Unique Trees of North Texas:

Hercules’ Club

(Zanthoxylum clava-herculis)

By Jeremy Priest

This aptly name tree certainly deserves a spot on the “unique” list. Hercules’ club, which is also known as toothache tree, tickle tongue, pepperbark, or prickly ash, is part of the Zanthoxylum genus. The genus has many species with overlapping or similar common names, but only 3 other Zanthoxylum species grow in Texas (Texas Hercules’ club, lime pricklyash, and tickletongue). Many of the common names associated with this species refer to the stimulating nature of the bark and wood of the tree. The tree was historically used by Native Americans and early settlers to numb the mouth, hence the name toothache tree. The name Hercules’ club is easily noticed in the scientific name and fits the appearance of large specimens. When young, the tree develops sharp spines on the bark (not just at leaf nodes as with some other species) which are usually distributed throughout the trunk and branches. As the tree ages, the prickles develop corky, pyramidal bases and eventually lose their sharpness. These corky structures may eventually develop strong ridges as seen below. No matter the age, the trunk of this tree always looks like it could be fashioned into a fearsome weapon.

To identify Hercules’ club look for the distinct spines on the trunk and branches and pinnately compound leaves. Typically 5-19 leaflets, including a terminal leaflet, that are 0.5-4.5 inches long. Small greenish white flowers form in the spring at the tip of branches and seeds are shiny black but not very conspicuous. Don’t confuse Hercules’ club with devil’s walkingstick (Aralia spinosa), which grows straight up with very little branching and has doubly compound leaves. While the leaves may be similar to other compound leaved understory species such as sumac, or the spines similar to species such as honey locust, Hercules’ club is quite distinct and easily distinguished from all except other members of Zanthoxylum. Texas Hercules’ club is more common farther south and west and is a smaller stature tree with only 3-7 leaflets (5 is typical) 0.5-1.5 inch long with the entire leaf less than 4 inches long. Texas Hercules’ club also does not develop the strong corky bases on older prickles. The national champion Texas Hercules’ Club is in North Richland Hills with a height of 22 feet and a spread of 20 feet.

Hercules’ club is an understory tree that typically grows to less than 20 feet in height; however, it is only moderately shade tolerant and relies on establishing with limited competition. The seeds are popular with birds and remain viable after being eaten allowing the species to spread profusely along fencerows and transition zones. Another way it establishes with limited competition for light is to grow on poor sites such as upland, dry, or very sandy areas. In the Cross Timbers region, the overstory on these sites often consists of post oak and blackjack oak. As the tree is adapted to grow on harsh sites with low overall plant density, developing spines reduce wildlife browsing and helps Hercules’ club outcompete other vegetation. If the tree is to be planted, select a relatively dry area with good sunlight. Also remember that the spines on the trunk can be a concern but can be removed or the tips cut if needed. The sharp spines usually do not grow back as the bark develops.

Hercules’ club is native throughout the southeastern US extending from Florida to central Texas. It is tolerant of acidic and alkaline soils. This species also prefers well-drained or sandy soils and does not tolerate flooding. Some of the largest Hercules’ club in Arlington are found at the Southwest Nature Preserve, which is known for being a dry, upland site with some unusual species. There may also be Texas Hercules’ club at the nature preserve as some of the trees develop drastically different bark texture and smaller leaves. Another Arlington park where Hercules’ club is prevalent is Cliff Nelson, which has similar site conditions to the Southwest Nature Preserve. Fully grown the tree may reach 20 feet in height and 15-20 feet in crown diameter, with a trunk diameter of 8-12 inches in 30-40 years. Given the poor sites on which these trees are found they do grow somewhat quickly, as much as 1 foot per year in height. The largest Hercules’ club reported in Texas is in Nacogdoches County (East Texas) and has a height of 38 feet and a trunk diameter of nearly 20 inches, although trees in the Cross Timbers would not likely grow this large.

Sources

Vines, R. A. (1982). Trees of north Texas (Vol. 14). University of Texas Press.

Unique Trees of North Texas:

Mexican Plum

(Prunus mexicana)

By Wes Culwell

The native range of Mexican plum consists of a large portion of the midwestern United States.

