Euphorbias for landscaping - the cold-hardy columnar, tree speciesBy Geoff Stein (palmbob)
February 3, 2008
One of the most annoying things I run into as I collect plants I know little about, is how little everyone else seems to know about them, too. The information is out there, but either unpublished or impossible to track down. When I grow exotic plants, I like to put them in my various landscapes about the yard, and Euphorbias are one of the most interesting, eye-catching ornamental succulents one can grow in a southwestern landscape. But I do not like planting plants that are just going to die the very first winter, or fry in the sun, or wither in the shade. There are a vast number of Euphorbias available on the internet, through specialty nurseries and even periodically at outlet nursery and garden centers throughout the world. Rarely, if ever, however, is there any information or advice offered about their care other than the standard 'cop-out' recommendation to grow them at a minimum average of 40F-50F (about 4C-10C). What does that really mean anyway? It is not a very useful recommendation as one cannot control the outdoor temperature. What one really needs to know is the absolute minimums these can survive, what its sunlight/shade requirements are, water, soil etc. preferences. For most of these Euphorbias, most of the answers to those questions are probably the same, but I have not gotten too many straight answers to those questions, particularly about cold tolerance.
The other complaint I have about collecting these Euphorbias is how does one tell them apart. Many are misidentified at the more generalized nurseries (or not identified at all, more commonly). It is evident from searches on line that many others cannot tell them apart, either. Even some of the 'expert' sites on Euphorbias have misidentified plants in their files (truthfully, I cannot tell them all apart, either). And there are no publications available that summarize these sorts of things. This article will be a brief overview of the more commonly (and a few rare ones) columnar succulent Euphorbias that survive outdoors in Southern California, along with some cultivational tidbits and brief attempts to describe them (along with a photo) to help the reader learn to tell them apart as best they can.
this unknown Euphorbia was being sold as an outdoor plant and labeled as Euphorbia tortulana... but I can find no such plant in books/internet.. so what is it?
here are two more unknowns that do well outdoors in California (in my own garden), but no names go with these plants.
For starters, all the plants listed below, in general, prefer full sunlight (even the variegated ones), well draining soils, and lots of heat. They all seem to tolerate winter rains well (though they all would probably rather be drier during cold periods) and intense (over 100F) summer heat and sun, unless otherwise noted. Most also tolerate, to a surprising degree, being grown in soils with a high clay content (commonly encountered throughout southern California). Again, that is probably not ideal, but realistically unavoidable if using them for landscaping purposes. Many do well in pots, but some outgrow pots too quickly. And all have survived many years outdoors in southern California despite periodic cold waves, so all could be labeled as 'somewhat' cold hardy.
Euphorbias make great companion plants for most other xeriscape warm garden lanscapes... and even good companions for each other, as can plainly be seen here (Euphorbia ammak hugging a Euphorbia ingens)
Euphorbia abyssinica- this Ethiopian native is a tall (up to 20' or more), columnar deep green 5-7 sided candelabra-like plant with paired, short, stiff needle-like spines along the narrow edges. Columns/branches are usually very upright and close together, and are about four to six inches in diameter. This plant develops flat, tear-shaped leaves up to two inches long at the top and down the sides of each column in early summer (this characteristic helps tell this species from some other large Euphorbias) and in shady situations. Euphorbia abyssinica has excellent landscape appeal forming large, neat, very vertically oriented 'candelabra-like' trees. It is cold tolerant down to only about 28F- severe damage below that. It is a moderately fast grower and commonly available. It is not that common in cultivation and has an unfortunate synonym Euphorbia acrurensis, or just as commonly, a mispelled version, Euphorbia acurensis. The reason this is so unfortunate as there is a much more commonly sold Euphorbia in the nursery trade that is sold under that name as well, but is very little like Euphorbia abyssinica. As far as I know, that very thin, flatted, 3-4 sided species is undescribed in the literaturebut continues to lead to endless confusion about what the 'true' Euphorbia abyssinica is. I am hoping this article will help clear some of that misunderstanding up.
