Succulents are a broad category of plants that have adapted fleshy leaves or stems for water storage. The relatively large interior to exterior area ratios of these plants reduces loss of water into the atmosphere. They originate in all parts of the world where periods of drought are common.
Some of the over 2.000 species of Euphorbia are succulent. Often mistaken for cacti, some succulent Euphorbia are striking and interesting houseplants. However, all Euphorbia share the copious, toxic, milky latex that flows from a wound and can cause traumatic skin and mucus membrane irritation. Take care to use gloves and protective eyewear when handling this plant.
African milk tree (Euphorbia trigona) has three--sided, columnar stems that grow upwards. The branches feature short spines along the prominent ridges so in many ways this plant strongly resembles a cactus, except for the small oval leaves that may drop away. A close inspection reveals that the spines are not produced from an areole (a round, raised or depressed, cushion-like structure) as they would be on a cactus. This plant can grow tall in a home, reaching up to 6 feet or more.
African milk trees are native to central Africa where they grow in the seasonally-dry, tropical biome of Angola, Congo, Gabon, Malawi, and Zaïre. This makes them well adapted to the dry, winter conditions suffered by most New York area houseplants. They are somewhat difficult to grow indoors because they need generous amounts of daily sunlight.
African milk tree needs strong, extended light exposure and in the northeastern United States that usually means giving it the sunniest possible place in your home (south-facing is best), with 6 to 8 hours of direct sunlight per day. Your plant will benefit from being turned weekly so that all sides receive equal light. Without extended, direct light, your plant will begin to stretch and may have difficulty supporting itself.
If the sun in your sunniest window is not adequate, artificial lights should be considered, alone or in combination with natural light. Artificial light is not equivalent to daylight in strength and must be delivered for at least 14 to 16 hours per day. (For more detailed information on see our Guide to Artificial Light for Houseplants.)
Your watering routine should vary with the time of year.
Water your plant to make all the potting mixture just moist with tepid water, then allow it to drain completely through the drain hole before removing the run-off water. The soil should nearly dry before you water it again. If the soil is allowed to stay too wet, it can kill the plant. Keeping your plant in a container that allows it to drain and then removing the water is essential to its health.
Begin to taper the watering frequency in fall as days get shorter. Typically, in the low-light conditions of fall and winter (October through February), water only as often as is necessary to prevent your plant from slight wrinkling. Your plant is not in active growth at this time and prefers extended, nearly dry conditions.
African milk trees do not need humidity to prosper, and misting is not advised. It is best suited to a dry to neutral environment with humidity below 50%.
African milk trees are happy with the temperature conditions achievable in New York area households. Daytime temperatures of 65 to 90° F. and nighttime temperatures ranging from 60 to 75° F. are well tolerated.
Like other succulents, they will benefit from a slight temperature dip from day to night in order to grow most successfully. A decline in temperature during the winter months helps the plants to adjust to decreasing hours of sunlight. A winter rest at approximately 55° F. is ideal.
While it is young and growing most rapidly, move your plant up one pot size each spring. This plant needs excellent draining soil. A mixture of half organic potting soil (with no added fertilizer) and half horticultural sand or perlite will make a good growing medium. Commercial cactus and succulent mixes are acceptable but avoid those that have food already in the mix. Your pot must have a draining hole at its bottom.
A terracotta pot with a drain hole and without glazing helps to maintain a healthy root system. Do not re-pot this plant into a container that is larger than necessary for its root system or extended soil moisture from too much soil can lead to root damage. Do not bury the main stem any more deeply in the new soil than it was previously, or the stem can rot.
Protect your skin and eyes when working with the plant as the sap can cause irritation or an allergic reaction. Moisten the soil in the plant's container before attempting to remove it to loosen the growing medium. Take your time and be gentle. The root system is delicate, and you want it to remain as intact as is possible. It may help to use potholders to handle the Euphorbia's trunk(s) with a soft grip.
Minimal nutritional intervention is required. Succulents should be fed only during their growing season (March through September). A cactus or houseplant food, fed at half strength, three or four times per growing period is fine. Any plant food with a high nitrogen value should be avoided.
It is important to differentiate the non-active growth period of your Euphorbia and to give it a rest. From October through February, it will need reduced water, food and temperature, though strong direct sunlight should continue.
Pruning can work well to keep the size of the plant in check and to create a more balanced branching structure. You should trim carefully to protect yourself and the plant, and with the expectation that the plant will have a different appearance afterwards. When you make a cut to the stem, the stem will not regrow from that point or grow in precisely the same direction. As long as the plant is healthy, it should send out one or more shoots from a point below the cut in any direction. A Euphorbia may send out a burst of branches from the area of the cut that gives the plant a fuller top or it may grow just one or two new branches.
