Starting in the spring, the petals of rainbow shower trees burst with color, enveloping their branches in a soft crown of orange, cream, pink, red and yellow hues. As the wind ruffles the leaves, the colors of the trees shift as different parts of the petal are exposed.

In 1965, Mayor Neal Blaisdell proclaimed the plant Honolulu’s official tree. It can be seen in front of Iolani Palace, dotting Kapiolani Park and decorating streets at the University of Hawaii at Manoa.

But the tree is in trouble because it isn’t native.

A new city law bans the tree from being planted on city property. The city must plant indigenous or Polynesian plants, brought over to Oahu prior to western contact, at new or renovated facilities whenever “feasible.”

Ordinance 14-6 is intended to help propagate native species that have been devastated, according to the law’s supporters, by the influx of new plants in the centuries since Captain James Cook reached Kauai in 1778.

Flickr: Rainbow shower tree

Those supporters — who include some scientists and invasive species experts — say that native plants are not only better for the environment, but also carry cultural and emotional value.

The native plants “are what makes Hawaii so unique,” said Taylor Marsh, a field supervisor at the Oahu Invasive Species Council. “That’s a cultural thing and that’s a moral thing.”

The bill passed the City Council easily with a 9 — 0 vote. But Mayor Kirk Caldwell, who allowed the measure to become law last Friday without his signature, says that the measure is impossible to implement and carries unintended consequences for iconic plants like the rainbow shower tree.

“Imagine how Honolulu would look without these stunning trees lining our streets for tourists and residents to enjoy,” he wrote in a letter to Honolulu City Council members on Friday that outlined his objections.

David Haughes, who was head nurseryman for the Board of Agriculture of the Territory of Hawaii nearly a century ago, bred the rainbow shower tree, which is formally known as the Wilhelmina Tenney, in 1918. The Honolulu Star-Bulletin reported that Haughes crossed the golden shower tree, a native of northern India, and the pink and white shower tree, a native of Java and Sumatra.

The Tree Battle

Oddly, a state procurement law already requires that native plants be used in public landscaping whenever possible, suggesting that the city may have been breaking the law in recent years with its plant selection.

Councilwoman Ann Kobayashi, who introduced the bill, says she does not think the city has been following the law and needed a reminder.

“I thought we should pass this at the city level to remind us that this is what we are supposed to be doing,” she told Civil Beat.

The mayor’s office stressed that the city law was duplicative and unnecessary. But a spokesman for Caldwell didn’t respond to questions about whether city parks officials have been violating the state procurement code.

But Caldwell says that the law, while well-intentioned, is flawed and not just for aesthetic reasons. Certain native plants are more expensive and might not provide benefits over non-native species when it comes to the urban environment.

And then there is the question of alternatives, or the lack thereof. The city uses turf grass throughout city parks where people lounge, picnic and play sports. There is no suitable native alternative, Caldwell says.

“Turf grass should be of a fine texture, high density, green color, insect and disease resistant, require minimal fertilization, be pleasant to sit or lay on, drought tolerant and thrive being mowed at a two-inch height,” Caldwell told council members.

Then there is the rainbow shower tree, which is admired for its beauty, but it also provides helpful shade in the urban environment and reduces rainwater runoff. So does the monkeypod tree, which can be seen forming a dense canopy over Paki Avenue between Monsarrat Avenue and the Ala Wai Canal.

“Both these trees share the feature of having lovely, broad canopies,” the mayor wrote in his letter to council members. “Canopy size is highly regarded as conveying environmental benefits in the urban core because it lessens impacts caused by stormwater runoff, reduces energy consumption and heat and air pollutants.”

Turning Back the Ecological Clock

Joe DeFrank, a weed scientist at the University of Hawaii’s College of Tropical Agriculture and Human Resources (CTHAR), is working to plant native seedbeds along state highways. He says that native plants can carry significant cultural and environmental benefits, but he agrees with the mayor that it can be challenging to plant them.

Much has evolved on the island since western contact. “If you want native plants, you have to make native soil,” he says. “We don’t have a native environment.”

Urban Oahu soil can have high levels of arsenic, lead or other heavy metals brought on by pollution, he noted. It can contain less water because of road construction and have different levels of organic matter and nutrients, compared to centuries past.

PF Bentley/Civil Beat

Monkeypod trees

Native plants also have to compete with other plants, including invasive species. Some thrive on the island, including the destructive albizia tree and miconia, a thick, shrub-like flowering plant that can reduce the amount of rainwater that seeps into forest watersheds.

DeFrank estimates that no more than 20 percent of Oahu’s landscaping consists of native species, and that the actual amount could be as little as 1 percent.

Shade and Safety

When it comes to shade trees, there are native varieties, DeFrank notes, but they can have drawbacks.

“There’s the kukui tree, but they make that big old nut and you don’t want that bumping people on the head,” he said.

The mayor says that two other shade trees native to Oahu, the alahee and lonomea, also have hard nuts that can fall on sidewalks and pedestrians, creating potential safety hazards.

But Marsh, of the Invasive Species Council, disagrees. He says that there are good native shade trees, such as lonomea, koa, hoawa and kamani.

He went on to take a dig at the mayor’s trumpeted monkeypod tree.

“It (produces) leaves, fruits or flowers non-stop,” he said. “So if you’re talking about lots of rubbish at parks, monkeypods are a bad alternative to native species.”

Despite the push to cultivate native species — whether for ecological, cultural or emotional reasons — some people question whether humans would ultimately thrive in such an entirely native environment.

The law “has good intentions and feels like something that we should attempt to do when we can,” said Ken Grace, associate dean and director of research at UH’s CTHAR.

“The problem is we are invasive organisms. People are invasive. We might not do well in a pre-contact environment because we have shaped (the environment) in a way that we can do well in.”