An Abundance of Acorns

Texas Red Oak displaying fall color.

Texas is famous for its oak trees, with over 50 different species found in the Lone Star State.  Each of these oak species produce acorns, also called oaknuts or mast, which is a collective term for fruits or nuts.  The word acorn is related to Gothic term akran which has been interpreted to mean ‘the fruit of the unenclosed land.’  When oaks are dominant in the landscape, as they are here in central Texas, they play an important role in the ecology of the forest.

Acorns usually contain one seed enclosed in a tough, leathery shell, and have a cup-shaped cap or cupule. Depending on the oak species, acorns can take from 5 to 24 months to mature.  As a general rule, acorns mature in late summer, turning green to brown, and start falling from oak trees in September and October.  Acorns produced by trees in the red oak family (such as Texas Red Oak, Quercus buckleyi) take two years to mature, while acorns produced by trees in the white oak family (such as Bur Oak, Quercus macrocarpa) only take one year. 

Texas Red Oak Acorns

Some years, known as mast years, trees produce copious amounts of acorns with smaller crops in the years between.  Scientists have proposed a range of explanations for the mystery of what might trigger a mast year, but they do know that it is not resource-driven as annual rainfall and temperature fluctuations are generally much smaller in magnitude than the variation in the crop sizes of acorns.  Some scientists hypothesize that masting trees are trying to maximize pollination efficiency.  If these trees flower and release pollen at the same time in order to increase their chances of reproduction, since large amounts of pollen correlate with larger amounts of germination, they ultimately produce more acorns.  Large, occasional outputs of seeds like acorns appears to be more favorable than frequent, smaller outputs.

Bur Oak acorns are our largest acorns.

Boom and bust years of acorn production actually benefit oak trees from an evolutionary perspective, because acorns are an important, highly nutritious food source for many animals including squirrels, mice, turkeys, blue jays, pigeons, ducks, deer, and bears.  In a mast year, these animals can’t consume all of the acorns produced, so some are left to germinate and grow into future oak trees.  In leaner years, animal populations are kept in check so there are fewer animals to eat the acorns in the mast years.  Over time, a higher proportion of acorns survive to become oak trees.

Copious amounts of acorns are produced in a mast year.

Too heavy for wind dispersal, acorns need other ways to spread beyond the mother tree into a suitable area for germination.  Jays, squirrels, and some woodpeckers serve as the main dispersal agents, as they gather and hoard acorns in caches.  While they are remarkable in creating mental maps of their cache locations, these animals rarely eat every single acorn, so a small number manage to germinate and produce the next generation of oaks.

The Blue Jay is one of many species that cache acorns.

As autumn arrives and acorns mature and fall from our oak trees, think of the potential in the abundance of acorns produced.  As Henry David Thoreau once wrote, “Every oak tree started out as a couple of nuts who stood their ground.”  

Resilient Trees

The beginnings of Winter Storm Uri

With winter storm Uri well behind us, many observations are being made regarding which tree species successfully survived the 8-day stretch of extreme winter weather conditions.  First and foremost, all trees that are native to Central Texas generally fared much better than their non-native counterparts.  Over the centuries, native species have adapted to all sorts of weather conditions, from cycles of droughts and floods, heat and cold, and even accumulations of ice or snow.  These adaptations greatly increase each native species’ resiliency over time.

A second factor that determined the survival of a native tree species was whether it was evergreen or deciduous.  The foliage of evergreen species increases the surface area that can trap ice and heavy snow, causing limbs to bow, split, and break, which can weaken the entire structure of the tree.  While deciduous trees can be covered in ice and snow, in winter they do not have foliage that can trap the extra weight that can cause permanent damage.

Third, established plants have a much better chance of survival in extreme winter weather conditions, so now is the time to plant!   While there are many desirable native trees that are deciduous, there are several that are also attractive bloomers in the spring, including Escarpment Black Cherry (Prunus serotina var. eximia), Roughleaf Dogwood (Cornus drummondii), and Rusty Blackhaw Viburnum (Viburnum rufidulum).

Escarpment Black Cherry in bloom

Escarpment Black Cherry fruit

Found only in the limestone-based soils in Central Texas, Escarpment Black Cherry is a distinct and isolated subspecies of native Black Cherry (Prunus serotina). Growing up to 50 feet tall, it is distinguished from other subspecies in Texas by its intermediate height and its virtually hairless, pointed elliptical green leaves with finely toothed margins that turn golden yellow in fall. It has attractive silvery bark and long showy clusters of tiny white flowers in March to April followed by purplish-red fruits that are favored by wildlife.  This native cherry prefers sunny areas in wooded canyons and on slopes, and is a larval host plant for many species of butterflies and moths including Eastern Tiger Swallowtail, Two-tailed Swallowtail, Cecropia Moth, and White Furcula.

