Trees that have thorns

Pink floss-silk tree (Ceiba speciosa). Photographs by Don Walker

With their fat, thorny trunks and branches, tropical-looking foliage, and exotic, hibiscus-like flowers, the floss-silk trees are among the most distinctive ornamental trees for regions where frosts are not severe. Formerly placed in their own genus, Chorisia (and still sold under that name), these showy South American members of the bombax family (Bombacaceae) are closely related to the tropical kapok tree (Ceiba pentandra) and have recently been reclassified within that genus. Like the kapok tree, floss-silk trees have palmate leaves with five-inch-long leaflets and are known for their large seed pods, which contain copious amounts of a cottony fiber that has been used as stuffing in pillows and as insulation in parkas and other cold-weather clothing. Successful in Sunset zones 12-24, they are at their absolute best in the warm, dry climates of Southern California, where they are popular in both public and private landscapes.

Flowers of pink floss-silk tree (Ceiba speciosa)

Pink Floss-Silk Tree

Of the two species in cultivation, the showiest and most commonly grown is pink floss-silk tree, (Ceiba speciosa, formerly Chorisia speciosa). This striking tree is an attention-getter at all seasons because of its thick, heavy trunk, studded with large gray thorns. Bright green when young the trunk becomes gray as the tree matures. In the fall (or as early as midsummer in hot-summer areas), mature trees are a spectacular sight, covered with a profusion of five- to six-inch-wide, bright pink flowers.

Cottony seed masses released from the fruits of floss-silk tree (Ceiba)

Pink floss-silk trees grow quickly from seed and can become thirty-foot-tall specimens in a few years. Then it conveniently slows in growth, with older trees reaching an ultimate height of forty to sixty feet. Seedling trees typically have straight, thorny trunks and a columnar growth habit, but become more spreading as they age. Although trees grown from seed form a better-shaped tree, they usually do not flower well until they are seven to ten years old. It has been observed that it is more the size and not just the age of the tree that facilitates blooming; once initiated, flowering becomes more profuse each year as the tree ages. The oldest and largest pink floss-silk tree in the United States is on the grounds of the Hotel Bel-Air, west of Los Angeles. That tree, now ninety years old and more than ninety feet tall, with a trunk thirteen feet around, has been called the single most spectacular flowering tree in the country.

Pink floss-silk tree is native to a large area of South America, typically in regions that have a cool dry season in winter. The relatively dry winters and warm summers of Southern California encourage heavy flowering. As with many other subtropical trees, this one drops most or all of its leaves just before it blooms, which makes for an even more dramatic floral display. Although most seedlings in California have thorny trunks and magenta-pink flowers, some seedlings may be nearly thornless, and flower color may vary to light pink, dark burgundy, or even creamy yellow on some specimens. To preserve these specific characteristics, a number of named varieties are grown and sold as grafted specimens. Grafted pink floss-silk trees have the advantage of flowering well as young trees, but they also tend to be smaller growing, with a more spreading, less upright form. Although thornless varieties are frequently sold, I personally prefer the thorns!

White floss-silk tree (Ceiba insignis)

White Floss-Silk Tree

Native to western South America from Columbia to northern Argentina, white floss-silk tree (Ceiba insignis, formerly Chorisia insignis) is white-flowered, stouter, and less commonly planted. It grows at a moderate rate to around thirty feet tall and wide (old trees may be somewhat larger), typically with a fat, thorny trunk and spreading branches. Like its pink-flowered cousin, seedlings typically take seven to ten years before they attain blooming size, but become more floriferous with age. The flowers of white floss-silk tree are five inches wide and creamy white to pale yellow, appearing just after the tree sheds its foliage in fall and winter. A mature tree in bloom is an arresting sight, with thorns, flowers, and seedpods fully displayed on bare branches, giving it the look of an odd, gigantic rose tree.

Flower of white floss-silk tree (Ceiba insignis)

Cultural Tips

Both species of floss-silk tree love heat and are drought-tolerant, with mature trees getting by quite nicely on no more than monthly deep soakings. Although they grow quickly in deep, well-drained soils, they may be slower in heavier soils. Young floss-silk trees may suffer frost damage at 27° F, but older trees will tolerate temperatures down to the low 20s F for brief periods of time. Floss-silk trees have the ability to exist for many years in pots, but a large tree in a small nursery container is not necessarily a bargain, because its roots will be pot-bound and will take a long time to get established. As with trees such as eucalypts and oaks, it is always better to plant a younger specimen that is not pot-bound, so that a sturdy root system can more easily develop.

