The past year has seen several articles in popular media reporting that various sunbelt cites are dialing back on the use of palm trees in urban areas. The most recent CNN article (Florida is ditching palm trees to fight climate crisis) by Meteorologist Allison Chinchar) refers to West Palm Beach and Miami Beach.
Of course, cities have every right to adjust their plant palettes, but one would hope they are doing so based on comprehensive fact-based analysis and not simply assumptions, pretzel logic, and lingering discrimination.
The introductory sentence, a quote from Penni Redford, the Resilience and Climate Change Manager for West Palm Beach lays out some of the more common climate change-related objections to palms in urban areas:
“Palm trees do not sequester carbon at the same rate as our native canopy trees and do not provide shade, cool down streets, and sidewalks to help counter the urban heat island effect that canopy trees do.”
To begin with, there is no such thing as a “canopy tree” any more than there are “root trees”, “trunk trees”, or ” leaf trees”. Virtually all trees have canopies and all canopies produce shade. The exceptions are dead trees and deciduous trees during their leafless periods. All living palm trees have canopies all the time.
As for carbon, there are two issues. These recent articles feature carbon sequestration — capturing and storing atmospheric carbon, but the article fails to discuss the carbon implications of producing and managing (and retiring) a street tree.
The Big Picture — Life Cycle Implications
Consequently. it would be appropriate to look at the entire ecological footprint or carbon footprint over the life span of any urban landscape plant before making judgements. Let’s start at the front end: How did this tree arrive to be planted in an urban setting? What “carbon skeletons” are in its closet?
Is the plant in question nursery grown? If so, how much energy/carbon was consumed in propagating the plant from seeds or cuttings, the carbon implications of obtaining growing mix, the sequence of plastic pots of various sizes, the irrigation layouts, fertilizers, biocides, weed suppression, and the electricity demands for pumping irrigation water. Oh, and the carbon implications of the human labor involved. For a decent-sized oak I should think that could be quite a lot. Has anyone bothered to calculate these carbon implications? It would be arduous, but valuable to see what sort of carbon burden a given landscape tree arrives with.
Now compare that nursery grown oak with our native cabbage palm, Sabal palmetto, the landscape tree I am most familiar with. No one collects or plants seed. No growing medium is prepared. No plastic pots are used. There is no irrigation delivery system. No fertilizers are used. There is no weed suppression. No electricity is used to pump water. For 99+% of the tree’s life before being installed in an urban setting, no human use of carbon is implicated. All this suggests that landscape cabbage palms are the most sustainably grown native plant. But those facts didn’t creep into any of these articles.
And the reason cabbage palms are so sustainably grown is because virtually all commercial landscape cabbage palms in Florida are grown on ranches where they reproduce and grow naturally. The only carbon implications are when equipment is finally brought in to dig the palms, wrap the root base in thin plastic and then relocate it to a place with irrigation. Trucking to a final urban destination and planting should be comparable to an oak. And when the cabbage palms are removed from ranches, they create space for new palm seeds to germinate and grow — continuing the cycle of sustainability.
Then we have the implications of managing the tree in place. Dicot trees (the oaks and mahoganys mentioned in the CNN article) exhibit what is known as secondary growth. Unlike cabbage palms, their trunks get fatter every year. Unfortunately, so do the roots. The ever expanding girth of the roots reliably lifts sidewalks (or parking lot pavement), leading to tripping hazards that risk managers decide necessitate energy-consuming grinding down of the sidewalks, and, in worst case situations, removal of the trees. That reality must be factored into lifespan carbon implications of using dicot trees. Various fixes have been developed in effort to constrain or direct dicot roots, and the carbon implications of these more or less elaborate measures should be accounted for as well.
Then there are the leaves. All trees produce leaves and then shed them. Some of these leaves can produce sidewalk slipping hazards, others can coagulate to block stormwater drains. With dicots you end up with many small leaves — with palms, a few big leaves. What are the costs of dealing with each extreme? In addition to dealing with the leaves once they have fallen, the leaves (fronds) of many palm species need to be pruned before they fall naturally due to the implications of their possibly falling on someone. This is typically due either to the weight of the fronds, or the fact that some are heavily armed. While cabbage palm fronds are typically pruned, I have yet to run across data suggesting people have been injured by falling cabbage palm fronds.
