Hardy Yuccas

by Trevor Key
Chamaerops No. 43-44, published online 05-08-2002

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Left: Yucca gloriosa grows only short trunks. Old plants can form large clumps.
Right: Yucca recurvifolia flowers profusely.

Since I became interested in Yuccas I have been surprised by the number of people who say " I have never noticed them before" even from some keen and established gardeners. Yet they have been in Britain since the middle of the sixteenth century and some are extremely well established in Europe, particularly around the Mediterranean, so much so that many people believe them to be endemic to that area.

My own interest started in 1990 when I acquired a specimen of Yucca filamentosa in flower and was impressed by a flower spike some five feet tall with hundreds of flowers which although lasting individually for only 48 hours or so the panicle lasted for three to four weeks. I tried to obtain other specimens of the genus, information and seeds, which proved difficult and so as I gathered more data on these plants I became hooked as a 'Yuccaphile'. My own collection of plants and information has grown but it is still difficult for any major detail about these plants to be obtained by the average gardener.

There are in excess of 50 species of Yucca ranging from ground hugging rosettes to tall 40 foot trees. Most are endemic to the western half of southern USA down to California and Mexico and are extremely well adapted to life in the deserts and foothills of the Rockies. Examples of the western Yuccas are not normally found in gardens but appear as specimens in botanical collections since generally they are slow growing and take a number of years to mature and flower. Yucca glauca is one that is occasionally offered for sale and will produce a rosette similar to Y. filamentosa but having tougher and thinner leaves which can have spiked ends (Names and data to follow).

The examples most commonly seen and offered for sale belong to a group which I term the eastern Yuccas. They are the ones which were discovered first and have become the popular plants which we frequently see in our gardens and scattered around the Mediterranean: Yucca aloifolia, Y. filamentosa, Y. flaccida, Y. gloriosa and Y. recurvifolia. Their native habitat extends across the Florida peninsula, the Carolinas, Georgia, Alabama, Tennessee and to the eastern edge of Mississippi. All the above including a number of garden cultivars and hybrids were first put into a good semblance of order by William Trelease in 1902 and the following detail is mainly based on his criteria.

Yucca aloifolia (Spanish Bayonet)

A tree form which in its habitat can achieve a height of 7 m. In my experience I find that specimens can grow to 1.5 m within 5 years in a cold greenhouse. Not entirely frost hardy in northern Europe and requires some protection. Ideal for a conservatory and placing outside in summer. Occasionally available from nurseries and is easily grown from seed. The leaves are finely toothed along their edges and tipped with very sharp spines. Not recommended where it can be a danger to children or animals. It is very common on the east coast of Florida and Georgia and has become established on most of the Caribbean islands due to human intervention. A number of variegated garden forms exist but these are more difficult to obtain.

Yucca filamentosa

One of the most popular, very hardy, easy to grow and readily available. It forms a rosette at ground level and flowers well after 3-4 years and if planted in numbers or allowed to form a reasonably sized clump will produce one or more flowers each year. In habitat it is widely dispersed throughout Florida, Georgia, North and South Carolina. Leaves 3 - 5 cm wide, 60 - 75 cm in length. Flower stalk up to 1.5 m tall can be simple or branched.

Yucca flaccida

This species is as popular and as easily obtained as Y. filamentosa and although classed as a separate species is thought by some to be a more northern form of Y. filamentosa, widely dispersed throughout northern Alabama and Eastern Tennessee. Trelease noted a number of variations between the two above species. It is now very difficult to find any Y. flaccida or Y. filamentosa with proven provenance. Trelease (1902) admitted that these two, due to their long use as garden plants even in 1902 had led to the possibility of numerous hybrids but that the two were separable. In Y. flaccida the leaves are broader and recurve more strongly. The flower spike is downy when in its early stages. The most common form seen is "Ivory" having cream flowers much paler than any other and standing out at right angles instead of hanging down.