Fat thorns adorn the trunks of floss-silk trees (Ceiba)

Why do floss-silk trees have thorns? The usual answer is that, in their native South America, monkeys (and probably other animals as well) found the flower buds and young fruits of these trees quite tasty. The large thorns served to discourage animals from climbing the trees, especially when the trees are young and more vulnerable to damage. Since most monkeys in South America are arboreal with prehensile tails, they would likely not climb the trunks but would swing from neighboring trees to reach the fruit, making the thorns unnecessary. Yet, most structures such as thorns do have some evolutionary purpose. Perhaps, we should merely conclude that the thorns discourage some sort of activity that would be damaging to the trunk and thin bark of these glorious trees.

Adapted with permission from Ornamental Trees of San Diego, published in 2003 by the San Diego Horticultural Society.

Honeylocust

Introduction: Once an excellent lawn tree, honeylocust has been greatly overused and has developed a number of disease and insect problems that have made it a short-lived tree in many areas. The beautiful, fine-textured tree offers light shade, allowing grass to grow up to its trunk. Its bright green leaves turn yellow in fall. The long thorns of the species are not present on var. inermis. Honeylocust is not a long-lived plant for landscapes in the southern U.S.

Culture: Honeylocust tolerates a wide range of conditions but performs best in full sun on limestone soils or in rich, moist bottom lands. It will tolerate high pH, drought and salt, but any stress will make it more susceptible to diseases and insects. The tree, which should be pruned in fall, is easy to transplant, although its coarse root system requires a larger than average root ball. Honeylocust is susceptible to a number of problems including leafhopper, cankers, flat-headed borers, mimosa webworm, spider mites, powdery mildew, rust and leaf spot. Honeylocust can be a short-lived tree in our area.

Botanical Information

  • Native habitat: Pennsylvania to Nebraska, south to Texas and Mississippi.
  • Growth habit: Short trunk and an open, spreading crown; irregular silhouette.
  • Tree size: Varies from 30 to 70 feet tall with a similar spread. It can reach a height of 100 feet or more in the wild.
  • Flower and fruit: Greenish-yellow, perfect and imperfect flowers are borne on the same tree from May to June. Flowers are fragrant but not showy. Fruit is a reddish-brown to brown pod from 7 to 18 inches long and about an inch wide. It contains hard, oval seeds and is often irregularly twisted. There are cultivars that are fruitless.
  • Leaf: Alternate, pinnately or bipinnately compound, 6 to 8 inches long with 20 to 30 leaflets. Leaves are bright green in summer; fall color is yellow. Leaves drop early in fall.
  • Hardiness: Winter hardy to USDA Zone 4.

Selected cultivars:

There are many selections of honeylocust. All are thornless and most will not produce fruit. Popular cultivars of honeylocust include:

  • ‘Christie’ – Vigorous growth and a symmetrical, full crown.
  • ‘Emerald Kascade’ – A thornless weeping form with dark emerald-green foliage.
  • ‘Impcole’ or Imperial® – Small, rounded form with spreading branches and the most compact of the popular honeylocust cultivars.
  • ‘Moraine’ – Seedless, vase-shaped cultivar with a rounded top and better resistance to webworm.
  • ‘Shademaster’ – Has upright ascending then spreading branches that result in an irregular, vase-shaped form. It does not produce pods.
  • ‘Skyline’ – Pyramidal form with ascending branches and bright golden yellow foliage in fall.
  • ‘Sunburst’ – New foliage is bright yellow and contrasts with dark yellow-green interior foliage. Irregular form.

Additional information:
Honeylocust has been used extensively as a street and shade tree. It transplants easily at large sizes and casts a light shade. It complements many architectural features in the landscape. However, it has been overplanted in many areas and many insect and disease pests attack honeylocust. It may be a short-lived species in our area.

Gleditsia triacanthos has sharp thorns up to a foot long on the trunk and main branches; var. inermis is a thornless form, but will develop thorns about 3 inches long when the tree is stressed. Seed pods are rather unsightly as they remain on the tree into fall. They also cause significant litter problems. Many cultivars of honeylocust will not produce fruit until the trees are older.