Roughleaf Dogwood in bloom

Roughleaf Dogwood fruit

Roughleaf Dogwood is a small tree to 16 feet with opposite, prominently veined, oval, green leaves with an abruptly drawn-out tip, slightly rough above and slightly velvety below, that turn purplish-red in fall.  From April to early June numerous broad clusters of cream-colored flowers appear at the ends of branches, developing into small, fleshy, bright white, spherical fruits.  This native dogwood prefers partially shady, wooded limestone hills, and provides cover for various species of wildlife, especially birds, as it spreads from root sprouts and if allowed, eventually creates a natural thicket.

Rusty Blackhaw Viburnum in bloom

A shapely small tree, typically to 18 feet but sometimes taller, Rusty Blackhaw Viburnum has attractive, glossy dark-green paired leaves that turn reddish-purple in fall, reddish-brown twigs, and dark bark that separates into rectangular plates. Platter-shaped clusters of tiny, creamy white flowers in March to April are followed by small, oval, waxy, blue-black fruits that are relished by several species of birds and mammals.  This native viburnum prefers woodland edges and thickets in partial shade, and is of special value to native pollinators like bumble bees.

Rusty Blackhaw Viburnum fruit

Rusty Blackhaw Viburnum fall color

It’s important to note that while winter storm Uri was no doubt an exceptional weather event, it’s never too early to plant some of our more resilient native trees!

Astonishing Acorns

Texas Red Oak is one of many types of oaks that produce
acorns in autumn

Famous for its oak trees, there are more than 50 species of oak native to Texas.  In our region of Central Texas, some of the most common include Live Oak (Quercus virginiana), Texas Red Oak (Quercus buckleyi), and Bur Oak (Quercus macrocarpa).  

A familiar tree with a stately growth habit, Live Oak is commonly 50 feet tall but with several large, twisting limbs that form a low, dense crown that can spread over 100 feet.  Its leaves are oblong in shape, leathery, 2 to 4 inches long and 0.5 to 2 inches wide.  Slow-growing but long-lived, it appears to be evergreen rather than deciduous since its old leaves fall just as new leaves emerge in the spring.  The annual acorns of this tree are dark brown and shiny, about 1 inch long and 0.5 inch wide, half covered in a gray, downy cup borne on a long stem. 

Live Oak Acorns

Texas Red Oak, also called Spanish Oak or Buckley Oak, is a small to medium tree to 35 to 70 feet tall, and its habitat is restricted to limestone ridges, slopes, and creek bottoms.  Its leaves are deeply lobed and it provides good shade in the summer and deep red color in the fall.  Its acorns are biennial, or maturing every other year, but when they do occur they are plentiful.  They can occur singly or in pairs, are up to 0.75 inches long and 0.5 inches wide often streaked with dark lines, and set in a shallow cup covering one-third to one-half of the fruit.

Texas Red Oak Acorns

A large, deciduous tree reaching a height of 80 feet or more, the Bur Oak has heavy branches that form an open, spreading crown, and leaves with highly variable lobes that can grow to 12 inches long and 6 inches wide.  But what is most characteristic is its’ distinctively large annual acorns, up to 2 inches long, set into a deep mossy-fringed cup that gives this species its common name.  In fact, an alternate common name is Mossycup Oak.  Bur Oaks have a medium growth rate, and develop a deep taproot that allows them to draw water and anchor the tree, even in drought conditions.

Bur Oak Acorn

The origin of the word acorn is dubious, as several sources are possible including Old Norse akarn meaning ‘fruit of wild trees’, Gothic akran meaning ‘fruit’, and Old English aecern meaning ‘mast or oak-mast.’  Mast is a term often applied to the fruit of oak trees, especially when they are used as food source for animals.

In Texas, oaks are important trees for wildlife as they provide acorns for food, shelter in their huge branches, and both food and shelter as they slowly decay.  For humans, oaks protect against soil erosion, buffer homes from strong winds, and provide true beauty in the landscape.  All of these benefits are derived from the simple yet astonishing acorns!

