How Do I Grow a Healthy Locust Tree In Denver?

Gorgeous flowering locust trees are a Denver favorite for ornamental street-side beauty. Their beautiful blooms and ability to thrive in adverse conditions make them desirable for residential homes and public landscapes. Learn how to identify different locust tree varieties and how to help them grow strong and healthy!

Locust Tree Species Commonly Found In Denver

Locust tree species are typically classified into two categories – Gleditsia and Robinia. The most well-known Robinia locust species is probably the Black Locust, while the Honey Locust is a highly recognizable Gelditsia species. All locust trees are part of the pea family and create large clusters of pea-like flowers. Learn more about each species and see if you can identify them the next time you’re out for a stroll in downtown Denver!

Honey Locust

The Honey Locust is one of the most sought after shade trees in Denver. Growing to 30-70 ft tall, it also spreads up to 70 ft wide, adding plenty of cool shade to your landscape. This variety loves full sun and can grow in almost any conditions! Ability to withstand air pollution, alkaline soil, drought, and flooding, help this hardy tree to thrive in Denver. Its full rounded crown turns golden yellow in the fall, adding fabulous color to the landscape. The honey locust grows well throughout zones 4-9.

Hardwood Black Locust

This fast-growing locust variety is desirable for its hardiness and white fragrant spring flowers. Native to the Denver area, the rot-resistant hardwood looks amazing in all seasons and makes an excellent ornamental tree. It prefers full sun but can grow in a variety of soils. Standing 70-80 feet at maturity, the Black locusts creates beautiful wispy blooms that drip from its branches in the spring. The flowers even attract hummingbirds! This locust variety also produces honey and quality firewood.

Purple Robe Locust

Known for being an easy tree to care for, the Purple Robe Locust is a hardwood variety that makes a great street tree. It can handle many soil types and needs little water. Growing at a rapid rate of 2 to 3 feet per year, this locust produces vibrant purple flowers that cluster and cover the entire tree in color. Native to zones 4-8, this species thrives on the Denver front range. Fully grown it will stand 30 – 40 feet tall and its wisteria-like flower clusters can grow up to eight inches long!

New Mexico Locust

One of the smaller locust varieties, the New Mexico Locust is typically only 15 feet tall, although it can reach 25 feet high. Growing strong in zones 4-8, the New Mexico Locust produces pretty pink flower clusters that are nearly four inches long. Tolerant of many soil types, The New Mexico locust can handle the high alkaline soil that Denver is known for. 

How to Identify Locust Trees

Locust trees have distinguishable features in every season. Keep an eye out and spot them by checking for flowers, thorns, and tree size.

Flowers: The flowers on locust trees are pea-shaped and bloom white or pink in the spring. Their feathery textured petals are pinnately compound and fragrant-smelling. 

Thorns: Honey Locusts in particular are known for their fierce thorns that grow along the bottom of the trunk. The dangerous thorns can grow to be up to 20 inches long!

Tree size: Locusts trees are unusually tall for ornamental trees, standing 40-80 ft high at full maturity.

Locust Tree Disease

Locust trees are a mainstay in the Denver landscape primarily because of their hardiness, but they do face challenges. The Honey Locust in particular struggles with diseases that can cause injury and even death. Cankers and Root Collar Rot are the most common diseases that plague Denver Locust trees.  Find out what causes Cankers and Root Collar Rot and how to spot the first signs of disease with details in this guide from Colorado State University Extension:

Cankers

  • Thyronectria cankers: caused by the fungi Pleonectria austroamericana, these cankers have bumpy, cushion-like bodies that are light yellow-brown when fresh but blacken with age. It also produces additional smaller bumps that are reddish-brown and also darken. Thyronectria cankers are typically found in bark openings, such as lenticels (raised areas of bark that act as breathing pores) and scattered on bark surfaces in thin-barked areas. 
  • Black Spot Nectria cankers: caused by the fungi Nectria nigrescens, it appears very similar to Thyronectria cankers. This canker can be distinguished by cultural characteristics and the morphology of the fruiting structures. The cankers present as raised bumps on the bark and are creamy to peach colored when fresh, but normally turn dark brown to black within a few days of drying conditions. They usually form under a thin layer of bark or are exposed on the bark surface. The additional bumps are round flask-shaped and reddish-brown, solitary or up to 20 on a raised mass of fungal tissue (stroma) and can be found clustered at the base.
  • Coral Spot Nectria cankers: caused by fungi Nectria cinnabarina, it can appear similar to the two other cankers but Coral Spot Nectria cankers may contain raised smooth fungal masses (sporodochia) that are creamy to coral colored when young (usually for several weeks) and tan, brown or black when mature. Growing cankers can look like small round flasks (perithecia) and form in late summer. They may grow singly or in groups up to 15 and are bright red to reddish-brown.

Root Collar Rot

Root collar rot needs to be recognized ASAP because the disease can rapidly kill trees!

