Understanding Plant Nutrition: Poinsettias
Argo and Fisher pore over the details to systematically identify and correct common nutritional problems found in the greenhouse.
September 1, 2009
Profit margins are extremely tight for poinsettias, so minimizing shrinkage that results from crop losses is essential. If nutritional stress occurs, you will be struggling from that point onward to produce a high-quality plant for sale.
Poinsettias hang around your greenhouse for several months, much longer than short-term spring crops. Therefore, a preventative approach that breaks the crop down into stages can help avoid the usual problems. Here are some pointers to consider at each stage.
1. Poinsettias have four stages in which fertilizer needs vary: propagation, initial growth, rapid growth and flowering phases. Figure 1 represents the goals in each stage. The amount of fertilizer taken up by plants varies depending on how quickly the crop is growing.
2. During propagation, avoid solutions that have a high electrical conductivity (EC) (above 0.75 mS/centimeter) or fertilizers containing phosphorus to avoid foliar damage (Figure 3). Rooting can be delayed as mist-water EC increases, so use your purest water quality for propagation.
If stock plants were adequately fertilized, fertilizer is not necessary during the initial mist phase. If tissue nutrient levels in cuttings begin low (Figure 3), then it will be more important to apply nutrients early during propagation.
3. After one soil volume of water is leached out the bottom of the pot, any pre-plant nutrient charge contained in the growing medium will have been removed. Once roots emerge and misting is reduced, consider recharging nutrients with around 200 to 400 parts per million of nitrogen (ppm N) applied with a thorough irrigation to recharge the soil solution.
4. Control fungus gnats and pythium throughout the crop. Nutrient uptake is completely dependent on having a strong root system. If roots get stressed through pests, disease, high EC or overwatering, you will run into slow growth and sometimes deficiency symptoms.
5. During early growth, poinsettia elongation should not be "pushed" with extra fertilizer. Figure 4 shows an example of an experiment where fertilizer was either gradually increased (scheduled), or remained high throughout the crop (at 400 ppm N). Root systems were much stronger with a gradual increase in fertilizer EC.
6. Trends in media-EC with weekly in-house tests help you refine the fertilizer concentration. Choose whatever soil-test method you find most convenient and use the appropriate target EC values (pour-thru (1.9 to 4.1 mS/cm), 1:2 (0.6 to 1.5 mS/centimeter) or saturated medium extract (1.1 to 3.0 mS/centimeter)).
The fertilizer ranges in Figure 1 provide a reasonable guide, but we have provided a range not just to be vague, but because the "best" fertilizer rate depends on a number of factors in your situation. For example, light-colored foliage plants tend to require a higher fertilizer rate than dark-leafed cultivars. Slow-growing cultivars, such as variegated and Winter Rose types, have greater leaf expansion and vigor with moderate, rather than high, fertilizer rates.
Irrigation practices also have a big effect on required fertilizer concentration. For example, research at Michigan State University showed poinsettias could be adequately fertilized either with 100 ppm N and no leaching (such as subirrigation), or 400 ppm N with 55 percent of solution leached with each irrigation. Fertilizer rates in Figure 1 represent constant fertigation (fertilizer applied with each irrigation). If you apply regular clear water or fungicidal drenches, a higher concentration is needed whenever you do fertilize.
7. Target pH levels for poinsettias are broad (ideally between 5.6 and 6.4) because poinsettias are not as sensitive to media-pH issues as some other crops, such as petunia and geranium. At the extremes, however, symptoms of micronutrient deficiency or toxicity can occur.
8. Remember, the total nutrient solution is a combination of your irrigation water and the fertilizer solution. Therefore, the ideal poinsettia fertilizer varies between locations depending on irrigation water quality. For growers with high calcium and magnesium rates in their irrigation water (60 to 80 ppm and 30 to 40 ppm, respectively), that is already at the level of most calcium-magnesium fertilizers. However, growers with more pure irrigation waters will need to use a fertilizer high in NO3-N that also contains both calcium and magnesium.
9. Calcium and magnesium can also be supplemented to avoid deficiency symptoms (Figure 5) and also to reduce botrytis infection of foliage. Calcium can be applied as foliar sprays during the bract expansion phase. The Paul Ecke Ranch, Ecke.com, has a detailed bulletin on calcium sprays in its Tech Help section. Monthly soil drenches with magnesium sulfate (Epsom salts) at 1 pound/100 gallons supplement magnesium. Do not combine magnesium sulfate with calcium fertilizers such as calcium nitrate in the same concentrated tank, or precipitation will occur.
10. As the crop approaches the middle of October, avoid fertilizers that have most of the nitrogen in the ammoniacal or urea form, for several reasons: (a) most ammonium-based fertilizers are low in calcium and magnesium; (b) there can be increased soft growth with ammonium; and (c) in extreme cases with cool temperatures and dark conditions, ammonium toxicity can sometimes occur (which appears as a marginal chlorosis and downward cupping leaves).
11. Post-production quality for the consumer is improved when media-EC levels are low at the end of the crop. Growers sometimes cut off fertilizer too early in production, leading to deficiencies during bract expansion. A safe approach, especially if you are growing multiple cultivars in the same irrigation zone that mature at different times, is to drop fertilizer level to around 100 ppm N for the last two to three weeks.
In reality, poinsettias tolerate a wide range of nutritional levels. You can grow by the seat of your pants and usually get away with it. However, when you can't afford a mistake, it's good review your fertilizer strategy for each of the four stages of production.