As gardeners we come to know our plants, what to expect from them over time, how we can better meet their requirements, and their contributions to the garden. If we watch we learn when to expect their spring time resurgence, the extension of stems and unfurling of leaves. We anticipate their flowering often recomposing vignettes to best display them. They teach us over time. But with Agaves their flowering is so infrequent, that if we don’t study them in mass over a span of years, we won’t know what to expect. With some it can take as few as six years, some even less, while others will keep us waiting for 30 or more. When we buy a year old grafted Magnolia we are told to be patient and we understand that the tree will take some time to grow and mature. So we wait knowing that once it begins a Magnolia will increase in both size and floral performance rewarding us for many years to come…but with Agave, it is once and done.
Agaves are flowering plants, Angiosperms. Angiosperms all share a broad survival strategy forming complete flowers which produce seed which grow into juvenile plants. If you follow plants back in time you’ll that find that plants can be broken down into a few large groups. The largest such group, the Angiosperms or flowering plants, are also the most recent of these…and the most successful. Angiosperms have seeds encased in a ‘fruit’ that formed within the flower from the ovary. Gymnosperms, produce seed as well, though their’s are ‘naked’ without a covering fruit, as the plants have no ovary nor true flower. There are other differences between them as well both structural and in the details of their reproductive cycles. Other plants like ferns don’t produce seeds at all and instead rely on the more ancient process of reproducing directly from spores which grow into an intermediate form, a gametophyte, with one set of chromosomes. This is the ‘sexual’ stage, either male or female, which mature independently, the male form later fertilizing the female, the product of which grows into the sporophyte, with two sets of chromosomes. The sporophyte later releases the single sex, single chromosome spore that will grow into the gametophyte, continuing the cycle. Agave, as a genus, share much with other Angiosperms, they have several distinct peculiarities they share amongst themselves as well. They are all perennial taking more than a year to mature and flower. This is a very common attribute. What sets them apart is that they are monocarpic, they die after flowering once, and, they can take up to 30 years or better, depending on species and growing conditions, to flower..
All living organisms, have scripted within their DNA a ‘plan’ and ‘schedule’, if you will, which a plant beginning with germination follows. Some possess the ability to reproduce themselves asexually, bypassing flowering and fertilization growing on vegetatively or from stolons or specialized root structures like bulbs, corms, tubers and rhizomes. Annuals grow, flower, seed and die within a single growing season. Annuals forego the ‘investment’ in storage structures and instead stake their survival on this annual cycling, with some portion of a year’s seed production possibly available in subsequent, barren years to re-establish the population. 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 as 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. Others, like many of the nut trees, follow an irregular pattern of flowering, producing a heavy crop, masting, communally one year while none produce the following year or years. 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 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. Particular bamboo clones will bloom simultaneously around the world. Trees, such as Magnolias especially when grown from seed, can take many years to reach maturity and begin flowering. Other plants will signal this by changing the form of their leaves from a juvenile to mature form. Some plants, 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 came 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 being 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, being dormant during their hot dry summers, often coming into flower with shortening days, dropping temperatures and a change in precipitation pattern. Other bulbs are more dependent upon 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, for these plants, 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, though they too store energy away in the form of starches in a modified stem structure. Agave 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 collectively 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.)
What distinguishes Agave from other related genera? 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 118ºF which converts the starches to simple sugars; or, in some species, removing the flowering stalk and collecting the sugar rich liquid which it is converting itself to supply the growth of its large inflorescence (Understand that removing the inflorescence will not stop the flowering process, nor will it result in the growth of another inflorescence) ; 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 has been modified into an ‘industrial’ process to supply today’s commercial market. 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 into cool temperate areas. They are extremely scrupulous with their use of water, utilizing the Crassulacean Acid Metabolism, or CAM, pathway. (See earlier posts.) Compared to plants that utilize the oldest photosynthetic pathway, C3, which grow in more moist climates, growth is very slow. They are able to sit for long periods, 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. They patiently build their starch reserves up to the level they need to successfully flower. Many Agave benefit from shade in their low desert locations as it reduces their water requirements 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 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 utilize its stored carbohydrate as it is not able to produce it at the rate that it requires for the rapid growth of the inflorescence. 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 very 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.
