Plant Palette

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Photo of Jennifer Schultz Nelson

Jennifer Schultz Nelson
Extension Educator, Horticulture

Recent news reports showed the sad results of freezing temperatures hitting California–especially half-frozen citrus and avocados. Each report I saw mentioned that the cold temperatures not only cost growers millions of dollars in lost harvest this year, but that the crops were potentially affected for two years. My husband wondered why this would affect the crops for two years. The answer has a lot to do with avocados.

Living in the Midwest, it is easy to forget that many other parts of the world do not have to cram all their growing into a three to five month growing season. We are very conditioned to and used to plants flowering in the spring, and their fruits being harvested in the summer and fall.

Avocados are native to tropical climates. There are three "races" recognized, based on their region of origin: Mexican, Guatemalan, and West Indian. Since they evolved in tropical areas that never regularly experience freezing temperatures, they are not particularly hardy in freezing temperatures. Mexican types are tolerant of the coldest temperatures, down to as low as 19 degrees Fahrenheit. Guatemalan types take the middle road, tolerating down to around 26 degrees Fahrenheit by some accounts. West Indian types will barely tolerate 32 degrees Fahrenheit.

Although avocados are tolerant of varying degrees of low temperatures, they cannot withstand these temperatures for extended periods. Their life cycle demands a warmer climate, as they flower anywhere from January to March.

Avocado's flowers are unique, some might consider them wasteful. They are an indistinct yellow-green and about one-half an inch in diameter, borne in clusters of two to three hundred near the end of branches. Of these flowers, up to 5% are sterile and non-functional. Despite hundreds of functional flowers remaining, only one to three fruits will develop from each cluster of flowers.

There are two types of avocado flowers, called A and B. A particular avocado cultivar will have one or the other type of flower. Both types are perfect, meaning each flower has male and female parts, and how each type develops favors cross-pollination between the two types, which ultimately means two different cultivars cross-pollinate.

The female parts of type A flowers are receptive to pollen in the morning and the male parts shed pollen in the afternoon, and type B flowers are receptive to pollen in the afternoon and shed pollen in the morning.

Botanically speaking, avocados are berries, consisting of a single large seed surrounded by the usually green-yellow pulp that we consume. Depending on the cultivar, there can be anywhere from 3 to 15% oil content in the pulp. Typically commercially grown cultivars will have an oil content of about 8%.

A strange developmental quirk of avocados is that the fruits will not ripen until they are removed from the tree. Complicating matters is that the ripening process will not happen after harvest unless the avocado has reached a minimum size. Avocados harvested too early will become rubbery or rot rather than ripen. An avocado picked at the proper stage will ripen within about a week of being picked.

This quirk is an advantage to commercial producers as they are not forced to harvest their crop within a small window of time. Many avocado cultivars can remain on the tree for months after reaching acceptable size. This also translates to good quality avocados being available in the grocery store over a time period much longer than a couple of weeks of flavorful produce.

But the road to a harvestable avocado is a long one. Depending on the type of avocado being grown, fruit development can take anywhere from five to fifteen months. This is where the news media got their statement that crops in California would be affected for two years following the recent freezing temperatures.

California accounts for at least 86% or as much as 95% of avocado production in the U.S., depending on the source reporting. Over 85% of the avocados grown commercially in California are the cultivar 'Hass' (also written as 'Haas'). 'Hass' is a Guatemalan type, which can take up to fifteen months to produce fruit large enough to harvest.

Since 'Hass' avocados are so prevalent, the freezing temperatures that visited California recently were particularly destructive. This year's flower buds were likely subjected to the freezing temperatures, as well as the fruits that developed from last year's flowers, that would have reached mature size this summer.

Considering that such a small percentage of flowers actually set fruit, plus the extended amount of time needed for fruit to develop, it is very likely that the harvest for this year and next will be reduced. Barring some additional wrench thrown into the system, it would take until at least summer 2009 for yields to recover.

This assumes a given tree just lost fruit and flowers and didn't experience dieback of the canopy. Clearly, losing part of the canopy reduces the potential for yield, and re-growth of the lost portion will take time.

As seems to be the case with many popular crops, the 'Hass' avocado's success was largely due to chance. Historically avocados were introduced as a cash crop to California in the 1800's, and propagation by seed was not uncommon, but meant that there was a lot of variation among seedlings.

A mail carrier named Rudolph Hass purchased an avocado seedling in 1926 that proved to have fruit of superior quality. Seeing the horticultural potential, Mr. Hass patented his tree in 1935 and all avocados labeled as 'Hass' can be traced back to this original tree.

Prior to their popularity as a commercially grown crop, avocados had a very colorful reputation historically. Avocados have been called "alligator pears" because of their bumpy skin and pear shape. Along the same lines, the name avocado derives from the Aztec word "ahuacatl" meaning testicle. This descriptive name also hinted of the avocado's passionate associations.

There were various beliefs circulating attesting to the avocado's aphrodisiac powers. In addition to their suggestive name, the Aztecs referred to avocados as the "fertility fruit" and kept their daughters indoors during the harvest to protect them.

This reputation persisted and hindered initial efforts to introduce the avocado as a food crop in the U.S. It took a concerted effort by commercial growers to dispel the avocado's "bad" reputation and convince the public that even upstanding citizens could enjoy an avocado now and then.

No matter what your opinion on avocado's "reputation", it does make for interesting trivia with Valentine's Day around the corner! If avocado's sordid associations don't interest you, they do have other redeeming qualities.

Some researchers have found that avocados produce a compound that interferes with our body's ability to absorb cholesterol from food, in theory potentially reducing cholesterol levels in our blood. But don't gorge on avocados just yet.

Remember that avocados are still relatively high in fat, specifically monounsaturated fat. One whole avocado contains approximately 25% of the FDA's recommended daily intake of saturated fat. Increased consumption of saturated fats is associated with having higher cholesterol levels in our blood. But they are great sources of potassium, folate, and vitamins B, E and K.

They may have some nutritional drawbacks, but something that thrills me as much now as it did in kindergarten is to sprout an avocado pit. With the pointed end of the pit facing upwards, stick three toothpicks evenly spaced around the width of the pit. Set the toothpick-studded pit in a jar filled with water, pointed end up, flat end just covered with water. In a few weeks the pit should sprout, and the seedling can be transplanted to a standard potting mix and treated as a tropical houseplant. With a little luck you may have a flowering specimen in about six years!

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