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Little Barrels 1 - Thelocactus and Gymnocactus
This month we’ll begin discussing several cactus genera
related to the commonly encountered barrel cacti from last month.
These “little barrels,” unlike their massive cousins,
rarely exceed a foot in width or height, and many never grow
more than four inches tall or wide in any direction. The little
barrels inhabit a wide territory centered on the huge Chihuahuan
desert, which covers much of north-east Mexico and extends into
southern Texas and New Mexico, with a few also ranging far to
the northwest, reaching into the Sonoran and even Mojave deserts.
The Chihuahuan desert, the home of most of these plants, has
a climate marked by hot summers with occasional drenching rains
and fairly cold, almost bone-dry winters. This must be an ideal
climate for cactus, as there is an incredible diversity of species
in this region. The physical geography of the place, with many
isolated canyons and mountains ranges, a perfect setting for
speciation, also obviously contributes to the abundance of variety.
The little barrel cacti, including the plants we’ll be
discussing here, consist of species that have been placed in
and out of genera such as Thelocactus, Gymnocactus,
Ancistrocactus, Glandulicactus, Echinomastus
and Neolloydia. Currently, some authorities in the
field have lumped many of these plants into Sclerocactus
(a fascinating genus, extremely difficult in cultivation, associated
with the Four Corners region of the U.S. Southwest), moved others
into Turbinicarpus (mostly arid-growing, tiny, highly
endemic plants), and left some genera bouncing between acceptance
and dissolution. There won’t be space this month to discuss
all of them, but we can make a start.
Most of the little barrels start out appropriately with a more-or-less
globular shape, with some ultimately growing into small columns.
They bear their usually brightly colored flowers from near their
apices, and have their outlines separated into ribs which themselves
are sometimes divided into tubercles. Other than this, there
isn’t too much these plants obviously have in common.
Some are green-bodied and many others have skins that are gray
or brown in color, to help camouflage them or enable them to
resist the sun. Some are totally covered by dense spines, which
range in color from pure white to almost black, while others
are only sparsely protected. Some of these plants remain solitary
while others form small clumps. Some of the more unusual plants
(such as Gymnocactus subterraneus and G. mandragora),
have large underground stems and tuberous roots. And finally,
some of these plants are extremely easy to grow while others
are quite difficult and demanding.
As plants that often have an exotic look but generally respond
well even to less than optimum growing conditions, it’s
not surprising that the members of the genus Thelocactus
are probably the most widely distributed in cultivation of this
group. Thelocacti offer up a range of colors, from bright green
to gray, vary from total coverage by glassy, whitish spines
to barely adequate protection from sparse, adpressed (close
to the body) spination. They further vary a good bit in form,
a tall or short dome or a shape something like a pillbox hat
rimmed with long spines. The most widely distributed species,
as well as the only one that crosses the Rio Grande into Texas,
is Thelocactus bicolor, with two officially recognized
subspecies as well as a very wide range of shape in T. bicolor
ssp. bicolor itself. This basic model grows into a medium
sized little barrel, up to almost a foot and a half tall and
almost as wide, but also with much narrower forms only half
as high and no more than a couple of inches thick. The biggest
form seems to have no more spine clusters per plant than the
small plants, which, as a result, appear much more densely armed,
the yellowish, downward-facing spines making attractive patterns
on their narrower bodies. T. bicolor ssp. flavidispinus,
found only on a few patches of flint-like white novaculite in
southern Texas, looks like a miniature version of the standard
plant, but its four inch wide, bright magenta flowers are just
as large as those of the bigger forms. T. bicolor ssp.
schwarzii, from hundreds of miles farther south in Mexico,
also remains rather small and lacks central spines.