We should always keep in mind that plant’s have evolved in place. Their physical structures, their metabolism and internal processes, including their flowering, are tied to their places of origin. We tend to think of DNA as a strictly limiting, coded, structure within which a particular plant or animal exists, each plant expressing itself within its narrow range of DNA set parameters. In a sense it maybe more like a kind of organismic ‘memory’ of what works, of what’s been ‘learned’ by the organism over many generations. DNA does not ‘tell’ an Agave when to flower, if it did, there would be less variability. Conditions in any native landscape have a consistency and pattern that repeats in familiar ways to an Agave. Yes, there is a component connected to the amount of energy/starch stored in an Agave’s heart, that must set some kind of baseline for triggering, so that otherwise immature Agave don’t attempt to flower when a longer term precipitation pattern begins. Without this base many plants would attempt to flower and fail. An Agave’s metabolism, being slow, is a conservative force, the entire plant has learned to take a slow, wait and see approach. Time has proven that it is better to delay than fire off prematurely. Of course, as is true for all wild life today, the increasingly wide fluctuations in climate are beginning to upset this, with some areas getting drier and hotter, others suffering more extremes of cold. Development, agriculture and our movement of plants far beyond their natural range have all worked to breakup the natural conditions and cycles that wild-life and plants coexisted with. In many cases species cant adapt enough, while others are proving to be dangerously so. What has ‘worked’ for wild plants in the past may not in the future. Agave are not on a fixed flowering schedule. There has always been variability from plant to plant and site to site. This variability has helped plants survive…but too much variability???
Most Agave species flower between 8 to 30 years, each species fitting somewhere along that span. The demand for carbohydrate is high during this period. Once flowering is initiated enzymes rapidly convert the starches to sugars and draw on available moisture to form the more dilute nectar to successfully support the rapid growth of the structure of their inflorescence, its flowers and energy rich seeds as well as in some species, the production and ongoing support of bulbils, independently growing little plants, sometimes hundreds of them, that form in place next to the spent flowers aloft on the peduncles. In the case of bulbils, the mother plant provides this ‘sustenance’ for two and even three years after flowering supporting them before they can be rooted in the ground, increasing their chance for success. The ‘mother’ plant sacrifices itself, quite literally, for its ‘children’. While flowering each flower produces enough nectar to spill over. The pollinators, being desert residents themselves, benefit significantly from the water portion of the nectar that the plants make available to them. Many Agave will also form multiple pups or offsets clustered around the mother at the time of flowering as it declines placing an additional burden on the ‘mother plant’. Others routinely offset and form colonies over the years preceding flowering as the ‘mother’ grows and accumulates starch while expanding the clonal colony. 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. Presumably an Agave storage capacity evolved with this flowering strategy as it would be simply impossible without it.
Once the flowering process begins the plant starts converting its starches, utilizing enzymes that it creates, back into the simpler and more available sugars. It can’t be stopped or delayed once initiated. This process is much like the annual cycling of starches to sugars, that other plants utilize such as bulbs, herbaceous perennials and woody plants which utilize it for rapid growth when there are no leaves to photosynthesize enabling early quick flowering and leaf replacement from their stored reserves. An Agave, were it dependent on its regular production of carbohydrates and other synthates, to grow its inflorescence steadily over the years, Agaves would do so slowly, over the course of years, 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. 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 reproduction without such an extravagant structure? After all, the related Yuccas flower much more compactly and 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. Its height makes it a kind of beacon in the desert, getting it above the surrounding plants and providing it platform from which it spreads its scent to attract pollinators from a distance. Does its height aid bats in their search for it? The Lesser Long-Nosed Bat and the Mexican Long Tongued Bat are primary pollinators of Agave flowers as well as the tall growing Organ Pipe and Saguaro Cacti, following their blooming north a thousand miles through Mexico to the Sonoran Desert in southern Arizona, raising their young in the north before returning south following the blooming Agave resident in the Sierra Madre Orientale and Occidentale. Both bat species are under threat as natural stands of the Agave and Cacti, upon which they depend, are being replaced by farms and development breaking the migratory corridors. This is a double hit to the long term health and well being of Agave.
