If you follow plants far enough back up their evolutionary ‘tree’ you find plants broken down into flowering plants, that produce seeds (Angiosperms have seeds encased in a ‘fruit’; Gymnosperms have naked or bare seeds) and those that pursue other strategies to propagate themselves. Plants like Ferns have special organs that produce spores which in turn form an intermediate form, where fertilization takes place, which then grow into an adult plant. Another step back and plants are divided into vascular or non-vascular, those with specialized cells and structures that include phloem and xylem tissues to move water, nutrients and various metabolites around the plant and those without. Non-vascular plants are essentially single celled organisms that form colonies or ‘bodies’ with each cell the same, complete and undifferentiated. Plants as a whole are very unique and resist easy classification. The botanists among us, in an attempt to better understand this, study plants and their relationships. Systematics is the study to make sense of the Plant Kingdom, giving it order: what constitutes a plant, how they relate to each other and how they evolved…whom ‘begat’ whom. Sometimes it would seem that the only common factor shared by all plants is their utilization of chlorophyll, but even this is not universal, because their are saprophytic plants that live off of the carbohydrates and metabolites produced by other plants and some of these are relatively complex Angiosperms. As gardeners we mostly concern ourselves with vascular plants unless we cultivate mosses, liverworts and such. Most of us concern ourselves with the flowering plants, though we may also grow spore producers like ferns for their structure and texture. All of these grow from seeds or spores that carry the DNA that provides a particular species with its ‘script’, a detailed growth plan organisms attempt to follow throughout their lives. Agaves are vascular Angiosperms that share key flowering characteristics within the genera and more broadly within their family.
All living organisms, have scripted within their DNA a ‘plan’ and ‘schedule’, if you will, which a plant follows from germination, or juvenility, if it is propagated asexually, bypassing the seed or spore and germination stages, and is grown vegetatively or from specialized root structures like bulbs and rhizomes. Annuals grow, flower, seed and die within a single growing season. Others have a biennial timetable producing a robust plant the first season, then, after undergoing a winter dormancy, explode into flowering the following growing season. Others live longer building more structure, sometimes a permanent above ground structure, before they mature enough to flower. Of these, a relative few are monocarpic and die after flowering. Others may live for as many a thousand years or more flowering every year given supportive growing conditions, maybe skipping some years due to harsh conditions or because of the energy expended. Each of these plants follows its own strategy for survival and contain a ‘trigger’ which chemically begins the flowering process. These ‘triggers’ are not widely understood for all plants.
Triggers can be keyed to day length (light), there can be a temperature component as with many winter/spring flowering bulbs, or, more mysteriously, an age minimum which can be related to height and/or bulk of a permanent structure or its ‘stored’ starches that will assure that there is an adequate supply so that it can complete its flowering phase successfully. Trees, such as Magnolias can take many years to reach maturity and begin flowering. Other plants will signal this by changing the form of their leaves The same plant, when cloned, can be quite precocious when compared to a seedling grown plant, while other clones, depending on from where on the mother plant the cutting material may remain locked in a ‘juvenile’ phase. Some plants may flower annually, but cycle back and forth between heavy and light fruiting years.
I remember years ago learning about such a trigger in Onions and Garlic. Vegetable gardeners learn quickly that if they grow these it is very important to get them planted early in the season. The reason for this is that they are sensitive to ‘day length’. If you plant them too late in the spring they are only able to produce limited top growth before longer days force them into flowering, seeding and the formation of its bulb stunting them. Plants that form bulbs, across the many species, share similarities: spring bulbs producing their foliage first, before they next flower, then produce seed, and finally reform the bulb which provides the energy for the following years eruptive growth. Most produce ‘offsets’ adjacent,or as a part of the main bulb, increasing itself. Bulbs enable this particular cycle and are recharged during the growing season. Fall flowering bulbs change this cycle up often coming into flower with shortening days, dropping temperatures and a change in precipitation pattern. Other bulbs don’t renew themselves and depend on the production of offsets and seeds to continue the line. The seed enables the plant to scatter more widely, the ‘bulbs’ to survive and increase in the immediate area. It is a two pronged strategy that better assures survival. The genetic mixing that occurs during flowering and seed production assures that some of the progeny will possess different characteristics that may insure the survival of the species, better able to adapt to changing conditions or meet those elsewhere. Bulbs have adapted this strategy for a variety of different environments most often occurring in the ‘steppe’ regions of the world, with their generally limited precipitation, but also in wetter parts of Asia with its monsoonal rain pattern and even in some temperate and desert areas. It is an adaptive strategy.