Yucca gloriosa

Occurs as a mainly coastal plant in habitat from the southern edge of South Carolina through Georgia to the north west tip of Florida. The plant has been in cultivation since 1896 and several garden forms have been described in which the leaves vary in length, thickness, colour and also the flowers vary in the shape of the raceme, being simple or complex. Y. gloriosa will form a small 'tree' having a stem up to 2 m high with numerous rosettes.

(A recently published book, not available at the time this article was written, "Agaves, Yuccas, and Related Plants" by Mary and Gary Irish, claims that Y. gloriosa is a plant with multiple trunks up to 4.6m tall, thin and pliable leaves 30 to 50cm long with very minute serrations on the edges and an acute but spineless tip. It is said to be hardy to about -6°C. This description corresponds with plants commonly grown as indoor plants all over Europe and invariably labelled Y. elephantipes. The true Y. elephantipes, according to the Irish' book should be a similar but much larger plant with leaves around 1m in length and is very sensitive to frost. Such plants can be commonly found in cultivation for instance in southern Spain, southern Italy and California among others. If this is true, the hardy plants commonly cultivated as and treated in this article under the name Y. gloriosa would then have to be regarded as forms of Y. recurvifolia or possibly as hybrids between this and another species. Hopefully, a book currently under preparation by renowned Yucca expert Fritz Hochstätter will clarify this problem once it is ready for publication. In the meantime, please refer to "Agaves, Yuccas and Related Plants" by Mary and Gary Irish, which can be obtained through the EPS bookstore at www.palmsociety.org. ed.)

Yucca recurvifolia

This species is similar to Y. gloriosa except that the leaves are longer, softer and smoother. They recurve naturally forming a nicely rounded plant with the outer leaves drooping. Again there are a number of garden forms which have been developed and named over the years. The habitat area is more limited than that of Y. gloriosa, to the coast of central Georgia. I have found from my own experience that a number of cultivars of gloriosa and recurvifolia are often misnamed.

Cultivation and garden value

The Yuccas as earlier stated have been available as garden plants for a considerable time. They were extremely popular towards the end of the 19th century and on into the early 20th century. At this time they were put into some botanical order by Trelease at the Missouri Botanical Garden. Later work has been carried out by other botanists and though considerable it is not easy to locate. Much work was done at the turn of the century on hybridisation. However after a hundred years or so, most hybrids have been lost in cultivation.

Most of the large gardens have examples of Yuccas as specimen plants, the rarer ones being found in botanical gardens and I have found it ironic that where a considerable number of plants are named these are just known as a "Yucca".

Y. gloriosa and Y. recurvifolia are particularly impressive as specimen plants especially in flower. Y. filamentosa and Y. flaccida need to be seen flowering in numbers especially on a good moonlit night!

Soil and position are not demanding provided the area is well-drained and plenty of sunlight is available. Specimens can be grown for foliage value by deliberately keeping them in the shade.

Seed is not always readily available and has to be obtained from specialised sources. Propagation from non-rooted cuttings is very easy. Remove a suitable cutting, treat with rooting compound and place in a mixture of compost and mainly sand until roots are established, then re-pot or place in the garden. If grown in pots for the patio it must be stressed that they need a large amount of root space otherwise plants will be "bonsaied." Plants are available from most plant centres. A relatively large plant can be expensive but then it may be 15 to 20 years old.

Catalogue of names

Yucca aloifolia
can easily be grown from seed or cutting
The following garden forms are listed for interest and although sometimes found in botanical gardens, I have not yet obtained any specimens.

purpurea purplish leaved
marginata edges tinged with yellow or white
tricolor median yellow or white band bordered green
conspicua clustered trunks, leaves broader
tenuifolia leaves falcate and purplish


å Yucca filamentosa

Variegated forms. A number of variegated forms are available and more seem to be coming on to the market. They vary from being merely striated to having edges or median lines of yellow or white. Various names are used e.g. Bright Edge, Golden Sword. I have a number of these which have been planted out for a few years. Although attractive none have reached the stature of normal plants. They all appear to be somewhat less vigorous in size, number of leaves and size of flower. Leaves up to 45 cm long and flowers not more than 1 metre.

bracteata very large form
concava larger plicate leaves


Yucca flaccida

f. orchi odes simple flowered form
glaucescens more glaucous and large complex flower
"Ivory" a more recent, pale flowered form

Other names do exist but I have not been able to locate examples.