Every March, Mexican Plum trees proudly display their dainty white blossoms while they compete in early spring with maybe the more popular Redbud trees, as the Redbuds catch our eyes with their pinkish-purple blooms.  But with close inspection, we find that the Mexican Plum is really a beautiful senorita with her delicate and beautiful blossoms and a unique and wonderful bark.  Since neither of these trees grow as a monoculture, they can sometimes be found blooming in small groups together.  They are our first native trees to remind us that once again spring is here.

So, let’s look at this Texas native’s two most popular features: the flowers and fruit.  The flowers are fragrantly strong and sweet with white petals and about 1” in diameter and usually in more of a mass than redbud flowers. And of course, they bloom before any leaves are present.  The fruit then begins to grow in midsummer and matures in September.  The mature, purplish-red round plums are up to 1” and are edible and can be eaten fresh or used in jams and jellies.  However, one must beat the birds and mammals to the plums.

A mature Mexican plum with distinct spiraling bark.

Although Mexican plum is known for its flowers, the bark is definitely unique and beautiful.  The bark is tight and smooth and a metallic gray when it’s very young.  As the bark matures, it becomes a wonderful brown that is exfoliating, papery and scaly.  Very mature trees have deeply furrowed and rough bark that often develops a spiraling grain.

The Mexican Plum will grow in sun or moderate shade.  This small tree may reach 20 feet tall and wide and is deciduous with alternate, simple leaves on sort of irregular branching of the twigs and limbs.  I can’t say with conviction that the leaves are attractive, but they are unusual.  The leaves have a felt-like feel on the upper side and during the growing season they appear coarse-looking and usually about 2-4” long.  Most of the time, the leaves are folded over at the edges and appear water deficient even though it is a drought tolerant tree.  But the leaves make a big come back in the fall with beautiful yellow and orange colors.

The leaves and late summer fruit of Mexican plum.

This little Mexican maiden needs to be used much more as an ornamental tree in our landscapes.  It can be used as a great alternative to dogwood trees, especially in clay or alkaline soils and dry sites that are detrimental to other species such as dogwood.  The next time you think “This is a great place for a redbud,” try to remember this beautiful delicate bloomer.  If you have to plant a redbud, plant a Mexican Plum to go with it, you won’t regret it!

Unique Trees of North Texas:

Texas Ash

(Fraxinus albicans Buckey, formerly Fraxinus texensis)

By Craig Fox

Pinnately compound leaf of Texas ash with 5-7 rounded leaflets

Ash trees, particularly in Texas, are a hot topic of conversation in the arboriculture world. After the formal confirmation of Emerald Ash Borer (EAB) in Tarrant County, ash trees seem to be on everyone’s mind and rightfully so. Though not nearly abundant here as in the Midwest where EAB originated domestically, North Texas has a modest population (about 2-4% of the urban forest) of ash trees at risk to the destructive pest.

Perhaps the most common ash in the western Metroplex is Texas ash. Just as other members of the genus Fraxinus, they have opposite bud and branch arrangement and pinnately compound leaves. Texas ash typically feature five to seven leaflets (fewer than what is common for green or white ash) rounded in shape with possibly a slight point at the tip. They display excellent fall color in hues of purple, reds and oranges–usually in November–before shedding all their foliage. Texas ash have narrow samaras, or winged seeds, with wings that generally do not extend past the mid-point of the seed and which may persist through winter.

DFW Regional Champion Texas Ash, located inside Fort Worth’s historic Pioneers Rest Cemetery which was created in 1849

Though they may grow to forty feet tall or larger, most Texas ash in our area tend to be smaller, probably due to the thin, rocky soils they natively inhabit and their relatively short lifespan. Large concentrations can be found on fossiliferous limestone ridges through areas such as the Fort Worth Nature Center & Refuge, around Lake Worth and through areas near Aledo, Azle and Weatherford, though it does often appear elsewhere in the DFW region. They are often found intermingled with Texas red oak, eastern redcedar, post oak and cedar elm in our area. Texas ash may be single trunk or multi-trunk and frequently found in colonies. Ultimately, their native range spans from a few counties in Southern Oklahoma near the Arbuckle Mountains, along a thin band into North Central Texas, eventually skirting the Balcones Escarpment and crossing into Northern Mexico.

