We must love trees in Denver because almost any seen in the City are planted. Denver’s unpredictable spring and fall weather and clay soils make it hard to grow trees. Some trees, like Ash, should not be planted because of the recently arrived Emerald Ash Borer. Everyone loves Aspens, but they have plant care issues. We will list and describe trees that grow in the Denver area and discuss their pros and cons.
Kentucky Coffeetree (Gymnocladus dioicus)
Kentucky Coffeetrees grow slowly in their early years, but mature trees reach up to 50 feet tall and 40 feet wide. The tree belongs to the pea or legume family. Their name comes from the seeds being used by pioneers as a coffee substitute. However, the seeds and pods are mildly poisonous because they contain alkaloid cytisine. Roasting neutralizes the toxin. The leaves of Kentucky Coffeetrees have two-foot-long leaf segments consisting of several lance-shaped leaflets off each side. Kentucky Coffeetrees turn bright yellow in the fall. The tree is durable, easy to grow, cold-hardy, and tolerant of salt and pollution. The deep green Bipinnately compound leaves grow opposite on a larger stem giving the tree a feathery appearance and the tree grows exotic-looking dark brown seedpods. The effect is stunning and looks tropical.
Kentucky Coffeetrees naturally grow from central Ohio west to central Oklahoma and from Iowa to northern Arkansas. The tree leaves and seeds are slightly toxic, so there is little wildlife usage as a food source. However, mastodons and rhinos feed on the pods and seeds during the Pleistocene epoch. Their digestive enzymes broke down seed germination inhibitors, allowing their seeds to sprout in dung piles as these beasts wandered the plains back then. Today, since no wild or domestic animals feed on the pods, the seeds grow where dropped. The tree is not considered invasive.
- Hard, firm, with thin scaly ridges curling outward along their edges
- Dark grey
- Deciduous, alternate
- Doubly pinnately compound
- Composed of about 70 leaflets
- On 3–7 pairs of branches from a central stalk
- A central stalk is mistaken for a stem
- Leaflets ovate
- Seldom opposite each other
- No terminal leaflet
- The tree is leafless for more than half the year
- Leaflets about 2 inches
- Leaf and central stalk 12–35 inches
- Bluish-green color
- Smooth margin
- Husk hard, leathery
- Usually with a powdered appearance
- Hanging on a stout stalk
- Remaining on the tree through the winter
- Pod 5-8 inches
- Husk dark reddish-brown
Kentucky Coffeetrees are both heat and drought-tolerant once they become well-established. Newly planted saplings need two years of watering. Its adaptability to urban conditions with no serious insect or disease problems. The tree makes a strong contender to replace ash trees decimated by the Emerald Ash Borer.
- The tree is free of serious pests and diseases.
- Birds use the Kentucky Coffeetree for nesting.
- The young saplings of the Kentucky Coffeetree tend to be ugly ducklings.
- The trees grow slowly at first.
Catalpa (Catalpa speciosa)
When I moved to Colorado in 1979, two exotic things caught my eye – Magpies and Catalpa trees. I will never forget walking up to an enormous tree in full bloom in Washington Park covered with white azalea-like flowers. The smell was excellent, and the tree was a sight to see. When I arrived at the tree, a massive gust of wind hit just right, and it exploded in front of me, filling the sky with petals. It was a fantastic sight, never to be forgotten.
Northern Catalpa Tree Attributes
Catalpa trees are members of the Bignonia family, and they are related to the Trumpet vine and Royal Empress tree. In nature, the Northern Catalpa grows along rivers, creeks, and streams in forests from southern Illinois and Indiana to western Tennessee and Arkansas. Common names for this tree are Cigar tree, Indian Bean Tree, Catawba, Caterpillar tree, Hardy Catalpa, and Western Catalpa. The largest living catalpa tree is located on the lawn of the Michigan State Capitol, planted in 1873.
Catalpa trees are known for their fast growth rate and can reach over 80 feet in height. Its heart-shaped leaves are about 12 inches wide. Through the eyes of a child, the tree with its long pods, large leaves, and twisted branches could be the giant bean vine described in Jack and the Beanstalk. Northern Catalpa is a popular choice for homeowners because of its upright form, elegance, and shade.
Hummingbirds and bees hover around the trees’ bell-shaped flowers for the sweet nectar in the spring. Once pollinated, the flowers drop and grow long fruits resembling large green beans. Each tree produces about 100 seed pods. The fruits ripen from September to October and stay on the tree during the winter. Kids play with the pods once they drop.
With the expansion of railroads in the 1870s, companies planted large tracts of Catalpa to produce railroad ties. Unlike oak, it is easy to drive spikes into catalpa timbers. However, the wood is too weak to support rail traffic. So instead, the railroad companies started to use the wood as posts for barbed-wire fencing.
In the late 1800s, Catalpa became very popular as an ornamental tree and was planted all around the country. While catalpa trees are no longer as popular as during the 19th century, they have self-propagated extensively in most states.
What Are the Tree Characteristics of the Northern or Western Catalpa Tree?
Mature Size – The northern Catalpa grows to a height of 40–60′ and a spread of 20–40′ at maturity. Sometimes trees develop a unique twist in branches and the trunk.
Growth Rate – The tree grows fast, with height increases ranging from 13 to 24 inches per year.
Sun Preference – The tree prefers full sun and needs a minimum of four hours of direct, unfiltered sunlight each day.
Soil Preference – The northern Catalpa grows in acidic, alkaline, loamy, moist, rich, sandy, silty loam, well-drained, wet, and clay soils. A wide range of moisture conditions can be tolerated, including some flooding and extremely hot, dry conditions.
