In a 1936 speech for the Los Angeles County Farm Bureau, William Hertrich, Henry E. Huntington’s superintendent of grounds, recalled his initial encounter with an avocado tree: “When I came to California in 1903, I saw, for the first time, an avocado tree planted in the open. The following year I tasted the fruit for the first time but as it was a little overripe did not care for it. Soon thereafter I became associated with the Huntington interests. Early in 1905 Mr. Huntington came out one afternoon with a dozen avocado seeds in his pocket. He said, ‘Hertrich, do you know what these are? Do you think you could grow these here? Could we plant an orchard?’ I said, ‘I think we can grow them here, but I don’t know about the orchard. If you are willing to stand the financial strain, I will be glad to make the experiment.’”
More than a century later, the avocado experiment continues in ways that would have astonished Hertrich and Huntington. In addition to trees that remain from their initial trials, a special orchard comprises varieties that are rare in commercial agriculture, and The Huntington’s Conservation Laboratory maintains a collection of “test tube trees”—additional varieties and related plants in tissue culture.
“It is a rare and gratifying opportunity to work with plants that connect natural and cultural history so directly with Huntington’s legacy,” said Cryopreservation Research Botanist Raquel Folgado, who oversees conservation science projects on avocado and other wild plants at The Huntington. “This collection is an important living link between many communities, and our work with these trees reflects the expanding understanding of plant science.”
Huntington’s avocado orchard, one of the first in Southern California, was commercially harvested until 1957. Two of the trees planted in 1907 as part of Hertrich’s experiment survive today. Grown from seed, they were intended to be grafted later with selected fruit varieties, though this did not occur. The most impressive tree of the two stands in the pedestrian walkway near the Allen Avenue entrance and still produces fruit. The other tree is adjacent to the Shepherd-Brokaw Orchard, a collection of 33 different cultivated varieties, or cultivars, of avocado—a fraction of some 500 existing avocado cultivars, all developed from a single species: Persea americana.
The Shepherd-Brokaw Orchard was established in 2010, with heritage cultivars provided by the University of California South Coast Field Station and Ventura-based Brokaw Nursery, the country’s largest commercial producer of avocado trees. Superficially, these trees look identical, with only subtle differences in foliage and form, but the diversity of fruit shapes, sizes, colors, and textures is startling. In addition to the familiar pear-shaped ‘Hass’ with its pebbly, greenish-black rind, the collection includes the larger and more rotund ‘Reed’; ‘Mexicola’, which resembles a gleaming black plum when it ripens; and the shiny, long-necked ‘Pinkerton’. Many who have sampled these cultivars consider them to be superior in flavor to the ‘Hass’, but most have never appeared in markets. In some cases, the fruits have thin skins, which make them too delicate for large-scale shipping. In other cases, the trees don’t produce a high enough yield to make them commercially valuable. ‘Hass’ dominates the market because it behaves well in cultivation and produces ample quantities of consistently good fruit that can be easily harvested and transported. About 95% of all avocados sold in the United States (and about 80% worldwide) are ‘Hass’. The remaining 5% are predominantly ‘Fuerte’, noteworthy for its tolerance of cold weather.
All ‘Hass’ avocados produced today come from trees that were propagated by cuttings taken from the original ‘Hass’ avocado tree, which was grown in La Habra Heights, California. This parent tree grew from a seed obtained in 1926 by a mail carrier and amateur horticulturist named Rudolph Hass. (The tree eventually developed root rot in 2002 and died shortly thereafter.) ‘Fuerte’ came to California via the West India Gardens nursery, founded in 1911 by Frederick O. Popenoe in Altadena, California. One of Popenoe’s sons collected the rootstock (a tree’s roots and base) in Puebla, Mexico, and the tree that grew from it withstood an extreme frost in 1913, thereby earning its cultivar name, which is Spanish for “strong.” More than 95% of all ‘Fuerte’ avocados on the market today are descended from that parent tree.
