The roughly 200 different species of agave (number depends on how you count) are mostly from Mexico and the SW US. Knowing the origin of a plant can be helpful in predicting its behavior. Some agaves are from the high desert (more cold tolerant), some from the low desert (more heat tolerant), some from woodlands (more shade tolerant), others from wet tropical climates (less cold tolerant, more water tolerant). They share in common an affinity for rough, rocky environments and an ability to endure drought.
The usual life strategy of an agave is to survive for long enough, storing up energy over the course of years, to reach full size and produce an enormous bloom stalk (relative to the size of the plant) and then die. Some agaves can live on after flowering, by axial branching for example. Many agaves will produce offsets which grow alongside and then survive the mother plant after she flowers, producing monoclonal clumps which can persist for decades when conditions are good. Some agaves make little plantlets called bulbils on the bloom stalk which they sprinkle all around them. So take the “monocarpic” descriptor with a grain of salt. In any case, agave flowers are always an event, often a spectacular one.
Some plants can grow huge, including the most common agave in cultivation, americana (6-10 feet tall and 13 feet wide). The eventual size of a plant depends on the exposure (full sun gives the most compact form) and the care. Plants which are forced to endure droughts without irrigation, or plants in nutrient-poor soil, will be seriously stunted. On the other hand, plants which are spoiled with regular water and nutrients may grow twice as large and produce more offsets. Remove offsets from large and extra large agaves before they get a chance to take off, or you may be stuck with an impenetrable thicket later on. Pruning is best avoided as it destroys the symmetry, unless you're going for the pineapple cut and that works best with big plants.
Plan on having to remove agaves in the garden after they flower and die, which may take a few years or perhaps decades. There are some exceptions where the plant will live on.
Most small to medium size agaves make excellent container plants, offering the advantage of portability, and this may be the only option in areas with winter cold. Agaves tend to grow smaller and slower in pots. Seriously underpotting an agave (leaving it root-bound for years) may stunt or stall its growth. On the other hand, providing a little extra space in containers (within limits) can be quite helpful in getting patio or greenhouse plants up to landscape size.
Agaves in cultivation enjoy good drainage and regular water when the soil is going dry, but not much sooner. They are dry growing plants. Landscape agaves can survive months of drought once they are established, though they prefer occasional water. The reversible symptoms of water deprivation include inward-curled leaves and a slouching posture. Tip dieback can occur as these plants consume their lower leaves for the moisture reserves they contain, also as part of the natural process of senescence, sometimes exaggerated when a plant needs a bigger pot.
The agave snout weevil invades mature plants about to flower and triggers catastrophic collapse due to rot. Some species are more susceptible than others. Regular preventive treatment with a systemic is probably helpful if there is weevil activity locally. A second specialist pest is the agave mite, whose damage is much more subtle, often manifested in foamy or discolored leaf surfaces near the core. Careful surgery and application of an acaricide is indicated as most insect control products do not touch mites.
Agave snout weevil manner of infestation, recognition of symptoms, and treatment
While agave (Eriophyoid) mites are at this point probably a more widespread issue for agave growers and collectors, the agave snout weevil (ASW) is a more immediate and more deadly threat for people living in the SW US and Mexico, and even this pest is spreading across the world.
While ASWs are active throughout the year, the best time for preventive treatment is usually the period between January and May, which is when mature weevils are looking for suitable agaves to lay their eggs in. However, especially in areas where winters are mild, fall infestations are not unheard of. Anecdotally, ASWs are said to target only mature plants close to flowering, and while they might prefer those plants, any plant at any stage of its life, except for maybe small seedlings, can and will be a target.
Unfortunately obvious signs/symptoms of weevil infestation do not occur until long after the infestation has already taken place, as the initial attack of the weevils on the plant is often very difficult to notice. A weevil will bore itself a way into the core of the plant, usually from a spot between lower leaves, where it is difficult to observe the telltale 1/4" to 3/8" diameter hole. The weevil will lay its eggs in the core of the plant and then leave.
During the process of boring into the agave the weevil also deposits a variety of plant rot-inducing bacteria (chief among them Erwinia sp.). These bacteria begin the process of slowly but surely rotting the plant from the inside out, in the process weakening the plant tissue and making much more readily consumable by the weevils larvae, who once they hatch will start consuming the plant. Once the infestation gets to this stage it is extremely difficult to save the plant, and immediate complete removal of the plant upon discovery of such an advanced infestation is highly recommended.
The earliest signs of a weevil attack are the telltale holes where the adults have entered the plant. These are often difficult to observe and require close inspection of the plant in question, which is not always an easy task. However, if an entry hole is observed, immediate treatment with a systemic insecticide stands a good chance of halting the infestation and saving the plant.
Secondary signs usually occurring several weeks/months after the initial attack are drooping of the lowermost leaves, as if the plant requires water (if watering does not re-invigorate those leaves, that could be a sign to inspect the plant carefully) and sometimes the appearance of a not always stinky, gooey liquid (it can be thick like molasses).
At this point treatment with a systemic insecticide may still be effective in saving the plant from complete destruction, but the bacterial rot by itself is also quite capable of killing the plant. So if the infestation is halted at this point, the plant will likely suffer a significant setback. Products containing imidacloprid are very effective, but are becoming more and more controversial due to the possible negative effects on pollinators which may also be exposed -- though the advantage of spot-treating agaves is that they tend to flower only once in their lives.
The final stages of the infestation are very obvious and quite recognizable. All the outer leaves of the plant except for the central growth spike will be flat on the ground, looking sickly, while only the growth spike shows something resembling a healthy appearance. However, upon close inspection the growth spike can usually be pushed over without any issues often revealing a creepy crawly mix of mature weevils and larval grubs busily destroying whatever remains off the plant. At this point the plant is a total loss -- though offsets may have escaped the infestation and can be saved -- and complete removal of the plant, with as much of its roots as possible, is the only recourse.
The best treatment for ASW is preventive rather that reactive. In most cases when an infestation is discovered, it is difficult to save the plant. The suggested approach is to use systemic insecticide drenches or granules targeted at the agaves twice a year in the late winter/spring, once around February (as early as January) and then again in April/May. However, some people suggest treating at least once more in the fall. As mentioned above, imidacloprid is very effective, but controversial, so if that is used, it is strongly recommended not to treat plants that are about to flower, since those plants are going to die anyway.
Agaves are also readily grown from seed collected after the fruit has matured and broken open. Young seedlings enjoy some protection but prefer strong light, especially once they have formed a recognizable rosette. Growing these plants from seed allows you to appreciate the variability many of them tend to exhibit in terms of spines, size, color, offsetting behavior, and other features.
Mary & Gary Irish: Agaves, Yuccas, and related plants (2000)
Howard Scott Gentry: Agaves of Continental North America (1982)