Agave are a perennial, but not a bulb forming genera, that have bounced around in several different plant Families over the years as botanists have studied and attempted to classify them. Some place these in their own family, while others have put them in the bulb forming Lily family with which it shares its three part flowers: 3 petals, 3 sepals, often called tepals as they are indistinguishable., while most recently it has been placed within its own ‘order’ within the Asparagus family. A casual observation of its flowering shoot certainly suggests this, but these plants are unique to themselves. (The sorting out of species these days is being done at a genetic level as morphology, a plant’s physical structure/appearance can be both misleading and conflicting.) Some people have suggested that Agave descended from the genus Hosta over a period of many millenia that brought profound aridity with climate change.
Agave don’t form ‘bulbs’ but they do form a ‘core’ or ‘heart’ around which the plant grows. This core is a relatively large storage structure in which the plant sequesters a significant quantity of the starches that it produces over its life. (These are also stored in their substantial leaves along with moisture.) Depending on the species these can be quite large with the core’s of Agave tequiliana, weighing 80 -200 pounds. Several species were harvested traditionally by native people’s, some actively cultivated them, like the Hohokam did Agave murpheyi, harvesting them in late winter just as they were starting to ‘shoot’, having reached their maximum size thus containing the most carbohydrate, and roasting them in pits. This was a significant food source for them during a time when other food sources were very limited. The now popular Agave Nectar is the fructose rich ‘juice’ produced via several methods: often by harvesting the entire core after 8 – 14 years and extracting the liquid, filtering and then heating it at 118F which converts the starches to simple sugars; or, in some species, removing the flowering stalk and collecting the sugar rich liquid which rises in its place; a third method bypasses the heat step, after extraction, utilizing an enzyme derived from a mold that ‘ferments’ the starches into sugar. This last method is a modern ‘industrial’ process. The first method is that that still comprises the beginning of the stage in producing tequila and mescal. The second method is that utilized by natives in producing the traditional fermented drink, ‘pulque’. All of this goes to illustrating the sugar/starch storing capacity of the many different Agave that powers the growth of the prodigious structure that comprises an Agave’s inflorescence.
Agave are desert and arid adapted plants that are found in tropical, sub-tropical and warm temperate areas. They are extremely scrupulous with their use of water, utilizing the Crassulacean Acid Metabolism, CAM, pathway. (See earlier posts.) Compared to C3 plants growing in a moist climate, growth is very slow. They are able to sit for long periods, even during their active growing seasons, with their leaf stoma closed, waiting for available soil moisture without serious damage to their structure. Water rather than sunlight is the limiting factor in their growth. When it is available, rather than adding excessive bulk, they convert the sugars that they have photosynthesized into starches and store them. They form their basal rosette of leaves around their core and extend their relatively shallow roots to gather moisture. The few that form a ‘trunk’ or stem do so in a very limited way. There is no branching. A tall ‘permanent’ structure would provide no survival advantage. Doing so would create more surface area that would increase its water requirement. Many Agave benefit from shade in their low desert locations and a taller plant would put its structure above its neighbors in the full sun as what shade there is is often caste by a variety of leguminous shrubs like Mesquite that are relatively low growers. Other Agave populate arid hill and mountainsides sharing space with Oaks and Pine in open woodlands where they receive some protection from the sun’s intensity offered by the trees.
