How to Grow Columnar Euphorbia
By Tyson Curtis in Partnership with Flora Grubb Gardens
Got a very bright spot in your house that’s begging for a plant? Columnar euphorbia are intricate, sculptural succulents that will happily grow indoors with proper care and attention. Read on for some interesting facts about euphorbia and advice about growing these beauties as houseplants.
First let’s get one thing out of the way, the elephant in the room: Euphorbia aren’t cactus!
Cactus and columnar euphorbia may look similar, but they’re different plants. Look at a Saguaro cactus and you'll see the stiff upright trunk and arms. It has spikes to defend itself from predation and grows in the desert. Then look at a candelabra tree (a type of euphorbia), which also has an upright trunk and arms spiked to defend itself. Candelabra trees also grow in the desert. So what's the difference? Well, believe it or not, there are no cactus growing outside of the “New World.” Wait, no cactus in Africa? That’s right: no cactus in Africa.
To square this revelation we must understand convergent evolution, which is defined as "the process whereby organisms, not closely related, independently evolve similar traits as a result of having to adapt to similar environments or ecological niches." In short cactus and euphorbia look nearly identical because they were forced to evolve the same way under similar conditions. They’re columnar because this is an efficient way to store water. They have spikes to defend that water. While looking similar, they evolved oceans apart and are completely unrelated.
So then how do we tell the difference? The flowers are the main distinguisher. Cactus flowers, often pollinated by birds or bats, are large, soft, and often brilliantly colored. Euphorbia flowers, often pollinated by flies, are small, usually green, and frankly not very impressive. While at a glance the spikes look similar, a closer inspection shows that cactus have spines (a modified leaf) while euphorbia have thorns (a modified stem). The other notable difference is that most euphorbia have a toxic latex sap to assist in their defense.
Columnar Euphorbia make excellent houseplants when grown in a fast draining cactus mix. Here’s how to care for them in your home:
LIGHT
Euphorbia are plants that want warmth; south- or west-facing windows are the best place for them inside (four or more hours of direct sun shining on the plant), though they can also be grown in very bright indirect light. Since their skin is more delicate than cactus, they may burn if placed within 18 inches of a window. Arranging them at an angle to the window will help prevent this. If you are growing them in bright indirect light, make sure the plant has excellent drainage and is kept as warm as possible.
WATER
Since these plants grow during the summer, water should be gradually increased through late spring into summer, when they can go dry without being bone dry. This means that the soil throughout the pot doesn't feel wet, but also doesn't feel crackly and chalky. Don't let them sit around bone dry during this time; shriveling is a sign they’ve been under-watered. While fertilizer is usually unnecessary, you can apply an all-purpose fertilizer at 1/4 strength during the summer months.
Water should be gradually decreased during the fall. In nature euphorbia get very little winter rainfall. During the winter they should become bone dry between watering. It is often hard to reconstitute water into a bone dry plant (as it is like a dry sponge, where water will just run off) so make sure you let your euphorbia stand in a saucer of water until it has completely sucked up the water. Continue adding water and letting the plant suck it out of the saucer until there is standing water in the saucer that isn't absorbed. You can then either dump out the saucer or put a rag / sponge / turkey baster in the saucer to absorb the extra water and remove it. Don't ever let columnar euphorbia stand in water more than 12 hours.
COMMON PROBLEMS
The most common issue with these columnar euphorbia presents as brown spots. If they are light brown and hard like a scab, this is likely corking, which is the natural progression of the plant becoming older. Think of corking as the smooth green bark of a tree becoming woody over time. This is totally normal, don't worry.
However, if the brown spots are darker and squishy, this is a sign of rot. Rot needs to be surgically removed, as it will spread and kill the plant. The sooner you catch rot, the more likely you'll be able to save your plant. There are three possible scenarios here:
For just a little spot of rot, dig back to healthy vibrant tissue and let the plant scab and recover.
For rot that's circled the column midway up, you'll need to remove the top, clean the rot from both top and bottom back to healthy vibrant tissue and re-root the top. Do this by letting it scab over for 2-3 weeks and then replanting the top in very fast-draining soil (1/2 pumice or lava rock and 1/2 cactus mix) and place it in a warm bright spot out of direct sun. Rooting hormone is also useful for this, if you have it. Cement or tile floors are great for re-rooting as they transfer that heat to the plant, but you can also use a heat mat set at 72F if you have one. In this scenario you'll get a new plant from the top, and the bottom will sprout a new head and continue growing. (This is also how you would propagate the plant, usually done in late spring when the plant is actively growing.)
If rot happens at the base, circling the column, you'll need to propagate the top, but the bottom is done for and should be discarded. Remember when we said that the plant’s latex sap is poisonous? While doing this use gloves and DO NOT get the sap in your eyes, as it is a serious skin and eye irritant.
Other common problems with indoor-grown euphorbia usually arise from plants cultivated in cold rooms without enough sun. We see issues when plants are drastically under-watered during summer; however, even a little overwatering in winter can be disastrous. Watering without reaching full saturation is another common mistake.
Some non-columnar euphorbias also can be grown indoors, like Euphorbia leucodendron and Euphorbia tirucalli ‘Sticks on Fire’ (sometimes called pencil cactus), which both have a beautiful forking shape that almost seems to mimic aquatic flora. Like the columnar euphorbia, these need a lot of light when grown as houseplants and can even grow in direct hot sunbeams by a window. (Not a viable choice for homes with pets because of the aforementioned toxic sap!)
Of course, these amazing plants can also be grown outdoors in a mild climate like the Bay Area, even thriving out in the foggy avenues of San Francisco. These euphorbia like a warm sheltered spot with good sun and excellent drainage. They prefer slopes where water can run off, and also love being near radiant south- or west-facing walls or fences. They are summer growers and will need additional water during this time, when dry. Generally they aren't hungry, though if you'd like to fertilize, do so with an all-purpose fertilizer applied at 1/4 strength. They tend to fail in clay soil, in partial sun, or where they receive too much winter water (in addition to our natural rainfall). Mounding can help with clay soil.
Now that you’re a columnar euphorbia expert, help spread the word about these amazing plants (and why they’re not cactus!). At Flora Grubb Gardens plant store in San Francisco we carry lots of varieties of euphorbia for indoor and outdoor growing. Come visit us and choose yours!