The Genus Yucca does only grow naturally in the new world, it consists of about 50 species in the USA and about 30 species in Mexico south to Guatemala (Yucca elephantipes). In the United States the Genus Yucca range from the Atlantic seashore  (Yucca filamentosa) to the Pacific coast (Yucca whipplei) and north into the southern part of Canada (Yucca glauca ssp. albertana). Only in the Pacific north western states and in the Atlantic north eastern states there are no Yucca species (except for those in cultivation!). In the old world and other places Yuccas has been introduced and has naturalized, but only vegetatively, while Yuccas need a specific moth to be pollinated. The only species that seems to produce seeds outside the new world is Yucca aloifolia, which make viable seeds in the old world. I have reports of fruits from: Spain (on several of the Spanish isle's), Italy, Switzerland (Bodensee) and Hungary (Budapest botanical garden). During the last 15 years the number of available man made hybrids has increased the palette of varieties suitable for cultivation in the colder parts of the world, so now there are Yucca hybrids that can be grown anywhere except of the extreme cold areas like Alaska.

Some species specially in the colder and northern parts of their range, Yucca species tend to be small and trunk less, but in the warmer and tropical areas, some species may become large branched trees up to 10 meter tall (Yucca brevifolia). 

The leaves of Yucca can be rigid, pliable or even lax. The leaf end can have a sharp or blunt spine or no spine at all. Many of the species have marginal filaments which may be colored brown, yellow or white, strait, curved, or curly. Others may have serrate leaves with many small sharp teeth.

The flowers of Yuccas are quite alike, most are bell like and they all have three sepals and three petals. The sepals and the petals are so similar that they are referred to as tepals. The tepals vary in color from greenish white to creamy white or clear white or even purple. (The flowers can be used in salad!)

Yucca ovaries are superior with three chambers that contain many ovules. The ovaries very much like those in the Liliaceae. About half the Yucca species have woody dry capsules (dehiscent) similar to those of an Agave, while the other half have fruits that are large and fleshy (indehiscent). The seeds are black thin and winged in the dehiscent species and thick (1-4 mm) in those that are indehiscent.

What really separates the genus Yucca from other flowering plants are their special way to get pollinated. The pollination can only be dome by a special moth in the Prodoxidae, that has been "programmed" to do it! The female Yucca moth collects a ball of pollen and flies to another Yucca flower where she squeeze the pollen down the pistil. But before she leaves the flower, she lays an egg in the ovary, there the larvae develop by eating some, but not all, of the maturing seeds. 
The scientist used to think that there only were 3 different Yucca moths; Tegeticula maculata which is specialized to pollinates Yucca whipplei, Tegeticula paradoxa which is specialized to pollinates Yucca brevifolia, and finally Tegeticula yuccasella which could pollinate all the other species of Yucca. But the recent research (Pellmyr 1999) has shown that there are many more species of Tegeticula at least 16 different species, which all are specialized in pollinating their "own" species of Yucca or Yucca species. It's actually possible that there are even more species of Tegeticula, while the research of the Yucca moths is still in progress. (Read more about Yucca moths)

It is possible to pollinate Yucca flowers by hand pollination, if you use two clones of Yucca plants, you will have a better chance to succeed.

So far I have only specialized in growing the hardy species of Yucca, therefore I have not written any pages about the Yucca species south of the US/Mexican border. But when I run out of writing about the US species I will start writing about the Species from Mexico and south of there!