One of the fastest growing cycads, the Giant Dioon is also the largest cycad currently known. Although not as cold hardy as D. edule, these may survive some very light frosts. When watered properly, D. spinulosum are tolerant of nutrient-poor soils (although fertilizer will improve growth rates) and virtually all pests and diseases. Unfortunately, most cycads are endangered due to overharvesting of the foliage, which typically starves and kills them. These, along with many other cycads are native to Central America.
Dioon spinulosum (Giant Dioon)
Hardiness Zones: 9b-11
Height: These grow up to 15 meters (50 feet) in native habitats. Most cultivated Giant Dioon cycads grow up to 3.6 meters (12 feet) tall.
Diameter: The trunk may grow up to 41 centimeters (46 inches) in diameter. Most of these grown in cultivation can reach 1.8 meters (6 feet) across.
Growth Rate: Fast (improved by even Nitrogen-Phosphorus-Potassium balanced fertilizers with trace elements), some plants grow 4 one meter long leaves in the first year of growth.
Age: long-lived, like other cycads, presumably up to several hundred tears old
Root System: deep, extensive tap root with many thin fibrous lateral roots
Subspecies: none researched
Tolerates: low nutrient contents (in limestone-rich soils), drought, salt spray
Problems (major): root rot when over-watered
Problems (minor): mealybugs and/or scale when stressed/damaged
Poisonous: The foliage and seeds contain some toxins
Soil requirements: requires well-drained soil, prefers consistent moisture and a pH between 6.0-7.5 (neutral to slightly alkaline)
Air requirements: not sufficiently researched
Watering requirement: demands frequent watering
Sun requirement: part-shade to full sun
Cones (male): longer and thinner than female cones
Cones (female): These grow up to 31 centimeters (12 inches) in length, covered in a wool-like substance. The seeds are white/cream colored, 4-5 centimeters (1.5-2.0 inches) long, 2-3 centimeters (1 inch) wide, and easily germinate.
Leaves: The leaves are shiny, sharp, stiff, light green, pinnate, up to 2 meters (7 feet) long, and radiate from the trunk. The leaflets have tiny thorns on their margins. Leaflets range anywhere from 140-240 per leaf.
Seeds require stratification: no
Monoecious or Dioecious: dioecious
The Giant Dioon is fast growing, especially for a cycad. They produce offsets/offshoots/pups that can be removed and transplanted to grow another (identical) cycad.
Giant Dioon work very well as ornamental in gardens in appropriate zones. The growth rate allows for saplings to quickly mature onto decently sized plants, and they grow in easily predictable forms. These grow quite easily, especially when correctly watered and given a balanced fertilizer during summer.
A male cone with individual scales visible
A medium-sized male Giant Dioon
Foliage and a mature male cone next to an expired cone
A fairly large female Dioon spinulosum
A close-up of a mature female con
A sign at mobot describing the aforementioned female cycad
All of the images provided were taken by me and are from the Missouri Botanical Garden. They were taken inJune of 2014, and they may be used for educational/informational purposes only provided that this article or onlne journal is appropriately cited or referenced first.