Botanists believe that the triggering of the flowering process is some how linked to available soil moisture, oddly, the year before flowering initiates. They’ve tested tissues for carbohydrate content and find this linked to availability of water. Precipitation may fall but if it comes down all at once or in winter when the plant is not in active growth, flowering may be delayed. Our own rain cycle, heavy in the winter months, little to none in the summer growth period is not ideal for an Agave. Supplemental water in summer may not be completely necessary here but it will certainly aid the plant in its growth and perhaps increase the likelihood of flowering. I didn’t see any literature that definitively states that withholding water will delay any given Agave indefinitely in terms of its flowering. Neither have I seen that providing optimum conditions will greatly lessen the time required before an Agave may flower. These are tendencies. There are documented cases where Agaves, grown in the north, in greenhouses, have had their flowering phase delayed for as for as long as 80 years! Why, would be hard to say, because there are so many variables at work there. One study, of Agave deserti, by Palmer in Colorado, 1977, determined that if there is available soil moisture during the flowering stage the plant will use it rather than deplete its own supply. In fact they also found that the plants that flower tend to have more and larger leaves that contain more water than the a non-flowering plant of the same age. This goes to how plants tend to be ‘triggered’ by the last couple years of rain/soil moisture. It will however still have to ‘rob itself of stored carbohydrate as it is not able to produce it at the rate that it requires it. This again emphasizes the importance of available soil moisture preceding and during flowering. In general, the early more ‘explosive’ stage of the flowering, when the ‘stem’ is rapidly extending, here in the Maritime PNW should not be limited by inadequate soil moisture, By desert standards, our soil moisture levels in Spring are quite high. The plant’s ability to convert starch to sugar would be more of a factor, a process that will require the production of adequate enzymes driven by the plant’s metabolism. Our generally relatively cool temperatures may slow this.
Agave are sometimes still known by the common name ‘Century Plant’, an exaggeration of the number of years it takes them to flower. Overall most Agave species flower within the range of 8 to 30 years, each species fitting somewhere along that line. This period seems to be encoded in a specie’s DNA, but this is not a fixed figure. Because of their relatively slow metabolism and the environment which they must endure, it takes a period of years to accumulate enough carbohydrate to support the formation of the structure of their inflorescence, its flowers and energy rich seeds as well as in some cases, the production of bulbils, sometimes hundreds of them, that form in place next to the spent flowers aloft on the peduncles. Many Agave will also form many pups or offsets clustered around the mother as it declines. Many others will offset and form colonies over the years as they grow and accumulate starch. Their flowering stem structure is commonly known as a ‘quiote’ in Mexico which is the center of diversity for genus Agave containing 76% of all known species, all are limited to the the Americas. The mother plant, its core and leaf storage, power this process including bulbil production and survival, and are their source of energy and moisture. In the case of bulbils, the mother plant provides this ‘sustenance’ for two and even three years.The ‘mother’ plant sacrifices itself, quite literally, for its ‘children’. Without these large storage organs Agaves would be unable to flower, seed and form bulbils. Once the flowering process begins the plant starts converting its starches, utilizing enzymes that it creates, back into the simpler and more available sugars. This process is much like the annual cycling of starches to sugars, that other plants utilize such as bulbs, evergreen woody plants and deciduous plants, that must replace their leaves with the commencement of Spring growth from their stored reserves. An Agave, were it dependent on its regular production of carbohydrates and other synthates, to grow steadily over the years would do so slowly, putting its slowly forming flowering structure at risk to damage. By producing all of the energy that it requires up front in a more compact, defensible structure, it is able to go through the process of reproduction relatively rapidly, not leaving a partially formed structure aloft that is vulnerable to damage growing slowly over the years. Recall from above how some native peoples would remove the newly forming flower stalk and collect the sugar rich liquid that would flow up from the core. This adaptation permits the expression of very rapid growth, that depending on the species can see an extension of this primary peduncle, or flowering stalk, of as much as 6” or more per day.
Why do Agaves genetically ‘insist’ on such a tall inflorescence? Wouldn’t it be more effective to accomplish this without such an extravagant structure? After all, the related Yuccas flower much more compactly an are able to flower annually once a mature size is attained. In part because of the years taken to reach this blooming stage the flowering of Agave has not been thoroughly studied.