Though commercial avocado orchards did not arise until the early 20th century in California and Florida, the cultural history of the fruit stretches back thousands of years. The earliest European accounts of avocados were from Spanish colonizers, notably Martín Fernández de Enciso (ca. 1470–ca. 1528), whose journals included descriptions of the fruit, which was highly valued by the Indigenous people he encountered. Evidence suggests that Mesoamerican cultures had been gathering and consuming avocados for about 10,000 years and cultivating the trees for some 5,000 years. In addition to archaeological remains of avocado rinds and pits found in human habitations in the Tehuacán-Cuicatlán Valley of south-central Mexico, anthropological references to avocado appear in the form of pre-Columbian glyphs in proper names, indicating the importance of the plant. For example, the Maya city of Pusilhá was represented by glyphs that translate to “kingdom of the avocado,” and Ahuacatlán is a “place where avocado abounds.” The symbol for the 14th month of the Maya calendar incorporates the glyph for avocado.
Mesoamerican people long cultivated avocado in the forests of South and Central America, fostering the growth of trees that produced superior fruit and weeding out others. But these traditional agricultural practices were unfamiliar to Europeans. To Gonzalo Fernández de Oviedo y Valdés (1478–1557), who explored the Americas in the early 16th century, the influence of Indigenous people was invisible, and he attributed the quality of the fruit to the divine, as he wrote in his 1526 report to King Charles V of Spain: “Those who have these fruits [avocado] guard them and esteem them highly and the trees are wild as are the others which I have mentioned, for the chief gardener is God, and the Indians apply no work whatever to them.”
Developing palatable fruit of almost any kind is a demanding process. Most fruit tree seedlings are like human siblings: Each is an individual with unique traits. Were you to take all the seeds from a single apple and plant them, each resulting tree would produce fruit that differs from that of the parents and one another. Each might potentially have characteristics that could prove vital to withstanding changes in the climate or the arrival of new pests and diseases. And some would produce fruits that are more pleasing to the human palate than others. Eventually, early agriculturists discovered that they could propagate a plant that makes exceptional fruit by taking cuttings (called scions) from that plant and grafting them onto the base of another tree (rootstock) of the same species. And just as the scion is selected for its fruit, different rootstock varieties contribute specific traits to the tree, such as producing a dwarf form or resistance to pathogens and pests.
Avocado seedlings are not only exceptionally unpredictable, but each fruit contains only one seed, and it can take up to 15 years for the resulting tree to mature enough to bear fruit. As a result, developing new varieties of avocado from seed is resource intensive and often disappointing. Many trees produce fruit with huge pits or flesh that is fibrous rather than smooth and buttery. Consequently, commercial production relies on tried-and-true cultivars and that begins with an elaborate propagation process.
While most fruit crops are grown from scions grafted onto rootstock that is produced by seed or from cuttings, raising an orchard of avocado cultivars involves grafting each tree twice. First, nurse seedlings are grown, and then, when they reach a suitable size, cuttings from the avocado variety chosen for the rootstock (clonal rootstock) are grafted onto them. Avocado cuttings are difficult to root, so the nurse seedlings support the rootstock cuttings until they develop enough to overtake the seedling root systems. The desired avocado fruit variety can then be grafted onto these rootstock clones. The labor-intensive, double grafting technique is unique to avocados and necessary to produce consistent fruit. The resulting tree can produce fruit in as little as three years.
This propagation method results in vast numbers of genetically identical plants—the opposite result to what happens under natural conditions, wherein biodiversity and genetic richness are key to survival. The loss of diversity that occurs with this type of agriculture represents a threat to food security because new pathogens or other stressors, including extreme weather events associated with climate change, can cause the wholesale devastation of crops. Although the many avocado cultivars in The Huntington’s collection were all developed from a single species, their varied characteristics present a vital learning opportunity.