Make sure that your plant is in a healthy condition and getting the best care before you cut for best results. Spring is the best time to prune but early summer is fine for a vigorous plant. Pruning at the right time of year gives the Euphorbia the best chance to recover from the stress easily. Be careful not to remove too many branches at one time because that could shock the plant. As a general rule, no more than 1/3 of the plant should be removed during pruning. Pruning a branch at its point of origin will look the tidiest but that may not be possible for tall, plants with fewer main stems. Here is the procedure to follow.
After pruning, your plant will need some time to recover so give it the best possible care and don't subject it to other changes in routine or location.
African milk tree should not be rooted in water or it will rot. To propagate this plant, you can take a 5-inch stem cutting in late spring or early summer. The timing is important. The plant is in more active growth at this time and the long daylight hours and dry warmth support faster rooting. The most successful cuttings come from newer growth rather than older, thicker stems.
Protect your skin and eyes when making cuttings, as the sap can cause serious irritation or an allergic reaction. Stopping the flow of milky sap is important to the health of the parent plant and the cutting. Dip the cutting end in water and spray the parent's wound with water. Then let the cutting dry out ("callus") in a warm, dry space before planting. This plant may sometimes take a month or more to callus because the stems can be thick. The dampness of the weather, type of heat in your home and amount of air circulation will all have an impact on timing. Watch the cut area and give it as long as it takes to form a hard callus to reduce the chance of rotting.
Prepare a pot of moistened potting mixture, with added perlite for better drainage (or a cactus and succulent potting mix without added fertilizer), and a layer of horticultural sand or vermiculite on top to prevent rot. Insert your cutting into the potting mix and place it in filtered light at a room temperature of about 70 degrees. If nights are colder in your home (below 60 degrees), a heated propagating mat can be used to accurately control temperature.
Keep the potting mixture just barely moist for two to six months. At that point roots will have formed. Once new growth appears on the cutting, you can repot the plant in succulent potting mixture and treat the plant as a mature euphorbia.
This plant leaks toxic, milky latex that flows from a wound and can cause traumatic skin and mucus membrane irritation. Take care to use gloves and protective eyewear when working with the plant. Even a casual rupture of a limb can leak copious sap.
Dropping limbs indicates that the plant is under significant stress and often involves issues occurring at the roots of the plant. Check that the plant has the right growing conditions for strong roots and vigorous health:
These damp soil injuries can lead to plant death if not corrected quickly. Examine the roots and stem of the plant gently looking for healthy, firm tissue. If roots or stem have become largely slimy, soft or mushy, the plant is unlikely to survive.
Leaf drop: The leaves are a bit of a luxury for this plant which drops them naturally as they age but also when under even slight stress. It will grow and keep more of them when all of its environmental needs are met. Do your best with its care and be consistent with your watering routine and you should see more of them.
Deterioration, drooping and dried branches can be water issues too (see Dropping Limbs above) but also result from extended environmental stress, such as exposure to:
These environmental factors can be easier for a plant to recover from if adjustments are made in time. Do not make too many significant changes all at once or the plant will become further stressed. Instead, ease the plant from the current environment to a more appropriate setting. Be patient and keep the plant stable and well-cared for while it regathers its strength.
The ridges of the plant will brown and crack in areas when water is irregular or too little.
Suddenly browning branch ends occur when the plant is exposed to cold temperatures. This can happen when it is left outdoors in the cold, in a cold draft from a window or air conditioning vent, or from a cold micro-climate near a window in winter. While the brown parts have died, the plant may recover. Return the plant to the proper bright, steady environment and let it stabilize for a day or two. If there is any green remaining on the plant, follow the pruning instructions above taking care to protect yourself and using sterile tools. Cut off each of the brown sections until you hit live, green growth. The plant may or may not recover depending upon its state of health before the injury, the extent of the damage and the time of year it takes place. Pruning of limbs heals more reliably on a healthy plant, with limited damage in the spring or summer when it is in active growth.
Brown, bark-like changes to the color and texture of the trunk base is natural. The plant is growing larger and is thickening the base of the stem like a tree does to help support a larger plant.
Soft, brown tissue at the base of stem can be stem rot which occurs when the succulent stem has been buried too deeply in the potting mix or when soil has remained too damp.
Exposure to a dramatic shift in light can cause limbs to sunburn and die back. This injury can happen if a plant is moved outdoors to strong direct sunlight or without a period of transition. Even a very bright interior window allows much less light transmission that direct outdoor sun. Exposing the plant to increasing hours of light over a period of a week to ten days will allow the plant to adjust and enjoy the new location. Keep the plant in an outdoor location with bright filtered light but not direct, afternoon sun.
Crazy about Cacti and Succulents by 
The Timber Press Guide to Succulent Plants of the World by 