Euphorbia abyssinica in my neighborhood, southern California (left); photo by Xenomorf of closer shot (right)
Euphorbia abyssinica close up in a botanical gardens, Xenomorf photo (left); right is plant in some shade showing foliation commonly seen in this plant in less than full sun situations.
This is oft-photographed plant in Huntington Gardens- a hybrid between Euphorbia abyssinica and Euphorbia ammak- most of its form is much more E ammak-like
These two plants represent some of the commonly available species in the nursery trade sold under the name Euphoriba acrurensis, but which are obivously not Euphorbia abyssinicas.
Euphorbia ammak- originating from Saudi Arabia, this is also a very tall, angular, candelabra-like plant, but more spread out and most forms in cultivation are the yellow, variegated forms. However there is a green form that can be hard to distinguish from Euphorbia abyssinica at first. This plant also has short, stiff, paired spines but along a much wavier edge than E abyssinica. Columns are about 4"-5" in diameter and the plant has relatively thin walls on cross section. This species also develops summer leaves- the green forms have pretty long, wide leaves 2"-3" along the top 1/4 or less of each limb, but the variegated forms have much smaller leaves at about 1/8"-1/2" or so and only at the very tops of the columns. For some reason these nearly chlorophyl-free plants have little problem with intense summer suns (unless not acclimated to them). Cold tolerance is 27F-28F with moderate damage below that. Frozen plants develop necrotic areas along the thin edges of the columns and no more growth can occur there from then on (this is true with nearly all the columnar Euphorbias). New 'arms' have to grow to continue the plant's enlargement as the damaged columns and branches will never get any bigger. This plant is OK for pot culture for 5-10 years, but eventually outgrows most pot sizes. Euphorbia ammaks tend to get top heavy and toppling old plants are not unusual, so some trimming of top heavy plants is recommended (can require very tall ladders for this). This is a moderately fast grower and is very commonly available.
Green form of Euphorbia ammak for sale at nusery (left); in my garden (right)
Old mature variegated form in botanical garden (Huntington Gardens, Southern California) and close up of garden plant
This plant survived 26F but all the edges have been seared by the cold preventing further growth anywhere along those edges.
Euphorbia ampliphylla- this a a very tall, gangly, peculiar plant up to 30' or more. It is a 4-sided, extremely flattened in cross-section, irregularly branching pale green to yellow-green spineless species with very little cold hardiness. Tall, old plants only exist in nearly frost free areas of southern California. Temps below 30F usually damage plants, and the walls are so thin that further damage usually kills the plant. However, this is not too surprising as it originates from tropical Africa. This plant has only modest landscape appeal, but most grow it in pots as an oddity more than anything else- that way it can be moved indoors during cold winter nights. It is a moderately fast grower. This is a rare plant, and relatively pricey, but obtainable. It is usually sold under the synonym Euphorbia obovalifolia
Old plants growing in Lotusland, Santa Barbara, where obviously they have not been exposed to much frost (yet). These plants are listed as Euphorbia obovalifolia (left); photo on right shows a plant still lookig nice and green and even leafing out a bit in late spring
This is a deceased landscape plant in a normally frost free climate, after a freak cold snap (left); right shows a less severely damaged plant, but is a good representation of how this species often looks when still alive in Southern California. Note than in many areas, the ridges are even worn through, demonstrating how really thin they are.
Euphorbia angularis- not many grow this South African plant due to its less than ideal ornamental appearance. It is a moderately tall (6'-10') many branchend, 3-sided, deep green, very thin-walled plant with paired long, sharp spines along the edges. Column diameter varies every few inches from 1"-3". It develops very tiny leaves (1/4" or less) briefly in early summer. This plant seems to be cold tolerant to at least 26F and maybe even more so. It is a slow growing plant, but unable to support its weight well, so it is best grown in corners or against walls. It is relatively rare but obtainable and inexpenseive species.