The avocado genome was published in 2019, providing scientists with a map of P. americana’s DNA that shows the location of genes governing different aspects of the plant’s anatomy. Genotypes are the variations of how these genes are expressed in individual trees; the range of cultivar characteristics demonstrates the potential diversity within a single species. Studying genotypes contributes to our understanding of the genome. So, maintaining living collections of as many varieties of avocado as possible is valuable to both agricultural and botanical science.
In addition to the Shepherd-Brokaw Orchard, The Huntington’s Conservation Laboratory maintains an in vitro collection of more than 50 Persea genotypes, including adult and seedling forms of P. americana as well as three wild relatives: P. palustris, P. indica, and P. podadenia. Micropropagation (also known as tissue culturing) is a technique that involves culturing small amounts of plant tissue in a synthetic nutrient medium. The gel that nurtures the plantlets can be adjusted to control the speed and manner of growth, making it possible to maintain the plantlets in a state of suspended animation. For longer-term storage, cryopreservation (cooling and storing plant cells, tissues, or organs in liquid nitrogen) makes it possible to maintain seeds, pollen, shoot tips, or dormant buds indefinitely. These materials can eventually be thawed and used to generate new plants through micropropagation. Because conventional avocado propagation methods are so labor intensive, nurseries have found it difficult to grow enough plants to meet the demand of commercial orchards. Further refining micropropagation methods for avocado could eventually boost the market by enabling nurseries to more easily produce large quantities of both rootstock and cultivar plants.
Huntington staff has used micropropagation to collaborate with colleagues at other institutions who are interested in conserving wild Persea species. Three plants propagated from a P. podadenia in The Huntington’s collection were sent to Marie Selby Botanical Gardens in Sarasota, Florida. Naples Botanical Garden, also in Florida, wanted to propagate P. palustris in its collection and shipped cuttings to The Huntington, where they are currently being established in tissue culture. The Huntington publishes its micropropagation protocols to assist other facilities that want to establish their own labs. The same is true for cryopreservation. An important Huntington goal is to share expertise and protocols with facilities that have greater capacity for long-term storage banks. As part of an ongoing collaboration with the University of Queensland, Australia, botanical staff, led by Folgado, developed protocols for ‘Reed’ and ‘Velvick’ avocado cultivars.
“It is truly wonderful that our living collections have supported Raquel Folgado’s research on the propagation and long-term storage of this iconic food crop,” said Sean Lahmeyer, The Huntington’s associate director of botanical collections, conservation, and research. “Partnerships have been at the heart of her research, and we hope to expand this approach to other plant groups as we move forward. The in vitro Persea collection that she has built up may be the only one of its kind in North America and points to the value of maintaining unique collections that are, in this case, dedicated to research.”
The ability to cultivate and promote new avocado varieties will become increasingly important as the changing climate alters growing conditions. It may be possible to breed plants that can tolerate higher temperatures and require less water. Trees that thrive at lower temperatures might result in varieties that could be grown in cooler climate zones, thereby reducing the need for shipping. It is also essential to maintain access to a diversity of Persea species and avocado varieties in order to develop trees that are resistant to new pathogens and such pests as the Redbay Ambrosia Beetle (Xyleborus glabratus), which was unintentionally introduced to the southeastern United States in 2002. Since its arrival, few strategies have been successful in controlling the devastating fungal disease that the beetle spreads, but the wild avocado species P. palustris has demonstrated some resistance to the fungus, making this plant an intriguing candidate for further study.
Diversity enriches both the natural world and human cultures. Just as fresh perspectives lead to multiple approaches to the obstacles we face as a society, genetic diversity presents a wealth of possible solutions for the challenges of sustainability and climate change. The Huntington’s avocado collection embodies significant connections to the past and a wealth of potential for the future.
Sandy Masuo is the senior writer in The Huntington’s Office of Communications and Marketing.
- Huntington Frontiers: A Garden in Deep Freeze
- Verso: Strengthening Bonds with Mexico through Cryopreservation
- Verso: Mr. Huntington’s Garden: A Is for Avocado
- Verso: Lectures: The Fruits of Their Labors