In Florida, honeylocust is sometimes called Confederate pintree, because its spines were used to pin together the tattered uniforms of soldiers during the Civil War. People in the southern mountains once used the thorns of honeylocust in carding wool, and to pin the mouths of wool sacks.

National champion trees are in Michigan (116 feet), Virginia (104 feet) and Pennsylvania (90) feet.

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Thorns On Citrus Trees: Why Does My Citrus Plant Have Thorns?

No, it’s not an anomaly; there are thorns on citrus trees. Although not well known, it is a fact that most, not all, citrus fruit trees have thorns. Let’s learn more about thorns on a citrus tree.

Citrus Tree with Thorns

Citrus fruits fall into several categories such as:

  • Oranges (both sweet and sour)
  • Mandarins
  • Pomelos
  • Grapefruit
  • Lemons
  • Limes
  • Tangelos

All are members of the genus Citrus and many of them have thorns on the citrus trees. Classified as a member of the Citrus genus until 1915, at which time it was reclassified into the Fortunella genus, the sweet and tart kumquat is another citrus tree with thorns. Some of the most common citrus trees to sport thorns are Meyer lemon, most grapefruits and key limes.

Thorns on citrus trees develop at the nodes, often sprouting on new grafts and fruiting wood. Some citrus trees with thorns outgrow them as the tree matures. If you own a citrus variety and have noticed these spiky protuberances on the branches, your question may be,

“Why does my citrus plant have thorns?”

Why Does My Citrus Plant Have Thorns?

The presence of thorns on citrus trees has evolved for exactly the same reason that animals such as hedgehogs and porcupines sport prickly hides — protection from predators, specifically hungry animals that want to nibble away at the tender leaves and fruit. Vegetation is most delicate when the tree is young. For this reason, while many juvenile citrus have thorns, mature specimens often do not. Of course, this may cause some difficulty for the cultivator since the thorns make it difficult to harvest the fruit.

Most true lemons have sharp thorns lining the twigs, although some hybrids are almost thornless, such as ‘Eureka.’ The second most popular citrus fruit, the lime, also has thorns. Thornless cultivars are available, but supposedly lack flavor, are less productive and are thus less desirable.

Over time, the popularity and cultivation of many oranges has led to thornless varieties or those with small, blunt thorns found only at the base of the leaves. However, there are still plenty of orange varieties that have large thorns, generally those that are bitter and less often consumed.

Grapefruit trees have short, flexible thorns found only on the twigs with ‘Marsh’ the most sought after variety grown in the U.S. The little kumquat with its sweet, edible skin is primarily armed with thorns, like the ‘Hong Kong,” although others, such as ‘Meiwa,’ are thorn-less or have small minimally damaging spines.

Pruning Citrus Fruit Thorns

While many citrus trees grow thorns at some point during their life cycle, pruning them away will not damage the tree. Mature trees usually grow thorns less frequently than newly grafted trees that still have tender foliage needing protection.

Fruit growers who graft trees should remove thorns from the rootstock when grafting. Most other casual gardeners can safely prune the thorns for safety sake without fear of damaging the tree.

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Parts of a Tangerine Tree

tangerine tree image by Daria Miroshnikova from Fotolia.com

The tangerine tree is a tropical, fruit bearing tree. Tangerines originated in Southeastern Asia and are hardy in USDA zones 9 to 12. Commercial growers and hobbyists grow many varieties of tangerines successfully. Some growers use it strictly as an ornamental tree, selecting it for its dark green foliage and small white flowers. In some instances, growers can raise tangerine trees as a container plant indoors or in a greenhouse.

Foliage

Tangerine trees are dense evergreens with shiny leaves. The leaves of the tangerine tree are broad with small, rounded teeth. Slender lanceolate (lance-shaped) leaves have winged leaf stems. Trees will only produce fruit if they have produced a sufficient number of leaf nodes. Excessive pruning will reduce the number of leaves and the chance that the tree will bear fruit.

Fruit and Flower

The fruit of a tangerine tree is a tangerine. They are oblate, rather than completely spherical. The skin releases when only a minimal amount of force is applied to the fruit. The peel of the ripe fruit is orange-red or a deep orange. The fruit is produced at the ends of the stems, near the outer parts of the tree. Purdue University notes that the “fruit has long been appreciated for its distinctive and sweet flavor and aroma. They are used primarily for eating out-of-hand, as fruit sections, in fresh juice and to a limited extend for processing.” The fruit stops ripening when it is picked from the tree.

