Taxus

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Taxus

Yews
Taxus baccata MHNT.jpg
Genus: Taxus
Family: Taxaceae
Type: Small trees and shrubs
Bloom season: Mid spring
Pollination: Wind

Yews are small coniferous trees or shrubs in the yew family (Taxaceae). They are relatively slow growing and can be very long-lived, and reach heights of 1-40 m, with trunk diameters of up to 4 m. All species of yew contain highly poisonous alkaloids known as taxanes, with some variation in the exact formula of the alkaloid between the species. All parts of the tree except the arils contain the alkaloid. The arils are edible and sweet, but the seed is dangerously poisonous, and unlike birds, the human stomach can break down the seed coat and release the taxanes into the body. This can have fatal results if yew 'berries' are eaten without removing the seeds first. Grazing animals, particularly cattle and horses, are also sometimes found dead near yew trees after eating the leaves, though deer are able to break down the poisons and will eat yew foliage freely. In the wild, deer browsing of yews is often so extensive that wild yew trees are commonly restricted to cliffs and other steep slopes inaccessible to deer.

Description[edit]

They have reddish bark, lanceolate, flat, dark-green leaves 1-4 cm long and 2-3 mm broad, arranged spirally on the stem, but with the leaf bases twisted to align the leaves in two flat rows either side of the stem.

The seed cones are highly modified, each cone containing a single seed 4-7 mm long partly surrounded by a modified scale which develops into a soft, bright red berry-like structure called an aril, 8-15 mm long and wide and open at the end. The arils are mature 6-9 months after pollination, and with the seed contained are eaten by thrushes, waxwings and other birds, which disperse the hard seeds undamaged in their droppings; maturation of the arils is spread over 2-3 months, increasing the chances of successful seed dispersal. The male cones are globose, 3-6 mm diameter, and shed their pollen in early spring. Yews are mostly dioecious, but occasional individuals can be variably monoecious, or change sex with time.

Growing conditions[edit]

Species[edit]

All of the yews are very closely related to each other, and some botanists treat them all as subspecies or varieties of just one widespread species; under this treatment, the species name used is Taxus baccata, the first yew described scientifically.

The most distinct is the Sumatran Yew (T. sumatrana, native from Sumatra and Celebes north to southernmost China), distinguished by its sparse, sickle-shaped yellow-green leaves. The Mexican Yew (T. globosa, native to eastern Mexico south to Honduras) is also relatively distinct with foliage intermediate between Sumatran Yew and the other species. The Florida Yew, Mexican Yew and Pacific Yew are all rare species listed as threatened or endangered.

Uses[edit]

Male cones of Taxus baccata

Yew wood is reddish brown (with whiter sapwood), and is very springy. It was traditionally used to make bows, especially the longbow. Ötzi, the Chalcolithic mummy found in 1991 in the Austrian alps, carried an unfinished longbow made of yew wood. Consequently, it is not surprising that, in Norse mythology, the god of the bow, Ullr's abode had the name Ydalir (Yew dales). Most longbow wood used in northern Europe was imported from the Iberian Peninsula, where climatic conditions are better for growing the knot-free yew wood required.

Yews are widely used in landscaping and ornamental horticulture. Over 400 cultivars of yews have been named, the vast majority of these being derived from European Yew, Japanese Yew and the hybrid between them (Taxus x media). The most popular of these are the "Irish Yew" (Taxus baccata 'Fastigiata'), a fastigiate cultivar of the European Yew, and the several variants with yellow leaves, collectively known as golden yew.

The Pacific Yew Taxus brevifolia, native to the Pacific Northwest of North America, and Canada Yew Taxus canadensis are the sources of paclitaxel, a chemotherapy drug used in breast and lung cancer treatments. Over-harvesting of the Pacific Yew for this drug has resulted in it becoming an endangered species, though the drug is now produced semi-synthetically from cultivated yews, without the need to further endanger the wild populations. The more common Canada yew, Taxus canadensis, is also being successfully harvested in northern Ontario, Québec and New Brunswick, and has become another major source of paclitaxel.

Maintenance[edit]

Propagation[edit]

Harvesting[edit]

Pests and diseases[edit]

Foliage of Irish Yew; note the leaves spreading all round the erect shoots

Needle Blights

  • Herpotricha nigra
  • Sphaerulina taxi

Leaf Drop

  • Alternaria

Twig Blights

  • Phyllosticta hysterella (sexual stage: Physalospora gregaria)
  • Pestalotia funerea

Shpaeropsis Root Rot

  • Honey Fungus, mushroom rot: Armillaria mellea
  • Pythium

Dieback, Root Rot

Nematodes

  • Criconemoides:
  • Rotylenchus:

Scales

Mealybugs

Weevils

Termites

Ants

Mites

References[edit]

  • Ann Fowler Rhoads and Timothy A. Block (2000). The Plants of Pennsylvania: An Illustrated Manual. Anna Anisko, illustrator. Morris Arboretum, University of Pennsylvania Press. pp. 116. 
  • Christopher Brickell and Judith D. Zuk (1997). The American Horticultural Society A-Z Encyclopedia of Garden Plants. DK Publishing. pp. 1000-1001. 
  • Staff of the L. H. Bailey Hortorium (1976). Hortus Third: A Concise Dictionary of Plants Cultivated in the United States and Canada. Cornell University Press. pp. 1098-1099. 
  • Pirone, Pascal P. (1978). Diseases & Pests of Ornamental Plants (Fifth Edition ed.). John Wiley & Sons, New York. pp. 500-502. 
  • Cranshaw, Whitney (2004). Garden Insects of North America: The Ultimate Guide to Backyard Bugs. Princeton University Press. pp. 623. 
  • Pippa Greenwood, Andrew Halstead, A.R. Chase, Daniel Gilrein (2000). American Horticultural Society Pests & Diseases: The Complete Guide to Preventing, Identifying, and Treating Plant Problems (First Edition ed.). Dorling Kindersley (DK) Publishing, inc.. pp. 197.