Euphorbia angularis grown in private garden, leaning on walls (or else a plant this large would have collapsed long ago)
Euphorbia avasmontana- Also from South Africa, this is a moderately tall plant (5'-8') normally, though can get taller in some protected situations. It is a somewhat compact plant with closely spaced, non-branching, suckering columns that are 4'5 sided and having closely spaced, paired, 1/4"-1/2" spines in neat rows along its edges. This plant in nearly square in cross section with only the slightest flair along the columns. Thanks to its symmetry, it is a highly ornamental landscape plant. Cold hardiness is approximately 27F, but I have little personal experience with this species. This is a slow growing plant and difficult to find, even at specialty nurseries.
Euphorbia avasmontana in botanical garden
Euphorbia canariensis- there are few more elegant columnar Euphorbias than this moderately tall (6'-10') 5-sided columnar, blue-green to bright green plant. This native from the Canaray Islands has many low branches, but the branches themselves rarely branch again, giving the effect of a plant with numerous solitary columns. However, the branches come off but a few suckering trunks in a graceful curve upward, often with the lowest point of the curve touching the ground. The columns are perfectly symmetrical and thick-walled in cross-section with completely uniform diameters (about 3") from beginning to end, lined with a straight row of closely and evenly spaced very small paired spines. There is only the smallest hint of leaf formation (1/16" or more) at the very tips of the columns in summer. This plant has excellent landscape appeal for larger areas of the garden or park, and is arguably the most attractive of all the columnar Euphorbias. It is a good pot plant, but eventually outgrows most pots. Cold hardiness appears to be around 27F with colder temps damaging mostly the tips of the columns, forcing them to branch just below that point. It is a slow growing species and is moderately available but pricey.
Euphorbia canariensis in 3 different botanical gardens
Close up of flowering plants, and a view of a younger plant in a private garden
Euphobia coerulescens- another of the South African natives, this is a relatively short (3'-4' tall) suckering, many branched, sometimes segemented, irregularly growing blue-green to grey-green plant with 5-sided columns armed with thick, paired pale spines up to 1/2" in length. Short (1/8" or so) leaves can be glimpsed on the very tips of the columns in summer. Columnar diameter varies erratically from 2" to 3". This plant has moderate landscape appeal, tending to form large, disorganized colonies. However, it has one of the best cold-hardiness ratings tolerating temps down to 25F or less with minimal damage. It is a slow growing plant and is commonly available and inexpensive.
Colonies or Euphorbia coerulescens at the Huntington Gardens, Pasadena, California
Closer shots showing flowers and leaves in summer
Euphobia cooperi- this South African native is a wonderful plant/tree for landscaping, having a single trunk with many, striking repeatedly and symmetrically constricted branches. Each growing season produces a wide branch segment and it tapers smoothly until the next growing season is marked with yet another wide section of branch. Amazingly all the branches seem to do this with remarkable regularity, making this plant highly ornamental. The branches are fairly thin-walled on cross-section. Trees can grow over 15' tall, but seem fairly slow growing, so few plants this tall exist so far in southern California. It is armed with short, sharp paired spines. There is usually some banding of variegation across the branches, but it is subtle. This plant is hardy to about 27F-28F. It is rare but obtainable, and large specimens are very pricey.
older Euphorbia cooperi sort of hidden behind some aloes and agaves in Huntington Gardens (left), and another in private garden (right)
two more plants in the Huntington, one in the greenhouse, one outdoors
Euphorbia grandicornis- yet another South African native and another good landscaping plant for larger areas of the garden. It is a moderately tall (6'-10') but sprawling, often collapsing, irregularly and seemingly haphazardly growing plant with flattened, angular and irregular columns in cross-section. Though this plant is often twisted and very assymetrical, the overall effect is rather ornamental, perhaps due to the large size of the columns themselves. The color is a mint-green and the columns are 4-sided with stout 1" paired spines and the exaggerated wavy edges. Column/branch diameters vary from 3"-6" or more. Very small (1/4") leaves appear mid summer only at the very tips of the columns. Plants eventually form massive thickets that can take up many yards in all directions, so most growers prune this species as needed. This is a species that seems somewhat intolerant of clayey soils, particularly if heavily watered in summer heat. However they do seem to tolerate very wet, cool winters if the soil is well draining. It is a moderately fast growing species and is commonly available.
