Grey alder sheds its leaves annually and grows up to 20 metres. It is common in barren landscapes, woods and by lakes and streams, often in misty forests. It is used to enrich soil because of its ability to bind nitrogen. Alder has figured in mythology and folk tradition. Goethe wrote a summary of many beliefs surrounding alder in Erlkönig, the Alder King.
Grey alder has minuscule flowers and blooms in the early spring. Grey alder belongs to the same family (Betulaceae) as Birch. Common, shared allergens could be expected and have also been found in these two species. Cross-reactivity between grey alder and Oak has also been reported.
White elm is a tall deciduous tree up to 40 metres and is famous for its vase-shaped crown. It occurs in woods, hedges and by roads and streams. It is often cultivated and is a favourite avenue tree in cities and parks.
White elm can be found mostly in forests in central and eastern North America where it represents a major source of pollen. The corresponding European species, Wych elm (U. glabra) and Smooth elm (U. carpinifolia) are distributed or cultivated throughout the north temperate regions. Fossils of elm have been found in Greenland and Alaska.
White elm flowers in the early spring while other species of the genus flower in the fall.
The olive is a small, evergreen tree with a broad round crown and a thick and knotty trunk. It is one of the slowest-growing trees in the Mediterranean area, It is said that trees in the olive grove at Gethsemani were there in Jesus Christ's time. It is famous for its fruits and oil and by-products used in making soap.
Historically the olive tree has been cultivated around the Mediterranean and gives that region a special character. It was later introduced in North America, South Africa and Australia.
The olive tree flowers in the spring and the fruit ripens in the fall and winter. Cross-reactivity between olive and Ash has been observed. At least 10 allergenic components have been identified from olive pollen.
London plane is a tall deciduous tree. It is easily recognised by the bark which exfoliates in large flakes. It is very popular for planting as a shade and ornamental tree in cities as they do not suffer much from dust and smoke and can stand where other trees usually die.
The origin of this hybrid tree is uncertain. It is usually found planted in southern and central Europe to western Asia, North America, South Africa, Australia and New Zealand, especially in urban areas. In America the tree probably crossed with a native variety and thus looks different from the European species.
The London plane flowers in the summer. The different species within the plane tree genus are expected to be highly cross-reactive to each other. An allergenic glycoprotein compound has been isolated from the pollen extract of London plane.
Willow is a deciduous shrub or small tree, growing up to 10 metres in height. It occurs in woods, on shores, as scrub and in hedges. Some willow species grow very rapidly and are among the plants which are included for planting in extensive plantations as a source of energy. The wood is light and firm and yields salicine which has been used in head-ache tablets.
WiIlow is one of the first flowering trees in early spring when it is important for bees as it is rich in nectar. High cross-reactivity could be expected between different Salix species and to some extent to Populus species such as Cottonwood.
Cottonwood grows up to 45 metres in height, with upright branches forming an open broad crown. It occurs in woods. Several species are planted for shelter and for use as a timber tree because of their fast growth. It is important for the paper industry and used for production of matches.
Cottonwood flowers in early spring. The pollen hangs on reddish catkins. Extensive cross-reactivity could be expected between the species in the genus Populus as well as the species of the genus Salix such as Willow.
(Susan Erasmus, Health24)