Yucca gloriosa and Yucca recurvifolia
I have grouped these two together since I have found that due to misnamed specimens and earlier hybridising attempts there is considerable overlapping of the two. I have collected a number of Y. gloriosa and Y. recurvifolia specimens from various gardens which show the variations possible. The leaves of Y. gloriosa are much shorter than Y. recurvifolia and are very plicate. Y. recurvifolia leaves are at least twice as long and have a smooth surface often serrulate (finely toothed edges). I cannot give data as to differences in flower as I have not seen enough in flower. Trelease has suggested that Y. recurvifolia may be a long established hybrid of Y. gloriosa and Y. flaccida.

Additionally several hybrids between the preceding named species were apparently produced between 1874 and 1910 by Willy Müller, Andre Deleuil and Karl Sprenger. It is possible that some of these remain in cultivation somewhere. Although many of their original names and provenance have been lost, a few have retained their identity and include:

Yucca gloriosa nobilis Leaves very similar to filamentosa but with spiny tips
Yucca X floribunda data indicates this to be a Y. gloriosa X Y. filamentosa hybrid
Yucca Emmanuel II Vittorio this is apparently a popular and well distributed hybrid from 1907 by Sprenger and would appear to be a hybrid of Y. gloriosa and Y. aloifolia

Uses of Yucca

In Europe, Yuccas generally have been used for ornamental qualities in the garden. None of the Yuccas have any poisonous or toxic properties. In fact, most parts of the plants can be used for making household items or are edible. The Yucca was most important to the native Americans but their use has declined since colonisation from Europe. More recently Yucca juice has been used a treatment for arthritis in horses. Yucca leaves have also been tried on pig farms to reduce the ammonia content of their urine. Attempts to grow Yucca on a large scale as a source of fibre have not been very successful.

Propagation and seed availability

Propagation is quite simple involving removal of a suitable offshoot or semi-rooted cutting. Treatment with hormone rooting compound helps and a compost comprising at least 50% sand is suitable. Shoots can be obtained after flowering. The flowers appear from the centre of each rosette and after flowering that rosette will not produce further flowers until a shoot from beneath has grown to a suitable size. Removal of rosettes which have flowered is possible provided that this will not spoil the appearance of the plant. This will improve the growth of subsequent rosettes and the removed stem can have all its leaves reduced to approximately 2" and be potted up. Any shoots from these can then be removed when they reach a suitable size to produce more plants.

All the Yuccas can be grown easily from seed when available. Growth in the first few weeks is rapid and small plants will first produce a mass of roots compared to their size above soil level. Most of the Yuccas mentioned here will produce flowers within five years. In the case of the western Yuccas (see list below), growth is slower and some plants may not flower for a considerable number of years. I have found that seed is available from the following source:

Mesa Garden, P.O.Box 72, Belen, New Mexico 87002, USA.

Pollination

I mention and describe pollination since it is a specialised procedure carried out by moths which have a symbiotic relationship with the plants. The female moths collect a quantity of pollen and place it on the stigma of individual flowers and then pierce the ovary and lay one or two eggs. The larvae develop with the seed pod and utilise a small percentage of the eggs. They then drop off and pupate in the ground until the following year when fresh flowers and moths appear.

That the plants can be pollinated by hand is evident from the results of earlier enthusiasts. I myself have produced seed of Y. filamentosa in two years to date 1993 and 1995 and these have produced two batches of plants in the garden. One of these batches has produced a number of flowers in 1998 and the second should flower in 2000.