large colony of Euphorbia grandicornis in botanical garden (left), and young plant in private garden (right)
about a 4-6 year old plant in a botanical garden (left), and a close up of a flowering plant(right)
Euphorbia grandidens- this is moderately tall, many branched shrub up to 10' or more with one to several main trunks. The trunks are cylindrical and tree-like but the branches are much narrower and are 3-4 sided depending on their thickness. Branches have somewhat rounded edges with very sharp but short solitary spines. Large plants are only of modest landscape appeal as the branches usually form twisted, somewhat sloppy sprawling, dangling masses of greenery. Regular trimming is recommend for ornamental reasons. It is a slow growing species, but rather cold hardy (to at least 26F). This is a moderately common plant and inexpensive.
mature garden plant and close up of the limbs of Euphorbia grandidens
Euphorbia ingens- this is the big daddy of them all, growing well over 30' and forming huge, towering trees with immense, often collapsing branches that weigh hundreds of pounds each. The bluish green to grey-green or bright green branches are relatively thick with 4-5 sides divided by rounded edges lined with almost insignificant paired spines. Column/branch diameter ranges from 4" to over 8" and tends to vary depending upon season (thicker while growing in heat, and thinner during cooler times of the year?) giving a somewhat segmented look to them. This plant also exists in cultivation as a variegated and/or monstrose form. Euphorbia ingens is probably the most commonly grown landscape columnar Euphorbia in southern California. Surprisingly it is only hardy to about 28F with significant damage occuring below that. It is a moderately to very fast growing plant, and though does very well in pots for many years (with the pot somewhat resticting it's massive size), most plants eventually outgrow even the largest pots. This is a very common and inexpensive species. It is so easily grown from cuttings, which one can sometimes get from anyone growing this (since it quickly gets much bigger than most growers or gardens can handle so pruned off limbs can be available), buying plants may sometimes be unecessary.
Mature Euphorbia ingens throughout southern California
this Euphorbia ingens shows an unusual weeping form, growing in Lotusland, Santa Barbara. The second photo is the same plant after a freeze- almost half its limbs were lost
healthy plant starting to flower, and then a close up of necrotic branch end after freeze of 26F- the entire branch eventually rotted off as did most of the rest of the plant, but new growth has since reappeared from the trunk
Euphorbia kamerunica- also from South Africa, this is a rare species in cultivation and looks like a less compactly growing version of Euphorbia trigona (a very common plant). This plant's branches are normally 4-sided, instead of the more common 3-sided characteristic of most Euphorbia trigona limbs. The color and overall size are similar to that of the green form of Euphorbia trigona with the columns being 2.5"-3" in diameter and similarly sharply spined and variegated. Cold hardiness is unknown but suspected to be high 20s. Growth rate is also unknown. This is a very rare plant and hard to come by.
Euphorbia kamerunica in Huntington Gardens
Euphorbia lactea- this is a shiny-surfaced, many branched, 3-sided, markedly banded lime green, to nearly white (the 'Ghost' form) or cristate tropical Asian Euphorbia. This one is armed with tiny but sharp paired spines along the margins of the limbs/columns. This plant can grow up to over 10' but rarely does in California since most areas experience frosts that tend to cut this plant down, or kill it. It is only hardy to maybe 29F and most grow it either in pots (excellent potted plant) or plant it in very protected locations. Euphorbia lacteas have the tiniest hint of red leaves at the growing tips in summer. Being from a tropical climate, it is one of the few columnar Euphorbias that do exceptionally well in very wet, humid climates. The Ghost form can be badly burned in direct sunlight, though acclimating it to morning sun is not difficult. This is a slow growing plant, though the cristate forms seem particularly slow growing. Cristate forms are usually grown as grafted plants and that may have something to do with their slow growth rate. This is an excellent potted plant and requires little care other than cold protection.
