I have already published a few other articles on cycad genera: Cycas, Encephalartos and Macrozamia. Dioon (pronounced 'Die-oon') is not a particularly large or diverse genus of cycads but it is one that I am particularly fond of, as they grow very well in my southern California climate. This is a Mexican genus (with one species also growing in Honduras) consisting of about 11 species, though more are being discovered periodically and some have not been named yet.
Compared to all the cycads as a group, Dioons are considered small- to medium-sized plants with only a few species attaining an impressive massive size. These are slow-growing species (with a few exceptions), with stiff, flat lancelote to very narrow leaflets arranged closely and neatly along the leaves. Stems are relatively narrow for some cycads, but a few species can attain impressive heights; Dioon spinulosum has been recorded to have grown nearly 50 feet tall. Most species rarely grow over 10 feet tall and those are still very very old plants.
These are primarily dry-climate plants and can tolerate desert-like conditions in the landscape, though still appreciate regular watering and fertilization. A few species do not like full sun (at least here in Southern California), however; they do much better in partially shaded to even fully shaded locations with a lot of warmth and humidity. Some of the Dioons are among the most frost-tolerant cycads growing naturally in mountainous areas that experience frost regularly.
As is the case with most cycad species, Dioons are over-collected in nature, though currently only two are in extreme danger of extinction (Dioon caputoi and Dioon califanoi). The rest are still ‘stable' but their numbers are dwindling and many are threatened. Fortunately there are a few local projects to grow cycads in large numbers of sale locally, and someday perhaps those efforts will extend to abroad.
Dioon califanoi is a popular species with collectors as it is one of the most ornamental of the Dioons, growing up to 9 feet tall (in several hundred years, perhaps) and having stiff, slightly arching grayish green leaves with closely spaced, narrow, sharp lanceolate leaflets arranged in a ‘V' along the rachis. This is a mountain species and can tolerate a good deal of frost without damage. This species is often confused with Dioon purpusii which looks similar in most respects except for the ‘V' oriented leaflets (Dioon purpusii has leaflets arranged in nearly a flat orientation along the rachis with only the hint of a ‘V' shape on cross section.) In cultivation it grows best in full sun and draining soil, but it can handle some shade and quite a bit of water without too much problem.
Dioon califanois- mature plant, young plant
Dioon califanoi on right and Dioon purpusii just to the left of it; Dioon califanoi leaf detail
Dioon caputoi is the rarest of the named Dioons. Its rarity is due primarily to its extremely dry climate, which makes regeneration of the tiny population very slow and problematic. Collection by cycad enthusiasts does not help, though. This is a small plant with very narrow stiff leaflets that are spread relatively far apart along the rachis compared to most other Dioon species. Plants are very slow growing but ideal additions to a desert garden situation in a warm, arid climate. These do not do well in shade.
Dioon caputois (in Southern California, in Hawaii, and leaf detail)
Dioon edule is the ‘type species' and by far the most common of the Dioons in cultivation, at least here in California (Dioon spinulosum could be more common in the tropics.) It is certainly one of the most variable species with several recognized varieties and many, as yet, unnamed forms in cultivation. It is also one of the easiest of all the cycads to grow being very tolerant to cold, heat drought, and sun; it even does well in moderate shade. It is also not too picky about soil growing in poorly draining clays as long as it does not get to cold for too long. This is a moderate-sized plant with fairly straight leaves symmetrically adorned with simple, narrow, stiff pale leaflets that end in a very sharp tip. It is a moderately fast grower and a moderate off-setter. This is an excellent ‘starter' cycad for those just getting interested in growing cycads and is one of the least expensive species.
Dioon edules: adult plant, very old plants, and young plant (last one in my garden)
Dioon edule new and old cone, close up of female cone, and leaf detail
Dioon edule subsp. angustifolia and leaf detail Dioon edule var. Palma Solo
Dioon edule var. Queretero photos
Dioon holmgrenii is an edule-like species from the Pacific coast of Oaxaca and is similar to Dioon edule in its needs so they don't need to be repeated. The only notable difference is this species does not tend to do well in shade. Leaves look similar but are tomentose (fuzzy) when new and this tomentum can persist up to a year. Plants eventually get relatively tall (compared to Dioon edule) and are generally not prolific offsetters, usually solitary.