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 pollinating 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 smaller role on this group. 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-
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 form either unbranched spikes, each flower anchored directly or racemes, with each individual flower on a short pedicel) are pollinated mainly by insects and sometimes by the more acrobatic and maneuverable hummingbirds. As their pollinators are day active, this is also when their flowers open and also effects the scent of the nectar which is more sweet to attract bees, butterflies and 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 resident while Mexico is home to 50 of the western hemisphere’s 300 species. Agave, like all other plants, have co-evolved with their pollinators, as well as the species that feed on the fruit spreading the seed in the process as well as any herbivores that may feed on them. The inflorescences are thus strongly influenced by the size and habits of the pollinators. Bees, small insects and hummingbirds need less space to maneuver as they feed.
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 are 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, being ‘polycarpic’, spreading their reproductive efforts over many years. There’s is a more conservative strategy and it seems to work very well for them. Several occur in the summer wet conditions and sandy soils of the US’s southeast coast. Something pushed Agave down a different path. Only a few Yucca species are monocarpic. Both Agave and Yuccas pursued different strategies and succeeded. Agaves found success storing energy in a ‘gambit’, that is some how responsive to the patterns of precipitation of their region…but it isn’t just that. It is a complex process which must include the amount of stored carbohydrate in addition to a long term pattern of wetter than normal growing periods, but I have to think that temperature and day length are also a factor. A plant’s DNA has recorded this lesson in code increasing its odds of survival. Growing Agave in exotic climates like here in the Pacific NorthWest, with different soils, patterns and totals of rainfall, longer cool winter temperatures, lower intensities of sunlight and more variable photoperiod…so much is changed that flowering is probably even more difficult to predict. I have an Agave colorata in a large pot that began its flowering stage in late summer with a very slow extension of its quiote. Buds formed with the first flowers opening in later September with day length and temperatures shortening, slowing the process further. Honeybees were visiting over a mild October on drier days…but the process eventually stopped completely before all of the flowers could open. The plant sits on my front porch in its large pot on ‘hold’. Will it be able to ‘finish’ as temperatures warm? Was the process delayed with the heat of summer on the reflected heat of my concrete deck, by its having limited access to water in its pot? As I said, there have been so many factors out of its normal range that it is difficult to say much about it.
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.
Agave seed germinates quickly, when fresh, in as little as a week each species ripening it 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 just less than 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.
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 die or fail to reproduce. Many plants may survive beyond their normal range but be unable to reproduce themselves by successfully flowering, producing viable seed and some portion of that seed germinating and surviving. Sometimes, in Agaves, this may come down to an absence of appropriate pollinators. Those plants that adapt too readily we often classify as weeds and, in extreme cases, as invasive plants, capable of disrupting healthy native plant communities. Without considerable climate change here in the PNW, Agaves won’t do this. . In some arid regions of the world Agaves are invasive and able to disrupt native populations. 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.). 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.
Change in the natural world tends to be incremental over vast time periods and plants have shown their ability to adapt and meet these challenges with great success. When those changes have been too rapid, even catastrophic, faster than the advance and retreat of ice during the continuing cycling of Ice Ages, the record shows that plants, though in greatly diminished numbers have survived, while it is Earth’s animal species that have suffered extinction events with massive losses of species, genera, even entire families. But plants seem to better able to survive over vast periods of time, able to synthesize the energy and metabolites from what is readily available to them and to reproduce in the face of changing conditions much more effectively than animals.
See my previous postings “Growing Agave in my Maritime NW Garden” and “Following the Vascular Trail: The Path of Water From Soil to Atmosphere”