Care and Feeding. Crown of Thorns is a good choice for water conservation and ease of maintenance. Attractive and compact growers, they come in a wide range of colors for landscaping. They are salt and drought tolerant. Thick, fleshy leaves and stems have evolved for water and nutrient storage, and they can share areas with other plants that have similar growing requirements. They can withstand infrequent watering and full sun, although ideal conditions would provide moderate moisture and light shade. Indoors, bright sunlight for a good part of the day stimulates flower production and is less harsh to the leaves. Strong sunlight promotes and abundance of flowers but will burn the leaves if they are not used to it and if heat is high and humidity is too low for prolonged periods. A good happy medium is full sun in the morning and/or afternoon with protection from midday sun in the summertime. Roots love room to grow, but they can’t abide stagnant water; so keep them in a mix that will retain needed moisture. Decorative ceramic pots make a gorgeous display; but remember that your pots must have holes in the bottom to let the extra water drain out and away. You may also put your plants in the ground. If a chosen site in the garden does not drain well, a 12-18 inch raised bed of crushed rock and sandy soil may be made. Sandy, gritty soil combined with added organic material like rich compost is an ideal medium for these bedding plants. Space individual plants well-apart for good air circulation. They may be watered regularly if media and pots drain freely. Once established in large pots or in the ground, Crown of Thorns can go without frequent watering. The top 1” of soil may be allowed to dry out. Water pots until you see water drain out from the bottom of containers. The plants can survive periods of drought and low humidity. They may drop their leaves and go dormant under extreme conditions and recover when conditions are better. So, if you forget to water potted plants for a week, all is not lost. They might not look their best by then, but they can grow new leaves as watering resumes. Check the growing tips. If they are look alive, they plant will survive. Keep in mind that they must also have bright light and good air circulation to stay happy and healthy. They can still fall victim to bacterial and fungal diseases, so keep them and the area around them clean.
Remove those nasty caterpillars on sight!!! They are voracious eaters and killers.
Apply a balanced slow-release fertilizer (13-13-13) every 3 t0 6 months if you do not wish to apply liquid fertilizer every 2 weeks.
Remove dead and yellowing leaves and flowers to prevent fungal and bacterial rot --especially in wet conditions. Debris that’s allowed to remain on a plant can trap moisture and encourage the development of bacterial and fungal leaf spots and root rot. It is important to check and keep plants clean during prolonged rainy weather.                                     Occasionally, an insistent family of scale may try to infest a euphorbia plant. All that’s needed, in that case, is to simply remove and dispose of the infected leaves and hose off the plant each time you water in the morning. Scale insects remain vulnerable until they find a good perch on your leaves and form a hard shell. A good spray of the waterhose will knock them off and keep them from taking hold again. The plant will grow new leaves. Pesticides are not needed to keep these plants attractive and healthy. I prefer to water my plants by hand instead of using a watering system. Using the thumb to create large drops to knock off fallen debris and possible unseen bugs can be hard on the thumb; but flow regulation is more immediate. You can also spot caterpillar presence by seeing chewed leaves and frass.  As your prickly posies grow, you may find that they have become so luxuriant in growth that their branches intersect and compete for space. That is your opportunity to make more plants to grow or give away. The cut should be where the branch narrows to meet the main stem, at least 3” away from the growing tip. Immediately put the cut ends into water to staunch the flow of sap. Then allow the cuttings to dry for 3-4 days before you dip them into a rooting compound that contains fungicide and plant each about 2” deep in a 3” plastic pot of potting mix and sponge rock. Water them once a day in bright shade and repot to 6-8” pots after 3 months, when the roots are well-developed. They may be blooming by then. Repotting into fresh mix in the summer every 2 years is beneficial.