Fewer Palms for Miami Beach? Sounds Shady!

Yes, I saw the Martin Vassolo article in the Miami Herald: DUE TO CLIMATE CHANGE, MIAMI BEACH MOVING AWAY FROM PALM TREES TO CREATE MORE SHADE. And there’s somewhat derivative article in The Guardian MIAMI BEACH TO CUT BACK ON FAMOUS PALM TREES OVER CLIMATE CONCERNS.

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. Chinese fan palms have fronds 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.

But don’t tell me a City is cutting back on palms because they don’t produce shade.

First of all there is no such thing as a “shade tree” or “canopy tree” any more than there are “leaf trees” or “root trees”. All living trees have canopies, and all canopies produce shade. Obviously, a modest Live Oak will produce more shade and capture more carbon than a robust cabbage palm. No argument there.

And if you have a park with lots of lawn, I’ll be the first to support planting Live Oaks where they can stretch out and do their thing. But in a parking lot or in the verges* between the curb and sidewalk – nope. They either lift pavement or sidewalks to a point where they have to be removed, or else they are dwarfed and cruelly constrained. They grow for a bit and are then removed and we start all over.Go find me a photo of a 100 year-old (0r even 50 year-old) Live Oak between a curb and sidewalk. I’ll wait.

Meanwhile, here in Sarasota at least, we have cabbage palms that were planted in 1911 and are still providing shade.

There’s a pdf summarizing some of the City’s thinking and an intriguing chart, which, aside from the misspellings, seems problematic to me.

1. There are not a lot of 16″ diameter cabbage palms out there. Questionable data like this reflects the fact that a lot of the analysis of palms has been borrowed from dicots and may not be accurate 2. They project pounds of carbon dioxide stored “over lifetime”. Well, based on the discussion above, what lifetime are they using for their Live Oaks and Cabbage Palms? How does an oak that is pulled after 30 years compare with a cabbage palm that is a 110 years old? And are they considering the carbon costs of nursery growing, irrigating, fertilizing, transporting, planting, then removing and disposing of that oak while the palm soldiers on?

There’s reason to believe the data cited for palms is not as definitive as that for dicots, but the primary fallacy in this sort of trunk versus trunk analysis is the fact that when it comes to shade or rainfall interception the trunk doesn’t matter- it is canopy coverage. And when it comes to canopy coverage, the number of trunks shouldn’t matter. In other words, 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.

Ten cabbage palms are far more versatile (and no doubt cheaper) than one oak with a 38′ canopy.

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:

Palms need not be planted densely to create considerable shade.

Cabbage palms along N. Palm Avenue, Sarasota. The palms closest to the storefront were planted in 1911.
This circular cabbage palm hammock at the Naples Botanic Garden no longer exists, but provided a dense circle of shade that contrasted with the sunny native planting in the center.

Bottom Line:

• If the powers that be in Miami don’t like palms, there are plenty of reasons to dial back on them.

• But failure to produce shade is not one of them.

•In fact, there are many situations where a palm is the best choice for creating urban shade.

• Plant shorter palms closer together – you’ll have as much shade as you can tolerate.

• Life cycle costs need to factored into calculations about the benefits of various tree species. Frequent replacement of trees that outgrow their sites is a real problem.

• Calculations attempting to quantify palms contributions need more scrutiny because too often in the past they have been based on assumptions.

  • *AKA berms, boulevards, curb strips, sidewalk lawns, sidewalk plots, besidewalks, grassplots, park strip, hell strip, tree belt, planter zone and furniture zone. [Courtesy of Roman Mars’ book “The 99% Invisible City“.]