Poinsettias can be successfully fertilized using a number of different approaches. In fact, sometimes it seems there are as many different ways to fertilize as there are growers! Probably this is because there are a number of factors which interact to determine how much nutrition the plants get. Rate (ppm N) is only one factor because it interacts with frequency of application, volume applied, amount of leaching, and environment to affect plant response.
This article discusses basic fertilizer requirements and special nutritional needs of poinsettia.
Basic poinsettia programs
Some common fertilizers used for poinsettias are 15-0-15, 15-16-17, 20-10-20, and EXCEL Cal-Mag 15-5-15. But other fertilizer types can also be used to grow good plants. For example, I've been using 20-0-20 with superphosphate added to the growth medium for a couple of years with great results. The range of rates commonly used are 200 to 300 ppm N - I recommend 250 ppm N.
Although I tend to think in terms of Scott's fertilizers because I use them; Plantex, Masterblend, and Greencare fertilizers also grow excellent poinsettias.
Regular fertilization should begin at planting because it helps the plants become established and encourages rapid development of branches after the pinch. Fertilization should continue on a regular basis until 2 to 3 weeks before sale and then it should stop. Reducing fertilization at this time helps extend the postharvest life of the plants and prevents soluble salts injury to the bracts (this practice really works!).
Soil tests should be made at several times during the season to monitor the nutrition of the plants. Ideally the potting mix should be tested prior to planting and then after planting when the branches are about 2-3" long (early to mid-September), again when the bracts begin to show color, and finally just before fertilization is stopped late in the season.
One important piece of information from the soil test is pH. Although poinsettias are fairly tolerant of a wide range of pHs, the target range is 5.8 to 6.2. If the pH falls in this range it helps maintain the availability of calcium, magnesium, and molybdenum.
Specific nutritional concerns
Calcium deficiency. Bract necrosis, leaf edge burn on stock plants, and weak stems are disorders linked to calcium (Ca) deficiency. Most of the focus is on bract necrosis. Keep in mind that bract necrosis is also a symptom of Botrytis infection, pesticide spray injury, too much fertilizer late in the season, and simply "old age."
Calcium is most critical during bract expansion to avoid bract necrosis. The amount of Ca reaching the bracts is influenced not only by the Ca supply in the growth medium but by water movement into the roots, through the plant, and transpired out of the leaves. High relative humidity inhibits transpiration and Ca movement through the plant. So adequate ventilation to reduce the relative humidity during bract development is very important.
Ca deficiency can generally be prevented by adequate liming and the use EXCEL Cal-Mag or 15-0-15, 20-0-20, or calcium nitrate/potassium nitrate supplements to a phosphorus-containing fertilizer. However, because of the role of humidity in the movement of water and Ca in the plant,use of these fertilizers may not always prevent bract necrosis from showing up.
Ca sprays to the bracts are very effective in preventing bract necrosis. Weekly sprays should start when the bracts first show color and continue until the pollen shows. Solutions are made up to supply 200 to 400 ppm Ca using calcium chloride. A spreader-sticker compatible with poinsettias should be added to the solution. The bracts are sprayed with a fine mist.
Today Ca deficiency is the most widely accepted nutritional explanation of bract necrosis. However, research being conducted by Rich McAvoy and Bernard Bible at the University of Connecticut suggests a more complicated scenario which may include boron nutrition and other factors. Perhaps more effective treatments than those discussed here will be developed to prevent this potentially serious problem.
Magnesium deficiency. Interveinal chlorosis of the lower leaves on the branches of a pinched plant is the main symptom. Mg deficiency can be prevented by EXCEL Cal-Mag or monthly application of magnesium sulfate at a rate of 1-3 lbs./100 gal. Mg deficiency is most likely in November.
Phosphorus nutrition. Low phosphorus (P) may result in reduced plant size or small bracts. This might occur if calcium nitrate/potassium nitrate, 15-0-15, or other low P fertilizer is used for an extended time to fertilize poinsettias growing in a low P mix. Remember, most commercial soilless media don't contain superphosphate (0-20-0) or other P fertilizer.
Too much P may lead to excess growth. Several years ago I studied the affect of superphosphate rate on poinsettia growth and P leaching (Cox, 1999). Plants grown with 9 lbs./cu. yd. had larger bracts than plants grown with lower rates, but were too tall. In addition to stimulating too much growth in height, excess P has been linked to bract necrosis (Whipker and Hammer, 1994) and to nutrient pollution of water.
Molybdenum deficiency. Interveinal chlorosis and marginal necrosis occurs on the recently-mature leaves and middle-aged leaves. Mo deficiency is most likely in November, however this problem is rare nowadays and may have disappeared with the 'Heggs' and 'Lilo.' The best way to prevent this problem is by maintaining the recommended pH and the regular use of "peat-lite" fertilizers. Foliar sprays of 1 ppm Mo are very effective at preventing deficiency before symptoms show up or correcting the deficiency if it is caught when the chlorosis first appears.
Ammonium toxicity. This is another cause of interveinal chlorosis. This problem has all but disappeared since the widespread adoption of "peat-lite" fertilizers which are about 50% nitrate-N. Don't use 20-20-20 for poinsettias!
What about subirrigation?
Poinsettias watered and fertilized from the bottom can be safely fed with the same rate (ppm) of fertilizer as top-watered plants. I recommend the range of 200-250 ppm N for subirrigation of poinsettia. This recommendation is based on my observations of what experienced growers are doing in New England and my own work at UMass (Cox, 1998). Results of my study showed that 100 ppm N was also good, but 325 ppm N was too high. Yes, the soluble salts were higher at the surface of the growth medium with subirrigation, but in the root zone (the lower two-thirds of the pot) there was no difference in soluble salts level between subirrigated and top-watered plants. Leaf analysis revealed no substantial difference in plant nutrition between subirrigated and top-watered plants.
- Cox, D.A. 1998. Subirrigation vs. overhead watering for poinsettia. Floral Notes. 11(1):8-10. (July-August issue).
- Cox, D.A. 1999. Poinsettia response to phosphorus. Floral Notes. 12(1):5-6. (July-August issue).
- Whipker, B.E. and P.A. Hammer. 1994. Determination of injurious phosphorus levels in poinsettias. HortScience. 29(2):85-87. Douglas Cox, Plant and Soil Sciences, University of Massachusetts, Amherst.
Dr. Douglas A. Cox, Stockbridge School of Agriculture
University of Massachusetts, Amherst