This is reprinted from my weekly post as part of Bruce Bratton’s excellent weekly brattononline.com This post was modified from the original in response to critique by Gillian Greensite who has followed this issue for many years. My content was largely informed by a science conference on the subject, with a record of many materials here.

Many Californians have opinions about Eucalyptus. Either you are for them or against them. Its a subject like politics or religion that you hesitate to bring up at the dinner table. As with Covid-19 vaccination, you can’t predict who’ll be on what side (or why) – people of any political persuasion can surprise you with their beliefs. I’m betting that you know what I’m talking about…I didn’t even need to mention which of the hundreds of species of Eucalyptus I am talking about.

Eucalyptus Bad

The most common concern I hear about blue gum (Eucalyptus globulus) in California is how fire-dangerous it is. Locally, some recall the 2008 Trabing Fire just north of Watsonville, ignited on a hot day by a poorly running vehicle backfiring, and spitting out fire balls along Highway 1. Grass caught fire and quickly spread into nearby invasive pines, acacia and Eucalyptus (those have since regrown denser than before). That fire surprised fire-fighting professionals from how high embers were flying…hitting their fire monitoring planes at altitudes previously thought safe. They cited the architecture of Eucalyptus forests…the tall, close very vertical trunks create chimney-like conditions, hurling fiery brands much further than expected. Leaves with volatile oils and large amounts of bark and branches accumulated in the understory are other reasons for fire concern.

Eucalyptus Good

The most common defense of Eucalyptus I have encountered is its beauty. Our cityscapes have surprisingly few trees, but there are almost always huge Eucalyptus nearby. Many are fond of their massive trunks, shaggy bark, and towering, spreading canopies, shimmering with blue green leaves. I have seen many painters capturing the alluring patterns of rows of old Eucalyptus trees in many seasons, in many shades of light. A few people will dedicate their spare time and energy to protect big old Eucalyptus city trees from the too-frequent human desire to cut down trees.

What Do the Birds Say?

If birds are any indicator, Eucalyptus is good in some places and bad in other places. Birds like city trees including Eucalyptus. Eucalyptus adjacent to larger bodies of water are attractive to birds. You may have seen masses of herons and egrets using Eucalyptus as ‘rookeries’ where they raise their young. Trees near the Santa Cruz Yacht Harbor are roosting areas for herons. The Eucalyptus grove south and inland of the Elkhorn Slough Bridge in Moss Landing has a huge rookery, with so many birds that their guano is killing the trees. Peregrine falcons were using the talk Eucalyptus near the river mouth for a while. Raptors like the tallest trees for nests and perches.

Gum Gone Wild

Eucalyptus in our area is considered to have a moderate threat of invasiveness, with regionally specific higher rate of spread in foggy areas and in areas with more water availability, especially along the Central Coast. As with so many Eucalyptus issues, this was once a source of controversy before Eric Van Dyke at the Elkhorn Slough Reserve demonstrated an 8 foot per year rate of spread of groves in northern Monterey County. Since then, many other examples of the species’ ability to spread in our region have been documented. Where Eucalyptus spreads into streamside habitats, there is a particularly bad impact for bird conservation.

River Gum Bad

Riverside or streamside (aka ‘riparian’) habitats are by far the most crucial targets for bird conservation in California. Most of these habitats have been highly altered and are no longer good habitat for wildlife. Many migratory birds visiting from the tropics nest in those habitats. The loss of riparian bird habitat compounds with the loss of tropical forests, and so these birds are particularly imperiled. Riparian ecosystems host many cavity nesting birds that favor holes in the soft wood of riparian trees like willows, cottonwoods, and alders. Eucalyptus trees quickly invade and transform diverse riparian forest, and cavities become much less common. Bird conservationists say that controlling Eucalyptus in riparian areas should be a ‘no brainer.’

Euc Pests

Some types of birds have recently been newly attracted to Eucalyptus because they like at least one of its natural pests that found its way to California. The blue gum psyllid is apparently tasty for birds such as warblers. I’m less sure if birds are eating other ‘new’ Eucalyptus pests: apparently a number of blue gum eating pests recently found their way to California. It used to be that Eucalyptus leaves were perfectly shaped, no damage- nothing ate them! Now, those leaves look like someone took a paper punch to them. Eucalyptus tortoise beetle are eating blue gum leaves – does anyone know if birds like to eat it or other of the new Euc pests?

The Arrival of Eucalyptus

Eucalyptus has a long history in California. It was widely planted in the 1870’s to address the ‘hardwood famine.’ Hardwood was becoming scarce because of its use as fuel for steam engines and heat, so there was a Eucalyptus planting boom. Eucalyptus was soon advertised as the solution to many problems: a fast-growing hardwood for fuel, people thought its wood could be used for railroad ties and other lumber, people said the tree would dry up wetlands and reduce mosquitoes, and its fast growth attracted people to plant it for windbreaks. People were buying large numbers of seedlings. Some advertised, promising investors good returns from productive Eucalyptus wood lots.

Hardwood, though, eventually lost favor to petroleum in California. But, if you travel to Central or South America, where hardwood is still important for fuel, you will notice many areas managed for Eucalyptus firewood.

Heavy and Twisty

It turned out that Eucalyptus wood twists and buckles when drying, so it was eventually recognized as useless for lumber. Well, almost. 15 years ago, someone claimed they had a process for drying Eucalyptus “correctly” so that it could be used lumber, including for picnic tables. They donated one to the organization I worked for…it weighed 250 pounds and took 4 people to move! After a couple of years it was impossible to use. It was so warped that when people sat on it, it rocked wildly about, and created a balancing challenge with people bobbing around spilling their drinks at vastly different elevations.

Perhaps this would be different if the wood were kept dry, indoors. Woodlots for Eucalyptus hardwood are still around, but you are more likely to see Eucalyptus spreading from old, planted windbreaks. Look carefully for the oldest biggest trees in a row with many generations of younger trees spreading from there. One thing remains true from the old hype: Eucalyptus does well at drying wetlands!

Drink it Up

With its huge canopy thick with leaves, Eucalyptus is known globally for its thirsty nature. Deforestation in its native home in Australia led to salinization of the soil from the evaporating heightened water table. Here in California, people note the loss of springs where Eucalyptus grows. Although closer scrutiny is needed, using transpiration rates from Eucalyptus elsewhere in similar climates, it is likely that a grove of Eucalyptus drinks most of the rainfall falling on it along our coast. This is much more water than native trees use. One day, one mitigation for new development that demands more water might be investment in Eucalyptus control.

Thinning and Containing

Given the fire danger and negative ecological and water impacts of most Eucalyptus groves, it is sad that they are still proliferating. To be sure, Eucalyptus control is an expensive proposition. Having felled several large trees, I can attest to the work it takes to clean up a fallen tree properly. The wood makes great firewood and is easy to split if you split it soon after felling. But there is an enormous amount of slash to deal with…chipping or burn piles- either way a lot of work. The stand-out organization for Eucalyptus control locally is State Parks. They are ‘thinning and containing’ some groves that people like to look at while obliterating others in ecologically sensitive areas. They realize that Eucalyptus control will cost more each year they wait, so they do what they can with the (too few) resources that our elected officials budget for them.

Fer it or Agin it?

After reading this, maybe you will have a more informed opinion about this provocative tree. It is my hope that you be ‘for’ the ones that grow near bodies of water or are city trees and ‘against’ the ones in riparian areas or spreading through our other precious native ecosystems.