Thelocactus rinconensis develops a highly ribbed but
flattened top, surrounded by very long gray to black central
spines. It can have either a gray or green epidermis, and generally
remains solitary with a diameter up to eight inches, but I’ve
seen clustering forms, with each head more than a foot across. Thelocactus conothelos also resembles a sort of raised-up
disc surrounded by long, sometimes downward curving spines,
but these spines may be glassy white, and the plants, slightly
smaller growing than Thelocactus rinconensis, have
a quite different look. This species also has two distinct subspecies,
smaller and very rare in nature, the mountain-growing T.
conothelos ssp. argenteus, with a dense covering of short
white spines and pink flowers, and T. conothelos ssp.
aurantiacus, with longer, glassy central spines, less densely
covering the plant, and bright yellow to orange flowers. More
common, both in habitat and cultivation, Thelocactus hexaedrophorus has a gray body, distinctly rounded tubercles and thick spines
that may remain close to the plant’s surface or may protrude
straight out for over an inch. It lives in limestone soil in
stony deserts that it shares with plants such as Ariocarpus
retusus. Thelocactus leucacanthus and its subspecies
schmollii both live at lower altitudes. They have green
bodies, clump freely, and ssp. schmollii with large
tubercles rather than distinct ribs, looks more like a Coryphantha
than it does a Thelocactus. T. macdowellii
also strays from the standard Thelocactus look. A small
plant, either solitary or clustering, and rare in the wild,
it consists of an almost globular body completely covered with
white, glassy spines. It also will rot if unhappy, and requires
a somewhat higher level of care than most thelocacti.
The wide variation of spine color and texture and general configuration
within the thelocacti has led to quite a few species of dubious
validity being described in the past. Currently there are about
a dozen species accepted. All Thelocactus enjoy very
bright light, and most will grow in any decent cactus soil.
The easier ones will take water once a week when they’re
growing, and perhaps once every month or two in winter. The
fussier ones might do better in a quicker draining soil, perhaps
with a bit of lime or gypsum added, and more cautious watering
in winter. The species from lower altitudes (and T. bicolor
ssp. schwarzii in particular) may be a bit tender,
but most thelocacti have no problem with some frost as long
as they’re dry. In effect they give a lot of bang for
the buck, as they are striking cacti from the Chihuahuan desert
that are considerably easier to grow than many of their compatriots.
Gymnocactus, though its very existence as a genus is
subject to debate (currently it’s officially though controversially
lumped in with Turbinicarpus), consists of plants that
both in appearance and culture differ significantly form the “classic” Turbinicarpus types to such a
degree that makes it worthwhile to talk about them as a distinct
group. The plants resemble small thelocacti, generally being
sunken or elongated globes densely covered with spines. They
produce their typically bright carmine or purple flowers from
their tops. Most have a dark green epidermis, and their spine
coloration can range from pure white to dark brown or black.
One of the more common species in nature, Gymnocactus beguinii,
has a dense cover of upward facing spines that may be pure,
almost transparent white, with a distinct black tip. This plant
often grows in gypsum soil, or in limestone, as do most of its
relatives, but it also grows at fairly high altitudes, sometimes
in small clearings in otherwise quite thick pine forest. Another
species with a somewhat aberrant habitat, Gymnocactus
(or T.) knuthianus, is a low, globular, solitary
plant with a bright green body, white spines, and a rather dense
covering of white wool. It grows in relatively well-watered
grasslands, sharing its habitat with good sized bushes and shrubs
that don’t at all appear like desert flora. Other species,
such as Gymnocactus saueri, Gymnocactus subterraneus,
Gymnocactus zaragosae and Gymnocactus mandragora,
conform more reasonably to an arid habitat of alkaline soils.
Gymnocactus subterraneus, and the very rare Gymnocactus
mandragora have large tuberous roots, in the case of the
latter often flaring out after a narrow section that separates
the roots from the body of the plant. Most gymnocacti have small
ranges and are rare plants in the wild. They can be touchy to
grow, being particular to excess moisture in winter. But a beautiful
plant such as G. beguinii, from high altitude pine
forests, should be able to withstand more water than such a
densely white-spined cactus typically has any business doing.
Giving all of them very bright light, a perhaps somewhat quicker-draining
soil than average, and very little, if any, water in winter
is probably the best approach to growing them. There are a number
of other species of gymnocacti (or gymnocactus-like-turbinicacti),
most from extremely restricted habitats and all of them well
worth consideration by anyone interested in cacti.
These several genera of small “barrel” cacti include
some of the most interesting plants of Mexico and the United
States. I’ll keep them as the topic for this column for
another month or two, until I’ve touched on most of them.
The Garden has a decent collection of Thelocactus and some interesting
gymnocacti as well. From time to time a few plants may be available
for purchase at one of the plant sales. |