During leafless periods, ash species are notoriously difficult to differentiate. Texas ash have “C-shaped” leaf scars where the bud sits within the cup of the “C”, very much like the white ash to which they are closely related. In fact, some botanists believe that Texas ash is a subspecies of white ash (Fraxinus americana). The botanical name was changed to Fraxinus albicans Buckley from Fraxinus texensis to correct a complicated issue of proper nomenclature (I still tend to use texensis for obvious reasons).

Clockwise from top left: Year old samaras of Texas ash; older bark (l) contrasted with younger bark (r)-diamond pattern not well formed on either tree; “C-shaped” leaf scar-note how the bud sits within the cup of the “C”

The DFW Regional Champion can be found in Fort Worth’s historic Pioneers Rest Cemetery—a worthwhile visit for the history, if not the tree. Located just north of downtown at 620 Samuels Avenue, the tree is the lone example of its species within the grounds and is officially recognized by the City of Fort Worth as a heritage tree. The state champion tree is located within Lost Maples State Natural area in Bandera County. Curiously, American Forests claims that the National Champion Texas ash is perched just south of Lake Ontario near Rochester, New York—well outside its native range and far removed from what seems to be the preferred site conditions.

Fraxinus is the Latin word for “spear”, while the common name ash is derived from “aesc”, an Anglo-Saxon word. Norse mythology states that humans emerged from Yggdrasil, a great ash tree spanning the cosmos and that both Odin and Thor owned spears made from ash. Vikings were sometimes referred to as “aescling”, translating to “men of ash” for the spears their warriors carried. Planks for ships, oars and chariot axles were commonly made from ash timber. Today, the wood of ash trees is popular for use in tool handles and baseball bats due to its shock resistance. It is also popularly used to create flooring and millwork.

All Fraxinus species in Texas are believed to be susceptible to attack by EAB, but some research indicates that green ash (Fraxinus pennsylvanica) is the preferred host. The beetle, Agrilus planipennis, is native to East Asia where it is rarely problematic within its native range. Believed to be introduced into North America in the late 20^th century, the borer was formally discovered in Michigan in 2002. The confirmed location in Tarrant County occurred in 2018 along Ten Mile Bridge Road, just north of the Fort Worth Nature Center & Refuge. Due to the extensive activity and damage found at the site, it is believed that EAB was present, but undetected, in Tarrant County for several years prior to discovery.

Evidence of EAB occurs in the upper canopy first, which can be hard to detect, but an increase in woodpecker activity could help provide clues as the birds actively feed on the larvae. A thinning upper canopy and epicormic shoots near the base of limbs or along the trunk are also common symptoms.  After hatching from eggs laid in bark cracks and crevices, S-shaped galleries which gradually open in shape are created beneath the bark as the EAB larvae chews through the tree’s phloem. Due to the extensive feeding and destruction of cambium and phloem, the bark will eventually begin to peel, split, or fall away. After feeding and overwintering, the beautiful adult beetles emerge from the trees (probably from March through June in our area) creating a D-shaped exit hole approximately one-eighth inch in diameter. Once affected, ash trees decline quickly, often losing large limbs in the upper canopy. The tree’s death is caused by the widespread destruction of the phloem tissue supplying carbohydrates and dissolved nutrients.

For updates, additional information, or useful resources on EAB, please visit:

Texas A&M Forest Service:  https://tfsweb.tamu.edu/eab/

United States Department of Agriculture:  https://www.aphis.usda.gov/aphis/ourfocus/planthealth/plant-pest-and-disease-programs/pests-and-diseases/emerald-ash-borer

Bartlett Tree Experts Technical Report: https://www.bartlett.com/resources/technical-reports/emerald-ash-borer

Unique Trees of North Texas:

Live Oak

(Quercus virginiana or Quercus fusiformis)

By Eugene Gehring

You may be asking how one of the most popular and common tree species in North Texas could be considered unique. The fact that it is so popular and common is one of the reasons I think it is unique. Live oaks will grow in almost any soil type. Quercus virginiana (coastal or Southern live oak) is native from Virginia down the Atlantic coast to Florida and across the southern states to somewhere west of Houston. Quercus fusiformis (plateau or escarpment live oak) is native west of I-35 down into the hill country and North in a 3-5 county band to just north and west of Fort Worth. Live oaks are found in the transitional counties. Whether they are “Southern” or “Escarpment” is a good question. The primary morphological difference is the shape of the acorn. The escarpment live oak acorn is fusiform, or more constricted at the base. After 30 years working with live oaks, I still don’t have a clue what that means.