I haven’t mentioned branches. Palms don’t have branches, so the carbon impact of pruning palm branches is zero. Dicot street trees need to be pruned to avoid hitting storefronts and to get their branches up above truck height in the adjacent travel lane. In addition, many street trees are “lifted” — have their lower branches removed to reduce the temptation (and hence, liability) of climbing into the trees. (And where does the carbon in those pruned branches end up?)
Then we have the end game. Calculations about how well a tree performs need to take into account how often they need to be replaced. While most dicots eventually get pulled because they outgrow urban settings, palms need not. The oldest landscape trees in downtown Sarasota at least, are cabbage palms planted in 1911. There are landscape palms elsewhere in Florida that are 200 years old. Are there old oak trees? Absolutely, but they’re not found in urban areas surrounded by concrete, storefronts and paved streets.
Based on the CNN article, one might assume live oaks are outperforming cabbage palms. Let’s look at some data from Miami Beach. No figures are provided for total canopy coverage. But the numbers are clear — so far, at least, on Miami Beach cabbage palms are doing a better job at conserving energy, filtering stormwater, improving air quality, removing carbon dioxide, and storing carbon dioxide than live oaks. How is that possible??
Partially because some of the oaks were planted relatively recently and in part because there are far more cabbage palms. For every live oak in Miami Beach there are nearly two cabbage palms (1.88). So, in order to compare on a tree by tree basis (questioned later), the cabbage palms would have to roughly twice as beneficial as the live oaks. And, for the most part they are. 2.18 times better on energy conserved, 1.74 times better on stormwater filtered, 2.3 times better on air quality, 2.15 on carbon dioxide removed, and 1.46 on carbon dioxide stored to date. So removing cabbage palms at this point would be a mistake, as would removing them prematurely. So Miami Beach managers can consider replacing them in 2120.
These presumed values are based on iTree models, which may not be accurate for palms.
Here’s another quote from the CNN article: “Southern Live oak, Quercus Virginiana — large canopy trees, can withstand occasional floods and hurricanes and are resistant to salt spray, provide habitat for birds and a variety of moss and bromeliads in south Florida,” Redford said.
That’s interesting. Did you know that a study done in the wake of 2004 hurricanes Charley, Frances, Ivan, and Jeanne found that cabbage palms are more wind resistant than live oaks? You can read all about it in the book Stormscaping. Cabbage palms are also far more resistant to salt spray and salt water flooding than live oaks. And they are comparable to oaks in terms of the wildlife and epiflora they support.
Carbon Sequestration
Apparently, palms are primarily being dinged for their presumed minimal carbon sequestration. I’m not sure why we would be looking to urban areas to maximize carbon sequestration, but people are.
Here’s a tip — if you really want to maximize carbon sequestration, I’ve got a great tree for you. There are 850 of them in Miami Beach and those 850 are sequestering seven times the carbon of the 2,308 live oaks. I mean these things really pack the carbon away and they grow great in sand. Ask for them by name: Australian Pine.
In order for carbon sequestration to be valid, the carbon actually has to be sequestered. If some urban oaks or their pruned branches are sold as firewood or consumed at Big Jim’s Real Pit BBQ, then any carbon sequestration benefits are literally going up in smoke. Even if mulched, much of the carbon ends up back in the atmosphere. To really get us somewhere, the carbon stored in urban trees needs to remain stored. So, before anyone starts crowing about the carbon stored in urban trees, there needs to be some solid analysis about what actually happens to the trees when they are inevitably removed. Ideally they would end up as lumber, protected from the elements in buildings, but that seems an unlikely fate for street trees such as oaks and black olives.
What of cabbage palm lasting sequestration you ask? The century-old home below in Holly Hlll, Florida is rumored to have been built from cabbage palms that once lined a local road and which were cut down for widening. So, sometimes at least, the palm carbon tortoises outdistance the oak carbon hares.
Shady Reasoning
Then we have the argument that palm trees “do not provide shade” (that’s another quote from the CNN article from someone who should know better) despite the fact that they obviously do. Does a 30 year old cabbage palm in an urban setting produce as much shade as a 30-year old live oak? No. But a fifty year old cabbage palm might produce more shade than a downtown live oak planted fifty years previously. That’s because the live oak likely will have been removed before it ever gets to fifty.