Leafy Treasures

Rusty Blackhaw Viburnum

Fall is the time when the quiet, green palette of summer gives way to the crisp reds, vibrant oranges, and mellow yellows that paint the natural landscape.  During the growing seasons of spring and summer, our trees and shrubs use sunlight to convert water and carbon dioxide from the air into sugar.  Called photosynthesis, this process begins to wane in November in Central Texas, and the leaves on some plants begin to change color in preparation for winter’s rest.

Mexican Buckeye

Pigments are natural substances formed by the cells of leaves that provide the basis for leaf color. Most familiar is chlorophyll, which produces the color green, and is vitally important as it is required for photosynthesis.  Carotenoid, which produces the colors yellow, orange, and brown, is a common pigment in many fruits and vegetables, as are anthocyanins, which produce the color red. Both chlorophyll and carotenoid are present at the same time in leaf cells, but the chlorophyll covers the carotenoid and hence the leaves appear green in the spring and summer.  Not all trees can make anthocyanins, however, and most are produced under certain conditions and only in the fall.

Flameleaf Sumac

As the days grow shorter, the decreasing amount of sunlight eventually causes trees to stop producing chlorophyll.  When this happens, the carotenoid in a leaf can finally show through, turning the leaves into a myriad of yellows, oranges, and browns.  Red, on the other hand, is an entirely different matter.  Affected by temperature and cloud cover, red fall colors can vary greatly from year to year.  A lively showing of reds depends upon warm, sunny autumn days and cool, but not cold autumn nights.  This type of weather pattern triggers the production of anthocyanins, which the tree produces as a form of protection.  Anthocyanins allow trees to recover any sugar or nutrients left in the leaves, moving them through the leaf veins and down into the branches and trunk, and its presence generates the red color before the leaves fall off.  Rainfall during the year can also affect fall color, with too much lowering the overall color intensity, and too little delaying the arrival of color.

Bald Cypress

Fall leaf color can easily be used to help identify local tree and shrub species.  The most notable reds and oranges in our area are produced by Texas Red Oak, Flameleaf Sumac, and Rusty Blackhaw Viburnum.  Dotting the hillsides, roadsides, and upper reaches of wooded canyons, they contrast well with the surrounding greens of Ashe Junipers and Live Oaks.  Golds and yellows are represented by Eastern Cottonwood, Escarpment Black Cherry, Mexican Buckeye, Bald Cypress, and Little Walnut, whose colors transform the low-lying areas near creeks and streams.   

Little Walnut

While a tree’s trunk and branches can survive the colder winter temperatures, many leaves cannot. Made up of cells filled with water and sap, these tissues are unable to live throughout the winter, and the tree must shed them to ensure its survival.  As the days grow shorter, the veins that carry sap to the rest of the tree eventually close.  A separation layer forms at the base of each leaf stem, and when complete, the leaf falls.  Some oak trees are the exception, with this layer never fully detaching and the dead leaves remain on the tree until new spring growth pushes them off to the ground.  Once on the ground, the leaves slowly decompose with the help of earthworms, beneficial bacteria, and fungi, creating the soil necessary for the continuation of the cycle of life.  

Air Plants

Ball Moss

When the time of year arrives when leaves begin to fall and the landscape starts to appear a bit more barren, some things become more noticeable, even though they were present all along.  One such thing is epiphytes, or plants that grow harmlessly upon another plant (such as a tree), and derive moisture and nutrients from the air.  The word epiphyte comes from the Greek ‘epi’ meaning ‘upon’ and ‘phyton’ meaning ‘plant.’  Epiphytes differ from parasites in that they grow on other plants for physical support and do not necessarily negatively affect their host.  They are also called ‘air plants’ since they do not root in soil.  In central Texas, the most common epiphytes native to our area are Ball Moss (Tillandsia recurvata) and Spanish Moss (Tillandsia usneoides).  

Members of the Bromeliad or Pineapple family, neither of these plants are real mosses, but true plants with flowers and seeds.  Ball Moss is a scurfy herb with narrow leaves forming small, grayish ball-like clusters on the branches of deciduous trees.  In North America, it is native from Florida to southern Georgia, Texas, New Mexico, and Arizona, with a disjunct population in central Louisiana.  Slender, pale violet flowers appear on long bracts from June to August.  Ball Moss grows well in areas with low light, little airflow, and high humidity, which is why it is often found on shade trees in the South.  It photosynthesizes its own food by receiving water vapor from the air, nitrogen from bacteria, and other minerals from windblown dust.  Wind is also the main method of Ball Moss seed dispersal, and its plentiful seeds are armed with fine, straight hairs that cling well to wet or rough surfaces such as bark.