Some Euphorbia lactea 'Ghost's in greenhouse (left); outdoor green form of Euphorbia lactea (photo by knotimpaired- right)
Ghost form following a freeze.
Cristate forms of various colors at a nursery
Euphorbia ledienii- this is a single stemmed, multibranched shrub to short tree (6'-7' tall) with 5-sided, thick uniform limbs (no flattened edges) about 2.5"-3" in diameter with 1/8"-1/4" closely spaced paired spines along the edges. The shrubs have modest landscape appeal but due to its cold sensitivity (about 28F-29F), many established landscape plants have damaged limbs. It is a rare but obtainable plant.
outdoor Euphorbia ledienii in Huntington Gardens
Euphorbia magnicapsula- from central Africa (Tanzania and Kenya), this is a very ornamental but cold sensitive landscape plant. It has the exaggeratedly wavy limb margins and large, paired spines of Euphorbia grandicornis, but is a much more symmetrical plant in that the limb/branch diameters do not vary erratically as they do in E grandicornis. The plants are deep green, 4-sided, about 4"-5" in diameter, and have thin walls on cross-section. They grow up to 6' or more but then tend to sprawl as the large, twisting limbs get too heavy. Cold hardiness appears to be only arouund 28F-29F and unprotected plants often get badly damaged periodically, back to just the central trunk, or worse. This is a rare and difficult plant to find.
prefreeze look of a plant in southern California's Huntington Gardens, and same plant following summer (all limbs froze off- right)
Euphorbia nerifolia- this Indian Euphorbia barely makes the 3' cut to be included in this article, but is is a columnar Euphorbia, frequently grown as a potted plant, and occasionally as a landscape plant, here in Southern Californnia. Cristate and variegated forms exist and seem fairly tolerant of sunlight and heat as do the the normally dull, dark green forms do. This plant has ornamentally knobby, spineless columns often oriented in a spiraling pattern making this plant look like a twisting 4-5-sided plant, while really it is more a cylindrical plant with knobs rather than linear edges. Probably 27F is about as cold as this plant can go without damage, but prolonged cool also seems to be a problem with some plants. During the warmer months this plant has 2" flat, tear-drop-shaped leaves, mostly along the upper limbs, but not just at the growing tips. It is not a prolific brancher but it always seems to branch somewhat. Limbs are about 1.5"-2" in diameter, and main stem nearly double that. Euphorbia nerifolias are not rare and many specialty nurseries will carry them.
Euphorbia nerifolia- same plant two different seasons, southern Californnia
variegated form outdoors as landscape plant showing variegated leaves. The second photo is a plant grown as a cutting from the first. This plant is easily grown from cuttings
this is a variegated, cristate form of Euphorbia nerifolia- quite hardy in full sun
Euphorbia quinquecostata- though from central Africa (Tanzania and Kenya) this plant seems surprisingly hardy, handly freezes down below 27F, at least briefly, with no obvious damage. It is a single-stemmed plant up to 12' or taller with many 4-sided, upward curving, symetrical limbs, heavily armed with 1/2" stiff, stout paired spines very evenly spaced along the wavy edges of the columns. The walls of this species are somewhat thin and about 3" on cross section. Growth rate is unknown as it is a pretty rare species, and not very available.
Euphorbia quinquecostata in Huntington Gardens
Euphorbia polyacantha- this Ethiopian or Sudanese Euphorbia is a very uprightly oriented, moderately tall plant (over 10') in southern California. It's limbsare 3-5-sided (usually 4-sided) and moderately thin walled, with very closely spaced, tiny (<1/8") paired spines along the margins (some plants have larger spines, up to 1/4" long). Cold hardiness not known for certain, but probably a minimum of 28F. This is a rare plant, but sometimes available from internet specialty sources.