Agave are divided into two subgenera, those that are obviously ‘branched’ are in the larger group, subgenera Agave. The structure forms a loose panicle. Each ‘branch’, or secondary peduncle, branching again, forming tertiary peduncles that terminate in a cyme or umbel containing as many as 10 flowers each. The ‘branches’ provide more maneuvering space for the bats to approach and depart from the flowers. Nectar eating bats are one of the primary pollinators of this group, though others like hawk-moths are also common visitors to the night-flowering agaves and are probably effective pollinators; bees and other diurnal insects play a role. These flowers emit a musky fragrance at night, which usually smells unpleasant to humans, like ammonia or rotting fruit, depending on the species. It provides the bats with copious amounts of nectar and pollen. Agave nectar and pollen are major food sources along the bats’ migratory routes. After wintering in the Mexican tropics, bats migrate northward through the desert following the south-to-north wave of spring-blooming columnar cacti. They raise their young in southern Arizona, then return south via the mountains, the Sierra Madre Occidentale or Orientale, feeding on Agaves. This is becoming ever more problematic as Agave are ‘lost’ along these essential corridors. Some of the species in this subgenera have long occurred outside of these bat’s range or flower during a time of year when the bats are absent and are pollinated primarily by hummingbirds.
Species in the subgenus Littaea (which bear unbranched spikes, each flower anchored directly without a pedicel, or racemes, each flower having a short stem or pedicel) are pollinated mainly by insects and sometimes by the more acrobatic and maneuverable hummingbirds. This part of Arizona is rich in resident and seasonal hummingbird species with areas like the Santa Rita mountains, south of Tucson, having as many as a dozen of the western hemisphere’s 300 species.
Hybrids that blend characteristics of these two subgenera, like ‘Sharkskin’, are intermediate. As we are over a thousand miles beyond the northern limits of nectar feeding bats, pollination, if it occurs, will be left to hummingbirds, butterflies, bees and other insects, barring individuals willing to repeatedly climb ladders tall enough to get the job done. It will be interesting to note if our local Anna’s and Rufous Hummingbirds are attracted to the flowers.
Agave flowers themselves are ‘perfect’ in that each contains both male and female parts making pollination more likely. Their ovaries are ‘inferior’, or below the ‘corolla’ which is comprised of six tepals, or undifferentiated petals and sepals, that form a uniform structure. This is characteristic of genus Lilies though they are much larger and showier in comparison). These are ‘fused’ at their base forming a tube which contains the ovary below. The tepals length and size may be quite small with three long ‘filaments’ each carrying a single anther high above. The anthers are generally linear and can be relatively long defining the flower’s appearance from a distance, while the much less significant tepals may be practically ‘invisible’ in some species. This structure is what allows many bees to visit flowers without pollinating them as they can enter and exit the flower without contact the pollen on the anthers. Some have purposed that absent pollination, absent the appropriate bats, hummingbirds or effective pollinators, some species are more likely to produce bulbils, but this seems to be a highly speculative assertion. Agave have ‘chosen’ to go down the monocarpic path, putting all of their resources into one flowering.
Yuccas, sharing similar conditions across at least part of their range, don’t go ‘all in’ in their bid for survival. There’s is a more conservative strategy and it seems to work very well for them. Something pushed Agave down a different path? Only a few Yucca species are monocarpic. One could speculate that Yuccas pursued a different strategy and succeeded with it while Agaves found success in very arid climates, storing energy in a gambit, that is some how responsive to the infrequent patterns of precipitation of their region. If an Agave’s flowering is linked to the soil moisture of the previous couple of years it may be that it is attuned to a long term pattern of wetter and drier periods, that its DNA, has recorded this lesson ‘learned’ increasing its odds of survival, if it ‘waits’ for one of these relatively wetter periods. Yuccas, as a genus, occur across regions with wider ranging conditions. Several occur in the summer wet conditions and sandy soils of the US’s east coast.
Successful germination of seed and its eventual survival to maturity, in an arid environment, can be very low. To improve its ‘odds’ Agave produce a large amount of seed, one time, during a ‘wetter cycle’, when it may more likely germinate and establish, relative to some smaller, conservative, annual production of seed that is not keyed into precipitation and moisture cycles. Remember that, the mother plant, though it dies, is generally reproduced asexually in the immediate are in the form of clones, multiple genetic duplicates, in a sense, resetting its ‘clock’, to attempt to again produce seed. In effect this increases the odds that the ‘mother’ plant will successfully produce seedling survivors. The ‘mother’ by doing this takes advantage of its precise microsite, like a perennial, with multi-year intervals between flowerings, that can produce crops of seed over time, in an otherwise inhospitable environment, ultimately increasing its odds that its flowering/seeding events occur in a wetter year, when soil moisture is supportive, producing plants with the diversity of seedlings and the species survival over time. Their seed germinates quickly when fresh in as little as a week each species ripening over a time period commensurate with its bottom to top of inflorescence flowering period. Agave coming from northwestern Mexico and the American Southwest evolved in a monsoonal climate, that experience lighter sporadic winter rains with the often heavy and localized rains of summer beginning generally around early July and continuing into September. If seed ripening aligns with a local sites rain events in mid and late summer, and soil moisture is adequate, seedling survival will be more likely. But rains can be highly localized and on the ground conditions extremely variable.