Spanish Moss

Generally growing upon larger trees such as Southern Live Oak and Bald Cypress, Spanish Moss forms a cascading mass of slender, scaly gray leaves.  These scales help the plant absorb water and nutrients, mostly from the minerals naturally leached from the foliage of its host tree.   Its specific name ‘usenoides’ means ‘resembling Usnea’, which is also known as Beard Lichen, but this plant is not a lichen either.  It grows in chain-like fashion to form hanging structures up to 20 feet in length, and bears tiny whitish-green flowers from April to June. Its primary range is the southeastern US, but is found as far north as Virginia, and it propagates both by seed and vegetatively with fragments carried by the wind to neighboring tree limbs.  Spanish Moss has been used for various purposes, including building insulation, packing material, and mattress stuffing.  It is still in use today for arts and crafts, and even in the manufacture of evaporative or swamp coolers.  These coolers contain thick pads of Spanish Moss that are pumped with water, with the cooling effect of evaporation caused by a fan that pulls air through the pad and into the building. 

Little evidence exists that Ball Moss 

harms the health of a tree.

There is a common misconception that these epiphytes are parasites, and that they harm the trees that serve as their hosts.  While trees that are heavily infested with these plants can have increased wind resistance and result in fallen limbs, there is little evidence among the botanist community that a reasonable presence of these plants have a noticeable effect on the growth or health of the tree.  In fact, the presence of these air plants serves as a benefit to many forms of wildlife by harboring small insects that provide food, supplying nesting material, and serving as shelter from the outside elements.

Three Terrific Trees

Mexican Plum in full bloom

Trees are often planted for their ornamental value, or to provide shade, but there are many other reasons to plant them.  They improve air quality by producing oxygen and storing carbon, which offsets the harmful byproducts of burning fossil fuels.  They can moderate the effects of sun and wind, reduce air conditioning costs, and clean the air by trapping dust and pollen.  Trees can also be credited with increasing property values, lowering our heart rates and reducing stress, and providing shelter and food for many types of wildlife.

In Central Texas, three terrific trees that are native to our area include Escarpment Black Cherry (Prunus serotina var. eximia), Mexican Plum (Prunus mexicana), and Carolina Buckthorn (Frangula or Rhamnus caroliniana).  All three of these trees are medium-sized, deciduous, display fall color, and benefit wildlife by producing fruit.

Escarpment Black Cherry blooms

Escarpment Black Cherry is a distinct variety of Black Cherry, found only on the calcareous soils in our wooded hill country canyons, slopes, and floodplains.  Up to 50 feet tall, this tree is prized for its attractive silvery trunk and branches, five-inch long clusters of showy white blooms that occur in March and April, juicy summer fruits, and vivid yellow to red fall foliage.  While the small dark red to purple-black cherries it produces are edible, the rest of the plant is not, and the cherries are often eaten first by birds.  Several butterflies, including Viceroy, Eastern Tiger Swallowtail, Two-tailed Swallowtail, Red-spotted Purple, and Striped Hairstreak use this tree as their host plant. 

Escarpment Black Cherry fruit

Often called the ‘star of our native plums’, Mexican Plum is easily recognizable in spring, as it is an early bloomer.  Before the leaves appear, white to pale pink, five-petaled flowers cover the 15 to 35 foot tall tree from February to April, and they are extremely fragrant, attracting several species of native bees and butterflies.  Plums up to one-inch wide turn from yellow to mauve to purple as they ripen July through September, and they are edible for humans and wildlife alike.  Thick, five-inch long leaves provide food for the Eastern Tiger Swallowtail and Cecropia Silkmoth, and turn a showy shade of orange in autumn. Mature trunks are a beautiful satiny blue-gray with horizontal striations, typical of most fruit trees.

Mexican Plum fruit

A flowering Carolina Buckthorn

Lesser known is the Carolina Buckthorn, an understory tree 12 to 15 feet tall, with oval, shiny green leaves and small yellow clusters of blooms produced near the leaf stems in May and June. It prefers bottomlands, canyons, and streamsides, and in light shade it is airy and tiered.  Bright red fruits turn to black when ripe, and are relished by many birds and mammals.  The leaves stay green into late fall, turning various colors from yellow-gold to bronze-sienna as the weather cools. Carolina Buckthorn is also the host plant for Spring Azure, Gray Hairstreak, and Painted Lady butterflies.

Carolina Buckthorn berries

Consider adding one or all of these terrific trees to your property.  While the best time to plant trees in Central Texas is in the fall, it’s never too late to plan for future enhancements to your native landscape!