Dioon holmgrenii and close-ups of leaves
Dioon holmgrenii female with male cone placed beside it (to the left)
Dioon meijae is one of the three wide-leaf species (Dioon spinulosum and D. rzedowski being the other two) and is a beautiful plant with long, slightly arching dull green to lime green leaves with closely spaced, flat lancelote leaflets that, in mature plants, usually do not have prickles near the tips (compare to Dioon spinulosum leaflets which usually do have prickles). This plant also has spines/leaves the entire leaf length while Dioon spinulosum has a smooth, leafless petiole before the leaflets start. This is another adaptable species growing equally well in sun or shade, in tropics or warm deserts, although it prefers some sun protection in very hot, dry climates.
Dioon meijaes (first two photos in Southern California, last photo of plant in Hawaii)
Dioon meijae showing fuzzy petioles Leaf detail of new leaf male cone (photo by kokioula)
Dioon merolae is a beautiful species with narrow leaflets that tend to be set slight angles along the rachis like a partially set of open blinds or a heater vent. New leaves are tomentose but quickly become leathery, stiff and somewhat rough to the touch (not slick-smooth like Dioon edule leaves). This is a moderate sized plant (massive plants exist in Mexico) and a moderately fast grower as well. Dioon merolae's cold tolerance is not too impressive, however; nor does it perform well in shady locations, so it is a bit less adaptable than some of the other Dioons.
mature Dioon merolaes in southern California
Dioon merolae leaf detail Dioon merolae female cones Dioon merolae male in cone
Dioon purpusii is one of the most sought after Dioons as it is such a magnificent landscape specimen, as well as being somewhat rare in cultivation. This is one of the taller species and it holds a relatively large number of leaves in its crown. Leaves are straight, grey-green and either flat or slightly keeled (slight ‘V' shape in cross-section) and have many very closely spaced (sometimes overlapping) narrow, flat, stiff leaflets each which have some prickles near the tip. The leaflets are angled off the rachis somewhat similarly to Dioon merolae. This species looks somewhat like Dioon califanoi in size and shape but does not have arching leaves that are deeply keeled. This is a very drought and sun-tolerant species that has some frost tolerance. However it is not a good plant for growing in shade, nor does it seem to do well in humid climates.
Dioon purpusiis in southern California
Dioon purpusii in Hawaii on left; immature plant on right
Dioon rzedowskii looks very similar to Dioon spinulosum (a much more common plant) though the leaflets do not have prickles on them. However it is a tall plant--up to 15 feet--and the leaves have prominent petioles (setting it apart from Dioon meijae). New leaves are strikingly tomentose, and the tomentum lasts for months (again unlike the other two wide-leaf species). This species also does better in full sun than the other two species. This is also, by far, the rarest of the three wide-leaf Dioons.
Dioon rzedowskii in full sun, colony in shade, and new leaves in Hawaiian plant (last photo kokioula)
Dioon sonarense was, until the 1980s, included in Dioon tomasellii. But now it is a separate species and does not look much if anything like Dioon tomasellii. This is a dry-climate species adapted to full sun situations and it has the more typical very narrow, long stiff leaflets of plants like Dioon edule, though perhaps even longer and thinner. Often the leaves of Dioon sonarense This is rare species and unlikely to be encountered too often in cultivation.
Dioon sonarense in southern California
leaves of mature and immature plants
Dioon spinulosum is one of the most common of all the cycads and is one of the tallest growing as well; with some exceptionally old plants growing over 40 feet. This plant has arching leaves with closely spaced wide pale to bright green leaflets with prickles along both margins near the leaflet tips, and a prominent petiole. This plant can tolerate full sun, though it burns often here in inland southern California if not protected from full afternoon sun. But it also does well in very shady conditions. In a warm tropical location it is one of the fastest growing cycads and seems to enjoy being wet constantly (as long as the soils are well draining).
mature Dioon spinulosums in southern California (male in second photo) and in Hawaii in third photo
female cone and new leaves coming up
rare variegated plant
Dioon tomasellii is the last species recognized in this article and might be the hardest plant to confuse with any of the others. This plant has long, slightly arching leaves with very closely spaced, slightly curved and markedly glossy leaflets. Plants here in southern California tend to be very slow growing and even act deciduous, losing a set of leaves before producing a replacement set much later. Few plants hold more than 1 or 2 flushes at a time. Plants seem to prefer some shade (leaflets burn easily in hot, arid afternoon sun) but are very needy of heat and can struggle near the coast. This is also a fairly rare plant in cultivation.
Dioon tomaselliis in southern California
Dioon tomasellii showing leaf detail
undescribed species of Dioon in southern California growing naturally near Juchatengo, Mexico