The flowers are small, yet fragrant. The flowers are white and most often have five petals. The flowers tend to be both male and female. Bees, butterflies and other insects pollinate the flowers by moving the pollen around the flower. The flowers usually appear in March or April.

Trunk and Stems

The tangerine tree is slender. The tree is smaller than the orange tree. Some tangerine trees grow to reach 25 feet high. Tangerine trees have thorns; however, some varieties, including ‘Fallglo’ and ‘Dancy,’ are generally thornless. The tangerine has an upright growing habit with a rounded top. The University of Florida IFAS Extension program says that the wood of the ‘Robinson’ and ‘Murcott’ varieties are brittle and their limbs will break under a very heavy crop load.

About Floss Silk Trees: Tips For Planting A Silk Floss Tree

Silk floss tree, or floss silk tree, whichever the correct name, this specimen has superb showy qualities. This deciduous tree is a true stunner and has the potential to attain a height of over 50 feet with an equal spread. Growing silk floss trees are found in their native tropics of Brazil and Argentina.

About Floss Silk Trees

Known almost interchangeably as silk floss tree or floss silk tree, this beauty may also be referred to as Kapok tree and is in the family of Bombacaceae (Ceiba speciosa – formerly Chorisia speciosa). The floss silk tree crown is uniform with green limbs branching upon which round palmate leaves form.

Growing silk floss trees have a thick green trunk, slightly bulging at maturity and peppered with thorns. During the autumn months (October-November), the tree bares lovely funnel-shaped pink flowers that

completely cover the canopy, followed by woody pear-shaped, 8-inch seed pods (fruit) containing silken “floss” entrenched with pea sized seeds. At one time, this floss was used to pad life jackets and pillows, while thin strips of floss silk’s bark were used to make rope.

Initially a fast grower, floss silk trees growth slows as it matures. Silk floss trees are useful along highway or median paving strips, residential streets, as specimen plants or shade trees on larger properties. The tree’s growth can be curtailed when used as a container plant or bonsai.

Care of Silk Floss Tree

When planting a silk floss tree, care should be taken to situate at least 15 feet away from the eaves to account for growth and well away from foot traffic and play areas due to the thorny trunk.

Floss silk tree care is possible in USDA zones 9-11, as saplings are frost sensitive, but mature trees can withstand temps to 20 F. (-6 C.) for limited time periods. Planting a silk floss tree should occur in full to part sun in well drained, moist, fertile soil.

Care of silk floss tree should include moderate irrigation with a reduction in the winter. Transplants are readily available in climate suitable areas or seeds can be sown from spring to early summer.

When planting a silk floss tree, the eventual size should be kept in mind, as leaf drop and fruit pod detritus can be hard on lawn mowers. Floss silk trees are also often affected by scale insects.

While not terribly common, many trees bear sharp thorns along their twigs, branches or trunks. Often, thorns are seen as problematic traits, and trees that bear them are often avoided during installations. Some have even produced thornless cultivars to address the issue. However, thorns are not always bad, and they can even help you accomplish goals in some cases.

Technical Terms

Many people use the terms thorn, spine and prickle interchangeably, but they do refer to different things. Thorns are modified shoots, and characteristic of hawthorns (Crataegus spp.), citrus trees (Citrus spp.) and honey locusts (Gleditsia triacanthos), among others. Spines are similar to thorns, in that they feature internal vascular tissue, but they arise from leaf tissues, not shoots. Spines adorn a number of shrubs, including barberry (Berberis vulgaris).

Prickles are the most divergent of the tree types, as they only attach to the surface of the stem or trunk, and have no vascular tissues connecting them to the main body of the plant. Perhaps not surprisingly, prickles (as are found on roses) are easier to remove from a stem or branch than spines or thorns are.

Primary Purpose

The primary reason trees developed thorns in the first place was to deter hungry herbivores. These mechanical deterrents are not the only strategy plants use for defense; others have evolved chemical defenses and produce urticating, noxious or toxic compounds. Nevertheless, sharp pokey things have stood the test of evolutionary time.

Thorns are not effective for deterring all primary consumers — they fail to deter arthropods, for example – but they are effective enough that thorns are worth the resources they require, which could be used elsewhere. Most herbivores prefer the easiest meal they can find, so when faced with the prospect of being stabbed repeatedly by a tree, they look elsewhere for food.