Live oak typically have long, extending branches that give the tree it’s unique shape.

Live oaks are different from the other oak species in several ways. They are evergreen or semi-deciduous. They drop their leaves in February/March instead of the fall. Their leaves are simple nondescript ovals, 1-4” long with an entire margin. White oaks have rounded lobes, while red oaks have points or stipules on their lobes. Based on leaf morphology, live oaks fit more with the white oak group. Live oaks can be more than twice as wide as they are tall with long graceful branches. White oaks tend to be about as wide as they are tall, and red oaks tend to be taller than they are wide.

The wood of most oak species is ring-porous. They lay down different size vessels and cells depending on the season. Live oaks are diffuse-porous. There is not a seasonal variation in the size of the vessels or cells. One of the major differences between white oaks and red oaks is tyloses within the vessels of the wood. In white oaks tyloses occur naturally, while in red oaks they do not. That is why whisky barrels are made of white oak. Live oaks do not have tyloses naturally in their wood, putting them in the red oak group.

Unlike most oaks, live oaks are prolific at sprouting from their roots. This characteristic, along with readily forming root grafts, allows live oaks in a given stand to share their roots. This is one of the reasons live oaks are considered drought hardy. It is also why oak wilt is so devastating to live oaks. During the drought of 2011 – 12 I witnessed live oaks in the Hill Country that bloomed or tasseled in the spring of 2011, but did not leaf out until it rained in the fall of 2012.

Live oaks have a unique ability to adapt and grow in a variety of environments. The exception being they do not tolerate extended periods of saturated soils. They will grow in the 100 year flood plain, but not in areas subject to regular flooding or standing water. In the late 1990s, there was a flood event on Lake Travis where mature live oaks were almost completely submerged and survived because it was a short duration. The same event killed live oaks on Stillhouse Hollow Lake because the water was not lowered as quickly.

Live oak growing in an old cemetery. Although live oak can live an extremely long time, they also grow rapidly when young which often leads to the age being overestimated. A live oak can grow to over 20″ diameter in just 30 years.

Live oaks are hardy to the 7b/8a zones of North Texas. I think the escarpment live oak is somewhat hardier than the Southern live oak. The 60 degree swing in temperature in January 2108 caused damage to the leaves on some live oaks in North Texas. They were not prepared for the extreme change in weather. When I first moved to the DFW metroplex, I was told there were live oaks with basal cankers caused by the cambium freezing during one of the cold winters in the 1980s.

Another unique characteristic of live oaks is that they will grow in almost any soil type. They thrive equally well in the sandy acidic soils of East Texas and the alkaline limestone based soils of Central Texas. Although they are not native to the heavy clays of most of North Texas, they grow and thrive there as well.

Climate change is probably responsible for the split between Southern live oak and Escarpment live oak. At a warmer period in time water covered a lot of North Texas as evidenced by ammonite fossils in Parker County. As the climate cooled and the water receded live oaks were pushed south and the split occurred.

Live oaks have long been valued as landscape trees. Some of the oldest planted can be found at Oak Ally Plantation in Louisiana.  They were thought to have been planted around 1710. In North Texas, they can be found in almost every neighborhood. Most of the older planted live oaks are probably Southern live oak because they would have come from nurseries in East or Southeast Texas. As the population in Central Texas increased, more tree nurseries were started in the 70s and 80s resulting in more escarpment live oak in the industry.  That is not to say there were not wild trees dug in Parker or Hood counties and planted in Fort Worth or Dallas. There were also nurseries, like O. S. Gray in Arlington, that may have grown escarpment live oak.

A little known fact is that live oak was used in the construction of the USS Constitution (“Old Ironside”). Live oak branches were used to form the ribs of the ship. They used the natural curves of the branches for added strength.

I have had a love/hate relationship in the more than 30 years I have worked with live oaks. I have seen them survive the extremes of drought, flood, tornadoes and hurricanes. I have seen them growing in practically solid rock, sugar sand, black gumbo clay, from East Texas to Midland/Odessa, from Lubbock to Houston to Corpus Christi. More than half of the trees in “The Famous Trees of Texas” are live oaks. The species has been witness to treaties, hangings, fights, marriages and everything in between. I think live oaks are unique because of their strength, adaptability, and mystic.