Poinsettia Growing Tips By Dr. Claudio Pasian Department of Horticulture and Crop Science The Ohio State University Pinching By the time growers read this article, cuttings will have been already planted and growers will be getting ready to pinch them. Growers should keep in mind that the vigor of the cultivar and the pot size are determining factors on when to pinch. For example, a medium vigorous cultivar (e. g. Freedom) grown in a 6 inch container should be pinched in the northern part of the US around week 34. Pinch earlier for larger containers and later for smaller ones. Vigorous cultivars should be pinched 1-week later while compact varieties should be pinched 1-week earlier. Some very compact varieties can be pinched 2-3-weeks earlier. This extra care of giving each cultivar the right treatment at the right time is very important in order to produce a quality crop. The "one size fits all" mentality doesn't work – especially with all the new poinsettia cultivars we have in the market. The person responsible for the poinsettia crop will have to contact the breeders and ask for the proper cultural practices appropriate to each cultivar. Growing mix The mix used for growing poinsettias is very important in order to have a high quality crop. Some growers I know have tried to "cut corners" buying chip mixes and had a lot of headaches as a consequence. One of the most important characteristics of media is air porosity. If this property is low, water retention is high – a condition that favors Pythium a very common problem with poinsettias. To prevent this disease avoid over-watering and do a preventative drenching with a fungicide. According to Dr. S. Nameth (OSU Plant Pathology) preventative applications of chemicals are discouraged but Pythium on poinsettias represents an exception to such a "rule" due to its high rate of occurrence. Fertilization Fertilization should start soon after transplanting to reple-nish the nutrients washed out in the mist during rooting. Start applying fertilizer at a rate of 150 ppm N. After plant establishment increase the rate to 200 - 250 ppm. Some growers use much higher concentrations (up to 400 ppm) during this time. This not only can promote soft growth and long internodes but also can limit early growth if the growing mix has pre-plant fertilizer incorporated. Another important point regarding fertilizers is the fact that they have the potential of modifying the pH of the growing mix. Some of the new poinsettia fertilizer formulations are high in nitrate nitrogen and contain calcium and magnesium and have neutral to basic reactions. If you have been using a more common fertilizer such as 20-10-20 and switch to a 13-4-20-6Ca-2Mg, you'd better start checking your pH on a regular basis to avoid problems. The latter fertilizer has a potential basicity of 330 lbs/ton – enough for the growing mix pH to move up. Stem break To minimize stem breakage, growers should reduce the level of fertilization, provide as much light as possible and pay attention to how the crop is pinched. If pinching results in excessive branching, those at the bottom will be thinner, and grow weak and more horizontally.