Euphorbia is a genus of about 2,000 species, including an array of interesting houseplants with varied growing requirements. Euphorbia milii are spiny, evergreen, semi-succulent shrubs that look and grow differently from their houseplant relatives. All of these plants share the milky latex that flows from a wound and can cause skin irritation.
Crown of thorns plants are native to tropical Madagascar and now distributed in areas of Asia and South America. They are notable for their unusual, bare, spiny stems with leaves emerging near the growing tips. Leaves last for only a few months before dropping off and leaving a barer stem. These older leaves are not replaced so a mature plant looks quite different from a younger, leafier one and may surprise its owner. The amount of leaves on the plant varies from one cultivar to another and from plant to plant. Some are characteristically tall and stalky, others lower and bushier.
Flowers, surrounded by colorful "petals" extending from a cyathium of fused bracts, emerge only on actively growing (leafy) stems in spring and summer. A particularly happy plant in a great growing situation may reward you with a longer flowering season, from late winter through fall.
There are a wide variety of cultivars available including large-flowered hybrids and others bred to showcase unique flower-forms and colors. Newer cultivars include dwarf plants and stems with less vicious thorns
Crown of thorns needs strong, extended light exposure and in the northeastern United States that usually means giving it the sunniest possible place in your home, with 6 to 8 hours of direct sunlight per day. Full sunlight is needed to stimulate bud formation and flowering and plants with great light will have a longer flowering season. Your plant will benefit from being turned weekly so that all sides receive equal light.
Water your plant to make all the potting mixture just moist, then allow it to drain completely through the drain hole before removing the run off water. The soil should just dry before you water it again. Do not let the roots stay dry for an extended period or the leaves may fall off. 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. The plant flowers best if kept on the dry side.
After the flowering season ends, reduce water a bit but don't let the roots dry too long. If you have the plant in great sunlight, it may continue to flower into the fall and you can continue to treat it as an actively growing plant.
This plant prefers low humidity.
Crown of thorns likes warm household temperatures. Keep it in a warm, dry location with temperatures always above 60ºF and no door, vent or window drafts. If temperatures dip into the 50's, leaves will fall off prematurely.
Feed the plant lightly with a balanced, organic houseplant fertilizer once or twice a month in spring and summer, continuing until flowers are gone. Do not use a high nitrogen fertilizer on these plants and avoid fertilizer with boron micro-nutrient content, to which some cultivars have a sensitivity. Over-fertilization can result in reduced flower production. Some cultivars have a naturally pale leaf color and it does not indicate a nutrient deficiency.
Crown of thorns likes a rich soil with excellent drainage. Mixing one part perlite (for drainage) with two parts organic potting soil (no added fertilizer) creates an excellent soil for your Euphorbia milii. Plants that are potted in the wrong mixture will initially do well but may later suffer from root rot associated with excess water retention in the soil.
Re-potting your plant every two years in the spring is healthy for your crown of thorns plant. Generally, a clean pot of the next size up with fresh soil is fine as slightly tight conditions benefit flowering. Wear gloves and handle the plant carefully as you move it so as not to break off the stems, or cause injury from the thorns or sap. (Some gardeners use a pot holder to protect their hands while holding the plant.) Pack the potting mixture tightly around the roots. Once the plant is full size (about 2 feet tall for the species when grown as a houseplant but may be different for varieties and cultivars) you can replace just a bit of the surface soil with fresh potting mixture each year to enhance soil texture and nutrition.
Pruning your mature crown of thorns plant can help to retain a nice shape and fresh leafy growth. Some varieties naturally begin to arch and bend as they grow, and pruning can keep the plant more compact and suitable for the indoors. Some cultivars and some individual plants branch less than others and pruning may be used as a way to stimulate branching. The results you will get from pruning are as varied as the plants themselves. You may get multiple, sturdy branches from each cut or none. Make sure that you start with a healthy plant that has the vigor necessary to heal and grow. Pruning has best results in spring when the plant is in a period of strong growth.
The thorns and sap of the plant should be taken into consideration before pruning. Protect your hands, eyes, clothes and gardening tools. A sterile knife is easier to clean of sap after cutting and is recommended as an alternative to ruining your garden shears. Spray the wounds created on the plant with water to stop the flow of sap.