Bark – Gray-brown that becomes scaly with age.
Leaves – The structure of the leaves is simple, with large 6″ to 10″ long heart-shaped leaves. They grow in a whorled or opposite arrangement along branches. In fall, the leaves turn yellow-green to brown.
Flowers and Fruits – Northern Catalpa has showy flowers in 4″ to 8″ clusters. The flowers are white, bell-shaped flowers with orange stripes and purple spots and stripes. The fruit of the Catalpa looks like large green beans growing 8″ to 20″ inches long filled with winged seeds.
Wildlife Value – Hummingbirds and bees visit the flowers in the summer. The tree is the sole host of the catalpa sphinx moth, whose larvae make excellent fishing bait.
Insects, Diseases, and Other Plant Problems – Catalpa does not have any serious diseases or insect problems. Some trees have verticillium wilt, leaf spot, mildew, and twig blight. Large infestations of sphinx moth larvae can cause considerable damage.
Pros and Cons
- Beautiful ornamental shade tree free of pests and diseases
- Grows fast
- Grows in most soil types.
- Requires cleanup after the flower petals, leaves, and seed pods drop.
- Should not be planted where fruit and flowers can drop on sidewalks because the flowers are slippery right after they fall.
Thornless Honeylocust (Gleditsia triacanthos f. inermis)
Honeylocust trees come in two forms – thorns and thornless. So, when you order a tree be sure to “hold the thorns.” Thornless honeylocust has alternate compound leaves. The crown of the Honeylocust is open-spreading and leaves filter less light than many other species, making this tree a good choice for lawns. It is tolerant of drought, alkaline soils, and salt. Honeylocusts produce a nondescript greenish-yellow flower in May and June. From those flowers, the trees grow a long, flat, brown legume pod that tends to curls up. There are some disease and insect problems with it, but overall it is fairly resilient.
- Up to 90 feet in height
- Grows fast with height increases of more than 24″ per year
- Maximum age 120 years
- Develops a thin, airy crown that provides dappled shade while allowing grass to grow beneath
- Smooth, with horizontal lenticels (pores)
- Matures trees bark turns brown with deeply furrowed with scaly ridges
- Deciduous, alternate
- Features pinnately or bipinnately compound leaves about 8″ long with 8–14 leaflets
- Leaflets have a rounded tip, often with a small point
- The last leaves to emerge in the spring
- Yellow autumn color
Flowers and Fruits
- Produces small, greenish-yellow blossoms arranged around spike-like stalks that are notably fragrant.
- Yields large, brown leathery twisted seed pods that are 7–8″ long
Tree History and Wood Use
Honeylocust wood has been used for pallets, crates, firewood, railroad ties, and fenceposts. It was formerly used to make bows. Honeylocust trees were used by Indians to make sweeteners and thickeners. Native Americans also cooked the seeds to eat or roast them for coffee substitute.
- Heavy, hard, strong
- Reddish-brown color
Pros and Cons
- Native tree easy to plant and grow
- Small leave size means less fall cleanup
- Thin, airy crown that provides dappled shade allowing grass to grow beneath
- Tolerates harsh urban environments with wet and dry sites, salt, compacted soil, and pollution
Denver weather over the last few years has been tough of Honeylocust causing the trees to grow smaller leaves with twig and branch dieback. These stresses may have become cumulative, affecting tree health. Current stresses on Denver’s Honeylocust trees are:
- Dry winters
- Polar plunges in late spring and early fall
- Hail damage
- Spider mites
- Thyronectria Cankers
- Root Collar Rot
Homeowners who love this tree should plant if they are willing to water and fertilize the tree throughout the year. The December and January drought this year has been particularly hard on these trees. Health trees ward off Spider mites, Thyronectria Cankers and Root Collar Rot. These pests and aliments usually attack weather stressed trees.
Thundercloud Plum (Prunus cerasifera)
Homeowners looking for a fast grown tree might consider the Thundercloud Plum. It is a popular purple-leaved cultivar of the Cherry plum that grows to a height of about 20 feet. The tree has a rounded shape and dark deep purple leaves. It blooms delicate, fragrant pink blossoms in the spring. Large Thundercloud Plum in full bloom is a sight to see.
The plum tree will grow in Denver in full sunlight or partial shade. It is drought-tolerant and prefers well draining, acidic soil, but will tolerate our alkaline and clay soils. The tree does not do well in windy spots.
The Cherry plum has a small stature with ornamental flowers and foliage making it a very popular landscape tree. It attracts bees with its flowers and birds with its small, fleshy red fruits. The fruits are edible, but may not produce fruits in shady locations. The tree is one of the earliest spring bloomers.
These plums are short-lived with an average lifespan of about 20 years. The tree is susceptible to fungal diseases, so avoid spraying them by lawn irrigation systems. Scale, aphids, and borers are known pests. During prolonged droughts, spider mites can become a problem.
- Moderate growing; reaches 20 ft. tall and wide
- Moderate growth rate
- Purple foliage color
- Round growth habit
- Full sun or partial shade
- Tolerates urban pollution
- Moderate water needs
- Dramatic foliage color, edible, extreme cold hardiness, bird-friendly
- Showy flowers
Pros and Cons
- Showy beautiful tree
- Grows fast
- Drought tolerant
- Dramatic foliage color
- Edible fruit
- Cold hardiness
- Bird friendly
- Short-lived about 20 years
- Susceptible to scale, aphids, borers, and mites