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Adventures in Bird World By Kristen Sanders, Horticulture Coordinator, Denver Zoological Foundation

June 10, 2018 6:47 AM | Anonymous


Some of you may have heard me talk about how as zoo horticulturists we respond to many problems others in our field typically would not. For example, when a Komodo dragon decides to eat the four-foot Agave americana that has been in the exhibit for over a year or when the hot grass goes down in an elephant yard and the resident pachyderms destroy an entire tree. You can try to scold the animal, yet I have found dragons to be quite unrepentant. I often say to friends when describing my day that I had “tapir problems” or “crocodile problems.”

Speaking from experience, I would say that bird problems are the worst. The plants are coated in bird feces, the soil is either drenched or too dry, and mechanical damage is a given. New plants are either picked apart by the birds or crushed by keeper hoses. Pruning is a harrowing experience as birds spontaneously freak out and fly into the wall. Light conditions are abysmal.

I have been responsible for the Bird World exhibits at the Denver Zoo for four and a half years and have found the small triumphs of growing plants in the exhibit to be both extremely satisfying and frustrating. What makes working around birds so amazing is when you win. When you find that right plant for the right place. The plant that was defeated in one area can thrive when placed in a different location. I have killed a lot of plants in Bird World—a lot. It took years of trials to figure out a workable palette to represent each ecosystem held in the building. Five habitats are presented:

  • Rainforest: a large room with several full-sized trees, multiple pools, and a waterfall. Anona squamosal, Dieffenbachia, Dracaena, Codiaeum variegatum, tree ferns, cycads. Very dark and in need of continuous pruning.
  • The Jungle: smaller with one large Ficus, Alpinia, and Trichelia dregeana (Natal mahogany). Too wet and too dry.
  • Tropics: black olive, Citrus mitus, Crinum lilies, Anthurium hookeri, Codiaeum variegatum. Was over-watered and trampled by previous keeper—now doing quite well!
  • Aquatics: Ficus nitidia (dying from white fly), Alocasia portora and ordata, Agalonema ‘Shamrock,’ Dracaena reflexa ‘Song of India,’ Codiaeum variegatum, Trichelia dregeana. Constantly saturated.
  • Swamp: Ficus benjamina, Dracaena, Spathiphyllum, Zamia furfuracea, Spanish moss, assorted tillandsia. Ironically, the driest room of the building.


I have learned to manipulate classic mall plants such as Agalonema, Dieffenbachia, and Spathiphyllums to look somewhat natural. My favorite Agalonema to use in many of the exhibits is the cultivar ‘Shamrock.’ The leaves are solid dark green, not the typical plastic looking gray stripped. They work great as a filler and I have several that have thrived for years in the Aquatics room even though it is regularly blasted with a hose and in sitting water. There are also quite a few Dieffenbachias that do quite well and blend with the natural look ‘Camouflage’ and ‘Jungle Boogie’ are decent cultivars. Codiaeum and Cordylines are great for pops of color and do well if they can receive enough light.

The Ficus trees in the Aquatics room developed a white fly problem a few years ago that has yet to be fully controlled. We have used every tool in the IPM arsenal that has been approved by veterinary staff. The beneficial insect Encarsia formosa was released on multiple occasions with varied results. The material the eggs come on is difficult to keep in place and is also an eyesore. The paper tags would get wet and fall from the trees before the eggs could hatch. Delphastus pusillus has also been released in several exhibits, more successfully in the smaller rooms with less significant infestations. Averaging $500 per release, it is cost prohibitive to have a beneficial program that will truly control the problem in such a large building. Insecticidal soap was not approved as we have two Sloths and sensitive birds in the room. In a desperate attempt to save the trees, a low concentration Imidacloprid soil drench was applied and helped for a short time. We would like to avoid reapplication. Currently, we are consulting with Arborjet to find a formula that will be presented to vet staff for future use. A baby sloth was born a few months ago, so we may have to wait for some time. Sloth problems.

Three years ago I decided to give giant Alocasia a try. The first species planted was portora which has happily grown into an eight foot tall monster. We have also had great success with odora and macrorizos. After years of thriving untouched in the Aquatics room, one or both of the sloths decided to use the portora as a Slip-n-Slide, shredding the leaves with telltale two-toed gashes. Sloth problems.

Many of the original trees planted with the building’s inception are still doing well and have withstood continuous hard pruning and harsh growing conditions. Bucida buceras (black olive) tree continues to thrive in the Tropics room. It has shown resistance to white fly and other pests and has a lovely growth habit. Our arborist treats it yearly with Cambistat to slow growth and reduce the need for heavy pruning. I definitely recommend adding this tree to an existing exhibit or new design. Annona squamosa (a relative of cherimoya known as alligator apple) is doing well in the Rainforest exhibit and has even produced fruit. Natal mahogany, (Trichelia dregeana) works well as a filler in tight spots and can be cut back regularly. Although Ficus are famous for being the toughest of them all, I will not recommend them due to their susceptibility to white fly, especially in a closed environment.

We recently moved in two Cycas circinalis that were outside all summer and had received some cold damage in the fall. After losing the majority of their foliage, they were close to being removed when new shoots began to emerge from the crown and within weeks had grown several feet. They are doing well at this time.

While visiting the Houston Zoo with AZH in 2014 Corri White and I were impressed by the gravel mulch used in many of their bird exhibits. Upon our return to Denver, we introduced the idea to our bird staff. It was not met with much enthusiasm. After multiple negotiations they agreed to let us try it in the smaller Swamp exhibit. Then came the task of selecting the proper substrate. We presented them with multiple options from local purveyors as well as shipped in samples from around the country including what they use at the Houston Zoo. Every one of them was rejected for being too sharp, too round, too big, too small. Nearly a year had passed by with no progress when on a totally unrelated trip to a local hardscaping facility, I happened to see a bin of small-but-not-too-small gravel. I asked the rep what it was and he said it was just regular old squeegee! I took a sample back to the zoo and it was instantly approved. It had been hiding in plain sight the whole time. The installation was a success and we were asked to use it in the Tropics and Aquatics rooms. All was well until our female sloth decided to start eating it. We had to remove three tons of gravel by hand immediately. Sloth problems.

I will be handing over the building to a new horticulturist soon and although I will not say I will miss it, I had some interesting adventures and learned to never give up finding the right plant for the right place.

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