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.
Pencil tree (Euphorbia tirucalli) is native to Africa where it is believed to have originated in Madagascar and spread to much wider distribution. It is one of a number of spineless, succulent shrubs to trees in the Euphorbia genus. Found in somewhat less arid areas of the tropics and subtropics than other closely related Euphorbia, it has leaves clustered at the tips of its branches that soon drop off, with otherwise leafless stems, and can bear tiny flowers with colorful bracts on stem tips. Grown in the brightest sunlight, plants can develop gold to red branches.
This plant is only reliably hardy in USDA zone 11 and is grown as a houseplant in the New York City area, where it can reach a height of 3 to 4 feet. Its semi-arid native habitat makes it well adapted to the winter conditions suffered by most New York area houseplants. They are somewhat difficult to grow indoors because they need generous amounts of daily sunlight.
Pencil trees need 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 become weak.
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 regime should vary with the time of year. This plant dislikes wet soil but is also intolerant of extended dryness.
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 be just dry before you water it again. If the soil is allowed to stay too wet, it can kill the plant. If you have difficulty determining the moisture remaining in the soil near the root system with your finger, a simple plant hygrometer (moisture meter) will help. 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. Pencil tree experiences seasonal dryness in its natural habitat and needs that in your home as well. Typically, in the low-light conditions of 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.
Pencil 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%.
Succulent Euphorbia are happy with the temperature conditions achievable in New York area households. Daytime temperatures of 65 to 75° F. and nighttime temperatures ranging from 60 to 65° F. are well tolerated. They 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.
Move this plant up one pot size in the spring approximately every other year while it is young and steadily growing. 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 can lead to root damage.
The soil and pot you choose for your pencil tree play an important role in its health. Moisture trapped around the negligible root systems of these plants can lead to sudden death or damage roots so they no longer support the plant. The ideal soil should resemble the loose, free-draining mixture of the plant's native habitat, with only a small proportion of organic matter. Equal parts potting soil (without fertilizer additives) and coarse sand or perlite is generally best with a small addition of peat or leaf mold if the potting mix is a gritty type. Commercial succulent mixes are fine but avoid those that have plant food already in the mix.
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. Do not bury the main stem any more deeply in the new soil than it was previously, or the stem can rot.
Minimal nutritional intervention is required. Pencil trees should be fed only during their growing season (March through September). A balanced, organic houseplant food, fed at half strength, every four weeks during the growing period is enough for pencil tree.
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 timing 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 while it heals.
Pencil 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. 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 before placing the wound edge into a sterile, barely-moistened mix of sand and peat. Keep the plant warm and in indirect sunlight. If nights are colder in your home (below 60 degrees), a heated propagating mat can be used to accurately control temperature. Water only lightly. Transfer the new plantlet to a potting soil, sand, peat mixture once roots have developed which should occur in about two months' time. Tug the plant gently to confirm roots have formed.
This plant leaks 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 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 but 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.
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.
Suddenly browning branches 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 or heat can also cause limbs to sunburn and die back. This injury can happen if a plant is moved outdoors to heat and strong direct sunlight or without a period of transition. Even a very bright interior window allows much less light that direct outdoor sun and 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. Outdoors, give this plant a position that is protected from harsh, late afternoon sun.
Crazy about Cacti and Succulents by 
The Timber Press Guide to Succulent Plants of the World by 
The Complete Houseplant Survival Manual by 