Brief description & names of the western yuccas

angustifolia stemless, alternative for Y. glauca
angustissima stemless. Similar to Y. glauca
arizonica stemless
arkansana stemless
baccata eventually short stemmed. Leaves straight
baccata var. vespertina short stemmed, leaves falcate
brevifolia large desert tree
brevifolia var. jaegeriana tree, smaller than above
carnerosana tree
coahuilensis stemless, recent discovery
constricta short stem
elata small tree
elephantipes the supermarket yucca (enough said!)
faxoniana tree
filifera tree
glauca short stem
garneyi short stem, similar to Y. glauca
harrimanae small stemless variety
intermedia short stem similar to Y. glauca
kanabensis small tree similar to Y. elata
louisianensis stemless similar to Y. filamentosa
mohavensis short stemmed similar to Y. baccata
pallida stemless
radiosa short stem
queretaroensis new discovery
reverschoni short stem
rigida short stem
rostrata short stem
rupicola stemless
schipigera stemless
schotti short stem
thompsoniana stemless
torcellii short stem, similar to Y. baccata
torreyi short stem similar to torcellii
treculeana short stem
valida short stem
whipplei stemless, has 5 subspecies varying in size
whipplei ssp. parshii the smallest up to 30 cm high
whipplei ssp percursa up to 18 inch
whipplei ssp intermedia up to 2 inch
whipplei ssp caespitosa forms a number of offsets
whipplei ssp whipplei up to 3 inch


References

The following are references which have been published in Great Britain.

Bean W. J. Trees and shrubs hardy in the British Isles. pp 761 - 770 inc. Bloomfield, F. (1985) Miracle plants Jojoba and Yucca (small booklet) Century Hutchinson Ltd.

Bishop, J. (1994) Yuccas in cultivation. British Cactus & Succ. Journal, Dec 1994 pp. 136 - 143.

Borland, J. Yuccas. Growing from seed. Seed raising journal of Thompson & Morgan. Vol 4 No. 1 Winter 1989/1990, pp. 31 - 33.

Bowles, E.A. (1922) Yuccas for English Gardens. Journal of the RHS Vol 47 pp 105 - 109.

Bussell, J. (1971) Yuccas in Britain. Journal of the RHS pp 491- 495.

White, A. (1992) Yuccas not Yukkies. Chamaerops - Journal of the European Palm Society, July 1992 pp 17 - 20.

Anyone wishing to pursue research further could refer to the following references through the libraries system - these are the major works available to my knowledge.

Matuda & Lujan, P. (1980) Las plantas mexicanas del genero Yucca.

McKelvey, S. D. Yuccas of the southwestern United States Part 1 (1938), Part 2 (1947).

Mitich, L. W. Uses of the genus Yucca. Excelsa No 7 Dec 1977.
Molon, G. (1914) Le Yucche.

Trelease, W. (1902) The Yucceae. Report of the Missouri Botanical Garden No 13, pp27 - 133.

Trelease, W. (1907) Additions to the genus Yucca. Report of the Missouri Botanical Garden No. 18 pp 225 - 230.

In addition the pollination process by Yucca moths is detailed in:

Riley, C. V. (1892) The Yucca moth and Yucca pollination. Report of the Missouri Botanical Garden No. 3 pp 99 - 158 + plates.

Collections

In Britain, there are two National Collections of Yuccas available for viewing.

My own which is held at Renishaw Hall, Eckington, nr. Sheffield, by permission of Sir Reresby and Lady Sitwell is open to the public with the garden on Friday, Saturday & Sunday from Easter to September (10 am - 4:30 pm). Viewing by appointment can be arranged by writing to:

Trevor Key, 15 Newbold Avenue, Newbold, Chesterfield, S41 7AR, U.K.

The second collection is held by:
Colin Smith, "Spring View", 10, Spring Close, Burwell, Cambridge, CBS 0HF, U.K., Telephone: (01638) 742993
Open by appointment.

Editor's note: For further details consult the NCCPG National Plant Collections Directory 2000 or visit their website - www.nccpg.org.uk

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