The map below lays out the internationally accepted climate zones for Mexico. As a reference Portland is designated Csb, a mediterranean climate. It should be noted that Agave come from many regions of Mexico, and include some of those that we might try to grow here in the Maritime PNW. Mexico possesses multiple climate zones only a small portion of which are mediterranean, the bulk of its variable rainfall occurring over the months of June-October with February being the driest month. Mexico City and the Southern Alto-Plano has an annual average rainfall around 28″, nearly that of Portland’s, we’ve received over 50″ so far in this rain year, which is high for us, but its temperatures and timing are very different. Some areas along the southeastern coast can receive as high as 79” annually with the state of Tobasco receiving nearly 12” in the month of September. Remember hurricanes occur in the south and Mexico is divided by the northern Tropic at 24 degrees latitude. Agave victoriae-reginae comes from SE Mexico. It should also be noted that much of Mexico’s wild lands and many of its species are under threat from agriculture and development as well. Several species are extremely rare in the wild, though they may be used as an agricultural crop. Such plants are rarely grown from seed so genetic diversity is being lost.
Group A: Tropical/megathermal climates:
•Tropical monsoon climate (Am)
•Tropical wet and dry or savanna climate (Aw)
Group B: Dry (arid and semiarid) climates:
•Desert climate BW: Hot desert (BWh), Cold desert (BWk)
•Steppe climate (Semi-arid) BS: Hot steppe (BSh), Cold steppe (BSk)
Group C: Temperate/mesothermal climates:
•Dry-summer or Mediterranean climates (Csa,Csb)
•Temperate climates (Cwa,Cfa)
•Maritime temperate climates or Oceanic climates (Cwb, Cwc,Cfb, Cfc)
•Temperate highland tropical climate with dry winters (Cwb, Cwc)
The above section is copied directly from the Wikipedia page. Refer to it for more detailed information concerning climate.
So what does this all mean if anything when we grow these plants in our gardens, in regions with different seasonal rain patterns than the SW and Mexico?
To my knowledge no one has done the research here so it is difficult to say. We all know that if you change too many conditions, too far beyond a plants normal range, any plant, at some point, will have exhausted its capacity to adapt and it will not survive. Many plants may survive beyond their normal range but be unable to perpetuate themselves by successfully flowering and seeding. Those plants that do this too readily we often classify as weeds and, in extreme cases, as invasive, capable of disrupting healthy native plant communities. Without considerable climate change, Agaves won’t do this. In some arid regions of the world Agaves are invasive and able to disrupt native populations. Remember that Agave are New World plants. In the drylands of the Mediterranean and Africa, where Euphorbia and Aloe often fill their niche, Agaves can become problematic. (Note that researchers are projecting that under our current pattern of climate change it is likely that our conditions will change enough that many sub-tropical species may find amenable conditions here in the not so distant future.). There are many other plants that we have brought to the Pacific Northwest from steppe and mediterranean climates that have a difficult time adapting to our winter wet and rich soils. Many of these grow so fast that they are weakened and die relatively quickly. Too much of a good thing can be too much! Plants brought in from summer humid/wet climates have no chance at all without regular supplemental watering in our dry summers. Will Agaves take fewer years to accumulate the carbohydrates they need to flower here with our climate and the occasional addition of water in Summer? I don’t know. It will take a lot more research than I can do in my small garden, especially given the infrequency of flowering events.
See my previous postings “Growing Agave in my Maritime NW Garden” and “Following the Vascular Trail: The Path of Water From Soil to Atmosphere”