The primary fallacy regarding palms and shade is this sort of trunk versus trunk analysis, when the fact that is that when it comes to shade or rainfall interception the trunk doesn’t matter- it is canopy coverage that counts. And when it comes to canopy coverage, the number of trunks shouldn’t matter.
Flipping the argument helps reveal why comparing individual plants with each other is nonsensical. If I noted that a single palm frond intercepts more rainfall and stores more carbon than a single oak leaf, reasonable readers would point out that it is not appropriate to compare one to one — the proper approach would be to compare all the fronds with all the dicot leaves. Exactly. And that’s why we should be comparing equal areas of palm canopy with equal areas of oak canopy.
Yes, the oak canopy keeps growing in size, and, as noted, that’s how they get in trouble and end up being removed before they max out.
The other fallacy is that you can plant an oak wherever you might plant a cabbage palm. Not so. Look at these Google streetview photos below from Miami’s Calle Ocho. Notice anything strange? There’s no pervious surface anywhere in sight. These have concrete or asphalt radiating from the trunks in every direction, yet are healthy trees. Can a Live Oak (or any other dicot) do that? No. So, in this situation it is not a choice between a supposed $31 of annual benefits versus $6.48, but rather zero dollars versus $6.48.
And just look at that shade over the sidewalk. Planting slow-growing palms keeps the shade close. Had these palms been planted on 12 foot centers there would have been continuous shade along this highway, with no risk of the branches growing out over the travel lane to be whacked by large trucks.
7-11 on Calle Ocho, Miami
Here’s what a clump of cabbage palms can do with lawn, and in an urban street tree setting. Any shade?
Palms need not be planted densely to create considerable shade.
The fact is cabbage palms can produce so much shade you may have trouble finding the sun. You just have to plant them close together. And there’s no reasons you can’t do that. In fact, planting palm between a driving lane and a sidewalk can produce not only shade, but safety.
There are many reasons to not plant palms in urban areas. Coconuts are a liability due to the damage posed by those darn falling coconuts. As a result they require frequent pruning to avoid creating risk. Royal palms are self-cleaning, but their fronds are enormous and could also be problematic if they hit someone. Some palms have leaf stems with saw teeth a shark would envy. And the date palms have long spines on their fronds that can send people to the emergency room. If OSHA regulated Phoenix (date) palms, they might be illegal. And this relatively new disease, Lethal Bronzing, can kill expensive Sylvester palms and cabbage palms alike.
In addition, over-reliance on just a few tree species is a dangerous practice, since an unanticipated disease, pest, or weather phenomena can take out an entire cohort of a single species. That argues against only planting palms in the sunbelt. It also argues against only planting dicots where palms are an option.
Before concluding it makes sense to ditch urban palms in the Sunbelt, some additional research is needed to flesh out the complete picture related to carbon, shade, and other factors.
• Life cycle (carbon footprint) costs need to factored into calculations about the benefits of various tree species. It is not enough to start counting once a tree is planted in an urban setting. Its carbon skeleton needs to be accounted for. This analysis needs to incorporate management costs and how frequently trees need to be replaced and what happens to their carbon afterwards.
• Calculations attempting to quantify palms contributions need more scrutiny because too often in the past they have been projected on assumptions derived from dicot trees.
Bottom line: If cities have legitimate concerns about palms, I’m sympathetic. Right plant, right place. But embarking on campaigns to decrease palm use based on assertions about shade, rainfall interception, and carbon sequestration is problematic without considering overall life span implications, the many urban situations where dicots underperform, and the potential of using multiple palms instead of single plants.
One more thing
I don’t know about Miami Beach or West Palm, but the only native tree species that reliably invades downtown Sarasota landscapes and grows to maturity is the cabbage palm. These unbidden volunteers make up a significant portion of the downtown canopy and are doing all the great things trees do without having been planted. While we have to account for the carbon implications of dealing with the fronds, there are no other costs associated with these trees — great examples of the free services natural systems can provide. This is all bonus canopy courtesy of palms.
