Fertilization

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Fertilization

Poinsettia nutrition is a factor of production that requires planning, monitoring and reaction based upon plant response.

pH
The pH of the media is important because is controls the availability of nutrients for uptake by the plant. When growing in a soilless media, the optimum pH is between 5.8 – 6.2. At this range, all the essential elements are available to the plant and growth should be normal or free of deficiencies provided the crop is given a balanced and complete feed program. If growing in a media that contains soil, the optimum pH range is between 6.0 – 6.4. Monitor these ranges throughout the crop to assure plants are capable of utilizing the nutrients provided.

E.C.
The electrical conductivity (E.C.) of the media is the measurement of dissolved minerals in the fertilizer solution and media. These minerals influence the ability of the root systems to function. Excessive salts, or E.C. levels above 2.5 mmhos/cm (or mS/cm) based on saturated media extract can restrict uptake and potentially burn tender roots, increasing the opportunity for root rot diseases to get established. In contrast, low salts, or E.C. levels below 1 mmhos would indicate low fertility and most likely will result in deficiency symptoms on the crop.

There is no “one fertilizer mix” solution for use with poinsettias. Even though many commercially produced mixes are named the “poinsettia special”, best results are achieved through the use of several fertilizer mixes alternated during the crop. Some growers continue to mix their own fertilizers, but even these mixes must be adjusted in proportion and content at different stages of growth. Whichever method you decide to use, it is critical that the quality of water being used for production is evaluated and considered when formulating, or choosing the fertilizer mix to be used. For example, if the water source has traces of boron (B), the fertilizers used may need to exclude this mineral to avoid toxicity to the crop. If the pH of the water is high (excess of 7.0), evaluate the ppm of alkalinity present to determine if acid treatment will be needed to lower the pH into a more acceptable range for production. If the pH is high but alkalinity levels are relatively low (less than 125 PPM), acid treatment is not generally required as long as the fertilizers used create a mild acidic reaction in the media and help lower the pH to the desired range. For more information on acceptable water qualities for growing poinsettias contact a commercial testing laboratory for assistance in evaluating your supply.

When suggesting appropriate fertilizer rates for poinsettias it is important to monitor and adjust this through measurement of the media EC. Factors such as the method of irrigation, regularity of leaching, or other cultural practices influence how effectively the fertilizers are delivered to the root media and made available to the crop. Growers who irrigate lightly, minimizing the movement of water/feed solution from the soil will elevate the EC of the media and should use lower concentrations than the grower who provides drip-through of the solution at every irrigation. Sub-irrigation will reduce the need for higher concentrations of fertilizers as the constant uptake of the solution without regular leaching of the soil will also elevate media EC’s. To determine the optimum for each poinsettia cultivar, see the table provided in this document for specific recommendations.

The cultivar mix is another factor in determining fertilization programs. As a generalization, the dark leaf cultivars (Prestige, Jubilee etc.) require about 20-25% less feed than the medium green leaf cultivars. When fed at higher levels (generally 250 PPM +), root systems of the dark leaf cultivars are less efficient at the uptake of water and nutrients, resulting in slower, shorter growth. Likewise, medium leaf cultivars grown at lower levels appropriate for the dark leaf types are more likely to exhibit deficiency symptoms if not supplemented through periodic “booster” feedings or the use of mild rates of slow release fertilizers like Osmocote. Because greenhouses typically contain a mixture of the different cultivars, it is advisable to fertilize at the lowest possible concentration required and provide supplementation to those cultivars that require higher levels for normal growth. It is easier to add fertilizer than it is to correct the damage of high salts!

Assuming that water quality and irrigation methods allow for “normal” production, the following provides a summary of the progression of fertilization programs for poinsettias from the starting of the crop through finishing

Establishment through pinch
When establishing young rooted cuttings there is no need to provide high levels of fertility to the crop. Until root systems are more extensive in size and plants have become very active in their growth rate, levels of fertilizer between 150 – 200 PPM nitrogen are acceptable. During the first two to three weeks of establishing, cuttings are kept moist in the soil zone where the young cutting has been placed, allowing for roots to move into the new soil. Avoid oversaturation of this soil as the intent is to provide adequate aeration for rapid root growth. Lower feed levels assure that roots will not be inhibited from high salts, and can quickly penetrate throughout the container. As the plant approaches pinching, irrigation frequency may actually be reduced while total volume of water and feed applied should increase, starting a regular wetting-to-drying cycle. Fertilizers at this stage of growth should include a good balance of nitrogen sources, including ammoniacal forms. Provide calcium to the crop from the very beginning as well.

Pinch through mid October
Increased plant vigor and growth rates will require higher concentrations of fertilizer to be applied to avoid deficiencies. At this stage, fertilizer rates between 200 – 250 PPM are generally acceptable, keeping in mind that medium leaf cultivars most likely will require periodic boosts in feed (300-350 PPM) to maintain normal growth. During this stage of growth, nitrogen containing ammonium nitrogen is still acceptable, but the percentage of this form should be maintained below 40% of the total N applied. Regular feeding with calcium-based material should continue as a constant feed or on a weekly or every third feeding basis. If spray applications of calcium are used, these should also be done on a weekly basis. Spray materials should be selected for purity to avoid burn and damage to the foliage, laboratory grade calcium chloride has been used successfully. Also during this stage of growth it may be necessary to apply a minor element mix or supplement. By using the lower feed programs appropriate for the dark leaf cultivars, minor elements may not be supplied at adequate levels through the fertilizer mix. Use of soluble trace element mix (S.T.E.M. ®) or comparable mixes should be considered using one-half to one-third the corrective rate listed on the label. This same micronutrient solution may then be repeated next month.

Mid October through early November
During this stage of poinsettia growth, the fertilizer program needs to shift focus away from any ammoniacal nitrogen mixtures to an emphasis on nitrate nitrogen forms (potassium and calcium). This change in fertilizer encourages less vigorous growth and promotes “hardening” of the crop in preparation for shipping. Days are beginning to be noticeably shorter and cooler, reducing the frequency of irrigation. Provide an adequate drying cycle to the media to maintain strong root systems and plants. Poinsettias are now forming bract color and expansion, which require continued fertilization. However, as the growth rate slows and plants are primarily forming bracts and cyathia, the overall level of fertilizer needed will lessen. Growers should reduce the concentration of fertilizers and the frequency of application after November 5th. It is still important to monitor media pH and EC to assure the crop is not put at risk. Since minor element deficiencies impact the expansion of young leaves and bracts, their continued uptake is critical during early November. Repeat the remedial S.T.E.M. application from last month. Spray applications of calcium chloride should continue weekly during this period of growth to supply adequate calcium to expanding tissue. This is critical for medium leaf cultivars that are more susceptible to bract edge burn.

Mid November through shipping
As a rule, most early to mid-season poinsettias should have reached an acceptable level of bract development to allow fertilizers to be discontinued in mid November. By switching to clear water at this stage, the reserve of fertilizer in the soil is generally sufficient to allow completion of the crop. Depletion of soluble salts in the media now makes the risk of root damage from salts less likely. This is important as plants probably will not be watered properly through the shipping period from the greenhouse through the retail. Calcium sprays may continue up to the point of shipping to minimize the risk of bract damage. The rate of spreader-stickers used with these applications should be reduced at this point to help prevent damage. The calcium residue although not significant in its visible presence on the bracts does have some fungicidal activity on the plant tissue and may assist in preventing Botrytis.

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