Euphorbia polyacantha in Huntington Gardens
Euphorbia pseudocactus- probably one of the more popular of all the columnar Euphorbias, this is a highly variable South African species, and hybrids of it also are common, making precise descriptions of this species difficult. Most plants grow between 2' and 4' tall, with the Lyttoniana form usually the shorter one and not really qualifying as 3' columnar Euphorbia. The far more popular 'Zig Zag' form (sometimes called a Zig-Zag plant) is similar in form to Euphorbia grandicornis, but a much smaller and more manageable size. It is also ornamentally decorated with horizontal bands of variegation. On cross section, this latter form has very thin walls and irregular diameter (1" to 3"), as well as being well armed with stout 1/4"-1/2" paired spines along the exaggerated, angular wavy edges. Leaves only appear briefly, less then 1/8" at the very tips of the growing centers only, briefly in summer. The Lyttoniana form is more square in cross-section and hardly armed at all, forming very compact, dense colonies of upright, slightly variegated columns. Both plants seem cold hardy to about 27F and get damaged at the tips below that, but usually recover. This plant has moderate landscape appeal thanks to the marked variegation, but can require regular pruning to keep it in check. The same goes for pot care- easy, but will require constant trimmng back. The Zig Zag form is very commonly available, while the other is much rarer.
the two forms of Euphorbia pseudocactus growing in Huntington Gardens, the 'normal' on the left and 'Lyttoniana' on the right
Younger Euphorbia pseudocactus growing against wall where it gets plenty of protection from cold (this is not a normal form of this species- left). Second photo is of a flowering plant (right)
Euphorbia royleana- this is another Indian Euphorbia and an excellent landscape plant for smaller areas of the garden. It is a moderately tall plant (5'-8') with 5-sided, segmented branches, either without spines, or with very tiny 1/8" paired spines along the margins. Leaves appear nearly 1/2 the year all along the upper limbs and are up to 3" long. Columns are about 3" in diameter and straight up and down with a good degree of symmetry. It is a slow growing species and excellent potted plant. This plant has similar cold hardiness to most of the other species here with a minimum temperature around 28F. It is moderately rare but obtainable.
Euphorbia royleanas, in summer, and winter (last photo)
Euphorbia tetragona- as the name suggests, this is a 4-sided plant, though some limbs are 5-sided. It has a single trunk up to 12' tall and many 1" diameter, very symmetrically and heavily spined branches that project out and upright (older plants branch again and again, and eventually the branches droop or fall due to excessive weight. The spines are paired, 1/4" and start out red but eventually turn black and then pale. If this species makes any leaves at all, I haven't seen them. Euphorbia tetragona has moderate to good landscape appeal, depending on its age (older plants can look a symmetrical and sloppy) since the branches are so perfectly squared and needly spined. Good for pots. This species seems to be cold hardy to at least 26F and possibly more hardy than that. It is somewhat rare but obtainable.
These are botanical garden plants (Euphorbia tetragona)
smaller plant showing color of new spines in my own garden
Euphorbia triangularis- probably one of the least ornamental of the 'tree' or branching Euphorbias, this plant grows up to about 10' with a central trunk and many slightly spiraling, 3-sided, upright to falling very sharply armed limbs. The spines of this South African species are thin, needle like, paired and about 1/4" or more long. The color of the columns is a lime to bright green.. Branches grow rapidly but get heavy, eventually drooping, and then finally falling off, where some root and form new trees. It is a good pottted plant if kept trimmed regularly.
