Previously in Family Liliaceae they are now in Family Asparagaceae > Subfamily Agavoideae.
There are up to 50 species and 30 to 40 subspecies, varieties and cultivars.
A few of them are commonly known as Spanish Dagger or Spanish Bayonet.
Various species and cultivars are sold in Australia as garden ornamentals and for pot plants.
They are slow growing perennial plants with rhizomes (underground stems).
Some are shrub-like with a basal rosette of leaves on a woody base.
These have no trunk (above ground stem) or a short one hidden by the leaves.
Others are like trees with the rosette of leaves on a thick woody trunk up to 5 m or more high.
The leaves, with no petioles are alternately arranged in a tight spiral.
The sword-shaped, linear to lanceolate leaves can up to around 75 cm long and 6 cm wide.
Many species have straight rigid leaves but some are softer and drooping.
Some are semi-succulent.
The leaf margin can be smooth or toothed and the tip is usually a sharp pointed spine.
They can be green, yellow-green or bluish-green and some have a reddish tint.
There are variegated cultivars.
The typically erect inflorescence peduncle is thick and can be nearly 4 m high.
The lower part, often hidden by the leaves may have a few leaves while the upper half has side
branches with bracts at their base.
The branches hold masses of drooping bell-shaped or globular flowers.
Most yuccas continue to grow after flowering unlike agaves that usually die.
The 6 fleshy tepals may be joined at the base or free.
They are white to cream and may be tinged in green or purple to a varying degree.
There are 6 stamens with the fleshy filaments somewhat flattened.
The filaments may have some papillae on them.
The oblong superior ovary has 3 locules but false septa may make that six.
There are septal nectaries in the grooves at the base of the ovary.
There is a single thick style with 1 or 3 small stigmas.
The fruit can be a capsule or soft and berry-like.
The capsules open at the top both into the chambers and at the septa between them.
Each chamber has many flat round, black seeds.