Nevertheless, thick hides and long tongues help many animals avoid thorns, and feed on the delicate foliage. In some places with ample herbivore pressure, thorns become very common. Much of the African Savannah, for example, is blanketed in a carpet of acacia thorns.

Serving Human Needs

With a bit of forethought and careful planning, thorn-laden trees can provide important services for people. While you may think that deliberately planting these spiked species is an exercise in arboricultural sadism, it is possible to harness the repulsive power of these trees and use their powers for good.

For example, many police departments recommend using thorn-bearing trees (or other noxious plants) to dissuade criminals from targeting your home and land. While even the thorniest trees fail to provide an impenetrable shield, they often offer enough resistance that your average criminal will avoid your home and look for easier pickings elsewhere.

Thorny trees also make good wildlife management tools, but they do so in two entirely different ways. For example, a hedge of hawthorns or honey locusts may encourage local deer to keep their distance, or a tangle of blackberry bushes (Rubus spp.) may prevent cats from lurking beneath bird feeders. Birds may preferentially chose such areas for nesting, as they presumably offer some protection from predators. Some birds, notably shrikes, even impale their prey on the thorns and spines of trees. So, you while you can repel some forms of wildlife with thorns, you can also attract desirable critters with thorny trees.

As you can see, tree care often means dealing with thorns. But you don’t have to deal with pin-cushion plants yourself. For residents of San Francisco, tree care is only a few clicks away! Contact Arborist Now and let us weather the thorns, spikes and prickles covering your trees and shrubs.

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Monday – July 21, 2014

From: Austin, TX
Region: Southwest
Topic: Invasive Plants, Problem Plants, Trees
Title: Fast-spreading desert-type tree with thorns in yard
Answered by: Nan Hampton

QUESTION:

There is a fast spreading tree in my backyard – many multiple almost symmetric flat green oval leaves on either side of the stem (sort of like a moringa tree but this is not that). Grows straight up, has thorns on it. Have not been here long enough to see flowers if any, although an older one is in front yard as tall as a regular tree. The foliage is sparse as in a desert type plant. This is spreading like mad in the back of the yard after heavy rains. It is in front of some bamboo. I dug out all the small ones in my garden plot and left some at the edge thinking they would keep the invasive bamboo from encroaching more. Did I do the right thing? Do you know what this is? Cannot find anywhere on line.

ANSWER:

There are several native trees that grow in Austin that are similar to your description. The one that first comes to my mind, however, is Prosopis glandulosa (Honey mesquite). It is fast-spreading, has thorns and its leaves are as you describe. Here is more information from Aggie Horticulture and here is more than probably you wanted to know about mesquite from the Texas State Historical Society Texas Almanac. It is a problem plant in some situations. Ranchers and farmers are often eager to remove large stands of mesquite from their pastures and Texas A&M AgriLIFE Texas Natural Resoruces Server has advice on how to do this. This is a bit much for your backyard, but you can use herbicides judiciously to help eliminate mesquite (or whatever your unwanted tree is). Cut the stem or trunk of the tree and then immediately paint it with one of the herbicides (RoundUp, Remedy) using a plastic foam paint brush. Alternatively, you could pour a small amount of herbicide over the cut stem. Be caredul that none of the herbicide gets on any plants that you want to keep and be sure to read the herbicide label carefully and follow the safety instructions found there.

Here are some others that also could be candidates:

  • Parkinsonia aculeata (Retama) Here is more information from Floridata.
  • Acacia farnesiana (Huisache)
  • Acacia roemeriana (Roundflower catclaw)
  • Gleditsia triacanthos (Honey locust)
  • Mimosa aculeaticarpa (Catclaw mimosa) Here are photos and more information from and Northern Arizona Flora.

Now, for the bamboo issue, there isn’t really a plant that will keep it from encroaching. If you are interested in trying to eliminate bamboo, please read the answer to a previous question.

From the Image Gallery

Retama
Parkinsonia aculeata
Retama
Parkinsonia aculeata
Retama
Parkinsonia aculeata
Huisache
Vachellia farnesiana
Huisache
Vachellia farnesiana
Roemer acacia
Senegalia roemeriana
Roemer acacia
Senegalia roemeriana
Catclaw mimosa
Mimosa aculeaticarpa var. biuncifera
Fragrant mimosa
Mimosa borealis
Honey mesquite
Prosopis glandulosa
Honey mesquite
Prosopis glandulosa

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