left photo is a nice potted plant which is cutting form the older, untrimmed (middle). Close up of Euphorbia triangularis mid summer (rigth)
Euphorbia trigona- probably one of the best of the landscape or potted plants from the standpoint of easy care, eventual size and ornamental appeal. This South African native has very thinly walled, 3-4-sided (mostly 3-sided- hence the name) very upright columns arising from a single trunk and these limbs tend to grow so closely together there is hardly any space between them. The columns are about 2" in diamater and very thin walled on cross section. It has a shiny surface of either pale green with white variegation, or deep red-maroon with some variegation (this is referred to as the 'rubra' form). Leaves persist on the upper halves of the limbs most of the summer months as long there is some protection from the sun. This is one of the few species that grow equally well, if not better, in shade. However, shade-grown plants do sometimes got too tall and 'etiolated' in shady or indoor situations, and the 'branches' may start to bend or even break off. This rarely happens to hard-grown outdoor, full sun plants. This rarely ends up hurting the plant overall, but one may need to constantly remove the tallest, weaker limbs, or start pruning them shorter (which eventually ends up making the entire plant look a less ornamental, unfortunately) Euphorbia lactea is the only other columnar Euphorbia that seems to like shadier conditions but it has less problems with this etiolation weakness. Euphoriba trigona is very tolerant of being overwatered, but is also nearly as drought tolerant as all the others. The rubra form is by far the most popular because of the colors, and its leaves are as red, if not redder, than the limbs during the warmer seasons. Cold tolerance is similar to all the others, being about 27F-28F. It is very easily grown from cuttings, but plants are very common and inexpensive, so growing from cuttings is often unecessary.
two nice mature 'normal colored' Euphorbia trigonas in two different botanical gardens
Euphorbia trigona rubra form in my garden
good comparison of two forms at a nursery, side by side
Euphorbia virosa- this is a native to many southern African countries and another one of the more ornamental of the cold-tolerant tall columnar Euphorbias. This plant grows to about 10' tall, though very slowly, and has 5-8-sided limbs with neat, elegantly but closely spaced, very sharp 1/2" paired dark spines along the edges. Plants are a striking blue-green in color. I have not witnessed leaf growth on this species. Euphorbia virosas are excellent potted plants, which is usually how they are grown since most find them too expensive once of any size to risk in the ground. However, some do have larger ones in their yards, and they are one of the most cold tolerant, if not the most cold tolerant of all these larger Euphorbias, managing temps in the very low 20s with little or no damage. This is a relatively rare species, but obtainable. Large plants are very costly as they are quite old.
maturing outdoor Euphorbia virosa (left), and indoor potted specimen (right)
seedling Euphorbia virosa as an landscape plant
Euphorbia zoutpansbergensis- nearly impossible to spell or pronounce, this South African native is the last one on our list of the columnar 'tree' Euphorbias. This plant has one central trunk but dozens and dozens of arching, upright branches, closely spaced and segemented in appearance, about 2" in diameter, 4-5-sided and armed with 1/3" paired sharp spines. The plant color is a pale yellow to yellow-green. Trees grow up to about 10' or more. Cold hardiness is assumed to be at least 27F, but possible a bit colder. It is a modest to poor landscape plant due to its odd color and irregular, crowded limbs. This is a fairly rare species in cultivation and a difficult one to fnd.
Euphorbia zoutpansbergensis showing some flowers, summer, Huntington Gardens, southern California
Euphorbia is a massive genus and contains hundreds, if not thousands of species, many which have not been studied well or even described yet. There are dozens more columnar Euphorbias that may grow in such a climate as Southern California, but there is too little information on those species yet. Here are a few that we do know grow here, but there is too little information to discuss them separately in this article.
Euphorbia classenii performs fairly well outdoors in the Huntington, but there is very little information on this species
This is Euphorbia confinalis, a wonderful, but so far, way underutilized landscape species
Euphorbia grandialata is doing well in my garden (left) as is this cultivar, possibly related, called Euphorbia 'Watusi' (right)
Euphorbia knoebelli is normallly grown as an indoor plant, but may have potential as a landscape species. This obvious is one that could be a challenge to tell apart from some other species
Euphorbia ramipressa growing in Quail Gardens, southern California
This Euphorbia tenuirama looks very happy in this southern California botanical garden
Euphorbia vallaris seems to be a bit tender and needs to regrow its limbs following each damaging frost
Euphorbia venenata is reported very cold hardy (20F), but not too much other information about this plant
There are undoubtedly dozens more species growing in the ground or potted outdoors in southern California, or similar climates throughout the world, but I just don't have enough information to include them in this article. Perhaps in 10 years we will do some follow up articles on a similar topic and have a lot more data then