Begin by removing any damaged limbs to their base. Look along the remaining stems that you would like to shorten for evidence of healthy buds and cut back to just above that point, or cut a stem back to a branch joint, leaving the axillary buds and branch collar intact. A strong and healthy looking bud is the best indication that your plant will send out a new branch as well as the direction it will grow.
Do not remove all the leafing stems on the plant. It is safest to have ½ the leaves remain on the plant. Once healthy new leafy growth is present, you can cut back remaining stems in the same way as long as it is still early in the growing season.
Your cuttings can become new plants! Read below for instructions of propagating your crown of thorns.
Crown of Thorns (Euphorbia milii) can be propagated from cuttings. Water your plants one or two days before taking cuttings--this will ensure that the cuttings have enough water in their stems. With a sterile cutting tool, cut off the top 3 - 4 inches of a healthy, young stem. Older stems have less of a capacity to generate new leaves, so only take cuttings from young stems; thinner than your pinky is a good rule to follow. You may wish to cut off a longer piece to shape the parent plant, but then trim it to 3 - 4 inches for propagation purposes. Make sure the stems you cut have leaves on them. Spray the cut end with cool water to stop the sap from flowing. The sap can irritate your skin and stain clothes and furniture; wear gloves and protect your eyes.
Place cuttings on a paper towel for 2 - 3 days to give the wound time to callous over. Remove most of the leaves on the cuttings, leaving on 3 or 4. Keep the cuttings out of direct sun at this time. Pot up in a soil mix that has good drainage, like the potting soil and perlite mixture described above.
Once the pots are filled with damp potting mixture, make a hole with your finger about 1½ inches deep. If you have rooting hormone powder, pour a little in a small container and dip the cut end into it--just enough to coat it, not more. Place the powdered, cut end into the hole, then gently firm the soil around the cutting. Place the pot in an area with bright light but out of direct sun.
Water the cutting sparingly when the potting mixture has nearly dried out, but only enough to keep the soil barely damp. (Do not water as you normally would because the new plant has no roots yet to absorb the water from the soil and you might cause rot.) It will take several weeks before the cutting roots out and gets established. Once you start to see new growth you can begin to water normally.
Leaves on just the tips of the plant stems is typical. It is natural for your crown of thorns to lose leaves on the older parts of the stem and to have the stalky, prickly look that gives it its name. Each leaf lasts for just a few months before falling off. Some crown of thorns cultivars grow in a bushier, leafier form than others. A tall plant with leaves only at the tips may be its natural growth pattern.
No flowers: The flowers appear only on the ends of actively growing stems. To have good flowering, proper light is the most important factor. Your Euphorbia needs all the sun it can get to have the energy to flower. The more bright, direct sunlight it gets, the longer the flowering period will be. Without enough light, you will not get flowers.
Your plant also needs to be healthy to flower. Keep it in a warm, dry location with temperatures always above 60ºF. Do not over-fertilize or it will reduce flowers. Check the care routine above to see if your plant is missing any other important elements. The plant flowers most reliably when soil mixture is kept on the dry side.
Oozing, latex sap is an irritant but the sap flow can be stopped by spraying the plant's wound with cool water.
Leaves fall off naturally after a few months leaving a barer stalk with new leaves emerging at the tip. Some individual plants and cultivars have a taller, barer habit than others. Leaves may fall off more rapidly, however, if the plant is allowed to stay dry for too long, the temperature dips too low or there is not enough sunlight for the plant.
If the parent plant is having trouble recovering after you take cuttings, there are a few things that may have gone wrong. It is important to take cuttings in spring or summer when the plant is in a stage of active growth so that it has the energy to recover. It is safest to leave enough leafing stems on the parent plant to photosynthesize and restore energy to the plant, so remove less than ½ of the leaves of a healthy parent plant when taking cuttings. The plant needs to have the light and warm temperatures that give it strength. Use a sharpened and sterilized blade when removing the growing tip to reduce the chance of infection in either part of the cut stem. Spray the cut stem of the parent plant with water right after cutting to limit sap flow.
The Houseplant Survival Guide by 