  • Symptoms: in early fall coloration of a portion of the tree may indicate a large amount of damage. Small drops of gum on the stem near the ground or farther up the stem usually indicate that collar rot girdling occurred below that point. Loose bark and discolored wood (yellow to brown instead of white) just below the bark indicate initial collar rot and are the most indicative symptoms. Extensive death and discoloration of bark and wood can occur over several months. Black spot Nectria or Thyronectria cankers at the tree’s base usually indicate collar rot is active or was active in the past.
  • Prevention: Frequent watering in heavy clay soils may induce soil microorganisms to kill the bark and cambium at the tree base just below ground-line. Thyronectria or Blackspot fungi may then infect the weakened tree above the area previously killed by collar rot.

Locust Tree Infestation

The Black Locust and Purple Roble Locust seem to have all the luck, they have very few problems with disease or pests. The Honey Locust, however, is regularly attacked by a whole host of pests. Keep an eye out for possible pest attacks by looking for these signs and attributes described by the Colorado State University Extension:

Honeylocust Borers

  • Honeylocust borers are metallic or long-horned beetles
  • Beetles spend larval stages tunneling under the bark 
  • Injury caused by tunneling can contribute to honeylocust decline
  • Borers can attack and develop successfully only in trees already stressed due to drought, root pruning, disease or other causes
  • Most borer activity occurs in areas of existing cankers
  • External evidence of a honeylocust borer infestation includes “weeping” at wounds and the small circular to oval exit holes made by the adult beetles as they emerge from the trunk
  • Proper watering, tree care and, in particular, wound prevention are the most important techniques for reducing problems with honeylocust borers
  • Supplemental insecticidal controls should consist of maintaining a protectant insecticide on the tree trunk during the egg-laying and egg-hatch period in early summer 
  • Soil drench applications of imidacloprid can be effective if applied in spring to trees not already damaged badly by borers.

Honeylocust Spider Mite

  • Closely related and similar in habits to the common two-spotted spider mite
  • Spider mite is barely visible to the eye and feeds on the undersides of leaves
  • spider mite populations greatly increase in midsummer
  • The foliage of infested trees turns bronze and injured leaves often drop prematurely
  • Spider mite infestations tend to be much greater on street trees and in dry, drought-stressed sites 
  • Regular watering in the summer months helps to reduce mite populations and lessen the damage
  • Chemical controls may be needed to prevent injury
  • Dormant season oil treatments also can suppress honeylocust spider mites

Honeylocust Podgall Midge

  • Commonly cause distortions of new honeylocust growth 
  • Infested leaflets curl and thicken, forming small “pod galls” instead of expanding normally After the adult midges emerge, the galls drop, leaving bare leaf stalks
  • The honeylocust podgall midge has multiple annual generations
  • It is most active from the time of first shoot growth through midsummer
  • Infestations occasionally can be severe enough to temporarily destroy all new growth
  • Repeat insecticide applications will protect the newly expanding leaflets

Locust Tree Care And Treatment Options

Strong and healthy locust trees can fight off cankers,  disease, and pest infestation. Keeping up with tree care is an important investment in your tree’s longevity. Here are our top recommendations for annual care that will help your pine trees thrive:

  • Watering: Proper watering is an overlooked but very important tree care practice. Locust species prefer well-drained soil and can become compromised if their soil becomes too saturated with water. We can provide expert consultation on how much water you should be giving your pine trees, and how often.
  • Fertilization: Locust trees are known for adapting to different soil types, but even the ones commonly found in Denver, can struggle in the high-alkaline soil. The best way to neutralize this threat is with the right fertilizer application. Balancing the soil and getting your trees the nutrients they crave will keep them strong and healthy.
  • Tree trimming: annual pruning and trimming help Locust trees keep their beautiful shape and color. It’s also a good opportunity to check and trim away any parts of the tree suffering from infection or pests. 
  • Insecticides: Locust trees can be proactively protected from borers and mites with the correct chemical spray application. Spraying can also help an infected tree if the infestation is recognized early on. Pest control helps your locust trees maintain their vibrant colorful flowers and lengthens their life-span.
  • Bactistat: trunk injections are an effective proactive measure to prevent and treat both cankers and root collar rot. Trained arborists can administer this application in spring before the spread and onset of the disease during the summer months.

If you are concerned that your locust tree is showing signs of disease or infestation, contact one of our certified arborists immediately! At Fielding Tree & Shrub Care we take a four-step approach for treating locust trees:

  • Step 1: A complimentary on-site disease diagnosis of your trees and/or shrubs. Sending a picture in advance helps speed things up!
  • Step 2: We work to cure the disease or at least slow it down with the most viable treatment option available for your tree or shrub’s unique situation.
  • Step 3: A proactive approach is implemented to protect your existing trees from disease or infestation.
  • Step 4: Unfortunately, sometimes trees and shrubs die, even with our best, safest, and most proven treatment solutions. If your tree or shrub dies, we try to remove the dead plant as soon as reasonably possible. This keeps your property safe from unnecessary infestations and unsightly rotting trees or shrubs.

Keep your locust trees strong and healthy by building a custom care plan with one of our trained arborists. Schedule your complimentary on-site diagnosis today!