By Dr. Fred Hulme, ICL-Specialty Fertilizers Technical Services Director
With poinsettia season upon us, now is a good time to review the nutritional needs of poinsettias and the corresponding fertilizer practices that will maximize crop quality. From a nutritional standpoint, poinsettias can be a rather finicky crop that challenges even the best growers. Poinsettias readily exhibit both deficiency and toxicity symptoms linked to specific nutrients. Common poinsettia nutritional deficiencies include nitrogen, potassium, calcium, magnesium, molybdenum and general micronutrients. Poinsettias can also suffer from excess levels of ammonium or boron.
Some of these nutritional problems may be linked to the time of year when poinsettias are grown (when day length and light intensity are decreasing). However, since production timing for this crop is fixed, fertilizer programs must be compatible with typical environmental conditions observed during this period. Further complicating matters are the continuing proliferation of new cultivars, the lack of current nutritional research on new poinsettia cultivars and crop types and even the movement to grow poinsettias at lower temperatures to save energy. The keys to growing a high-quality crop are proper fertilizer planning that adequately meets the specific nutrient demands of your poinsettia crop and establishing a good nutritional-monitoring program to ensure optimal nutrient conditions are met throughout the crop cycle.
Irrigation water quality is the starting point for proper fertilizer selection. Plants are nearly 90 percent water by content, and nutrients are taken into the plant with the irrigation water. Test your irrigation water even before you pot up cuttings to help you select the correct fertilizer program. Start with a comprehensive irrigation water analysis (alkalinity, EC, pH, nutrients and other potential problematic elements) with a reputable testing lab to determine what nutrients are abundant or lacking in the water. Then you can select fertilizers that compliment your water and meet your crop needs. The Scotts Testing Lab, for example, tests irrigation water, growing media and plant tissue to help nursery and greenhouse growers choose the best fertilizer or diagnose problems with various plants.
There are a number of fertilizers labeled as “poinsettia fertilizers” to make the selection process appear easy from a marketing perspective. However, simply naming products this way does not necessarily make them the most suitable ones for your operation. Because irrigation water, growing media and other management practices vary significantly from greenhouse to greenhouse, the best fertilizer programs, when matched to these different situations, also should vary.
Fertlizer programs should abide by some general nutritional guidelines, such as the following:
- Fertilizer should provide the correct amount of acidification or buffering to maintain growing medium pH between 5.8 and 6.2; increase root zone pH to 6.5 if irrigation water has a high boron level.
- Fertilizer should have a high (50 to 85 percent) nitrate-N fertilizer formulation. Avoid a build up of high ammonical-N levels in the root zone during production. However, use of a fertilizer with higher ammonium percentage may encourage breaks during cutting production and after pinches.
- Fertilizer should have a low ratio of phosphorus; nitrogen (N) and a higher ratio of potassium to N such as Peters Professional 15-5-25 PL.
- Avoid excessive EC levels in the root zone.
- Avoid excess P levels to avoid plant stretch.
- Supply adequate calcium, magnesium and micronutrient (especially molybdenum) levels.
- Avoid high boron level in growing medium (>0.25 ppm B). Poinsettias can be sensitive to B toxicity that results in leaf edge burn.
One fertilizer formulation may not be able to meet all these demands. There may be a need to supplement your main fertilizer product with components such as STEM, sodium molybdate and magnesium sources and rotating in calcium fertilizers when appropriate. Some fertilizer formulations cannot be combined in the same stock tank. Thus, they need to be applied in rotation.
In the past few years, growers have reduced their fertilizer concentrations from historical levels, and in some cases, insufficient fertilizer has resulted in N and K deficiencies. For lighter, green-leafed varieties, ICL-SF recommends starting off the first two weeks of production at 300 and 350 ppm N on a continuous feed basis (to charge the growing media with nutrients). Then gradually reduce levels to 200 and 250 ppm N. Dark-leaved cultivars such as ‘Freedom,’ ‘Prestige,’ ‘Dynasty Red’ and ‘Lilo’ require lower nitrogen rates (100 to 150 ppm) at this point. After bract expansion occurs, further reduce fertilizer concentration to avoid soluble salts build up in the growing medium. Frequently monitor growing media EC and maintain soluble salts levels in the root zone below 2.5 to 3.0 mmhos/cm (based on an SME).
Poinsettias will respond favorably to combination water-soluble/controlled-release fertilizer (CRF) programs. Incorporate a low rate of CRF (e.g. 4 pounds of Osmocote Plus 15-9-12, eight to nine month product longevity per yard) as a base feed, then use water-soluble fertilizers at half the above recommended concentrations (100 to 125 ppm N). The controlled-release fertilizer provides some insurance in case something goes wrong with the liquid feed program, and it may also provide some nutritional benefits in the post-production environment. If you have a mixture of dark and light-leafed varieties and are feeding all plants at low fertilizer concentrations, supplemental CRF can be used to compensate for the higher nutritional demand of light-leafed varieties.
Some of the most effective supplemental nutrient treatments to fine-tune crops are:
- Calcium (Ca).Low Ca can lead to weak stems and leaves, bract edge-burn and depressed growing media pH. Maintain a continuous feed of at least 60 to 100 ppm Ca in your feed program. If your irrigation water does not supply enough Ca, supplement it by using fertilizer containing calcium in rotation, such as calcium nitrate or Peters Professional 15-0-15. Sometimes, even if Ca is adequate in the growing media due to high temperature or high relative humidity, the plant may not utilize it. Weekly foliar spray applications of technical grade calcium nitrate (15.5-0-0) at .25 ounces per gallon of spray solution (providing 338 ppm Ca) during bract formation (between first sign of color to until first pollen on flower) may prevent low Ca problems.
- Magnesium (Mg). Low Mg can lead to interveinal chlorosis starting on older leaves. Maintain at least a continuous feed of 30 to 50 ppm Mg in your feed program. If your irrigation water does not supply enough Mg, use a magnesium containing fertilizer or supplement with magnesium nitrate or Epsom salts (applied 4 to 6 ounces/100 gallons).
- Molybdenum(Mo). Low Mo can lead to marginal chlorosis and edge-burn on younger leaves, especially in late October and November. Avoid this problem by adding Sodium Molybdate from September through mid-November on a continuous feed basis at a rate of 1 gram per 100 gallons (to provide 1.2 ppm Mo).
- Micronutrients. Micronutrients can be limiting, especially if you’re feeding dark-leafed varieties at low fertilizer concentrations. To avoid micronutrient deficiency symptoms supplement with a remedial drench of STEM at 1 to 2 ounces/ 100 gallons on October 1 and November 1.
Monitoring and Follow Up
During crop production, growers should conduct frequent on-site testing of injectors and root zone pH and EC to catch problems before they result in damaging deficiency or toxicity symptoms. Additionally, growing media and tissue testing with an outside lab can be used to monitor the nutritional status of the crop in a more comprehensive manner.Growing media should be tested one week after planting and every four weeks thereafter to create a decent database. Test leaf tissue monthly after planting, more frequently if you observe problematic symptoms. These results should all be recorded and charted to help growers troubleshoot problems and to understand how specific fertilizer practices are working.