting is the only sure remedy). It should be noted that if the grower opts for this method that even though squirrels are classed as vermin in some areas, red squirrels (Sciurus vulgaris) are protected by law and penalties for interfering with them in any way can be severe. The North American grey (Sciurus carolinesis) is a non-indigenous species to Britain and can be removed as needed; but they must be properly despatched with minimal suffering to the animal. Failure to comply with the law in this regard is also a prosecutable offence. The nut crop should be laid out in trays in an airy room and disturbed and turned to ensure even drying. For prolonged storage, the nuts should be packed in layers in earthenware jars intermingled with coconut fibre mixed with a little salt. Protection from mice, and cool dry conditions, are essential.

Propagation is normally by rooted suckers of chosen cultivars. Grafting on to seedlings is possible but is not normally practised: seedlings will prove variable in performance and in the size and quality of nut produced.

A number of artificial hybrids have been produced, which include the Trazel, Corylus X colurnoides (Corylus avellana X Corylus colurna), e.g. ‘Laroka’, with large long nuts; the Hazelnut or Mildred filbert (Corylus avellana X Corylus americana), e.g. ‘Graham’, nut large, resistant to Eastern filbert blight; and the Filazel (Corylus avellana X Corylus cornuta), which are hardy into Z4, e.g. ‘Gellatly’s Earliest’.