How to grow poinsettias in a greenhouse, the right way

Shortly after poinsettia flowers are initiated, the plants go through a brief period of lower fertilizer usage. This is the stage where growers most commonly allow a dangerous rise in the growing medium electrical conductivity.

Coming up with a list of poinsettia production mistakes is really quite easy, narrowing it down to 10, well, that’s a bit tougher. For this article I’ll focus on producing a pinched poinsettia. The following is a list of 10 mistakes (I have personally made them all) that are common and can result in either crop losses or a significant reduction in crop quality. The mistakes are covered in sequence of the crop cycle, starting with mistakes in propagation:

1. Mistake No. 1: Too little mist early
   

   As a rule, I typically advocate as little mist as possible in propagation. However, with poinsettia the first three to four days are critical. The cuttings are extremely sensitive to wilting.

   Keeping propagation mist levels high and light levels low can help to better establish the cuttings. Allowing the cuttings to wilt early can result in leaf drop and Botrytis problems.

   After four days the leaf stomata are closing and the cutting stem ends are healing. Less mist will be needed to keep the cuttings turgid.

2. Mistake No. 2: Phosphoric acid

   When it is necessary to acidify irrigation water, many growers choose phosphoric acid. It is safer to handle than sulfuric or nitric acids, and it has the added benefit of providing a highly soluble form of phosphorus as a nutrient. However, I personally am no fan of phosphoric acid.

   The leaf cuticle is the plants’ primary form of defense. The problem is that phosphoric acid can strip the waxy cuticle off the surface of leaves. This can open the leaf up to attack from pathogens and other stresses, such as phytotoxicity from chemicals and sun burn. This problem is greatly compounded in poinsettia propagation. Every time the mist is applied, the cuticle is washed away.

   I feel the same way about wetting agents. Be very careful. Never apply these materials at higher than recommended rates, and don’t continuously reapply.

   
   
   When the callus is forming, it is critical to “force” the cuttings to root. These roots are crucial as they allow the poinsettia plants to begin to feed.
   

3. Mistake No. 3: Too much mist in week two and three of propagation

   I know I just advocated misting heavy in the beginning of the propagation cycle, but later, when the callus is forming, it is critical to “force” the cuttings to root. By reducing the mist at this time, roots begin to form in and around the callus.

   These roots are crucial as they allow the plants to begin to feed. The formation of a large ball of callus, with no roots, is a classic sign that the cuttings are being over-misted.

4. Mistake No. 4: Fungus gnats

   Fungus gnat problems can begin in propagation and continue into finishing. Fungus gnats are often thought of as a nuisance, not necessarily a pest.

   In the case of poinsettias they are definitely a pest as they aggressively feed on poinsettia tissue. They are particularly fond of callus tissue, and often become a problem even in propagation substrates that have little or no organic matter on which the fungus gnats can feed.

   Ultimately the biggest problem caused by fungus gnats with poinsettias is Pythium. I have found a strong correlation between fungus gnat feeding on callus and Pythium. Growers who have plants that die from Pythium, should dissect them and look for evidence of fungus gnat feeding on callus. It will be an eye-opening experience.

   There are many methods to control fungus gnats, including cultural, biological and chemical. Any is a good investment with poinsettias.

5. Mistake No. 5: Pinch preparation

   Attention to pinching has major ramifications on finished plant quality. Achieving the appropriate bract count can be controlled at this point. It is not as simple as counting leaves. Counting leaves on many varieties can achieve the correct number of bracts on the plants, but alone cannot assure that the bracts are in the “on top” position required to meet retailers’ finished specifications.

   Growth control during pre-pinch is important to help keep the internodes short on the base stem of pinched plants. This is really important as to how the branches develop.

   As natural apical dominance increases with internode length, keeping the internodes short keeps the bract height even. If there is a ½-inch interval between the leaves and a grower makes a pinch at six leaves, the bract formed by the lowest branch will potentially be 3 inches below the height of the top bract.

   Growth regulator sprays can be applied to help balance out the internode spacing, but stretched internodes is still a real issue. By controlling growth during propagation and pre-pinch the internode length can be reduced to create the desired “stacked plant.” This growth control also helps to reduce any apical dominance as an increased distance between leaves increases the apical dominance effect. Managing leaf internode length and the timing of the pinch are the keys to ensuring all of the bracts grow into a salable position.

   Keeping the humidity high and reducing light and heat stress after plants are pinched can help ensure all of the axillary branches develop normally.

6. Mistake No. 6: Post pinch stress

   The time after pinch during which the new developing branches are being formed is important to ensure that the axillary shoots become established. High temperatures, high light levels and low humidity at this stage can result in some of the axillary shoots being blind or at least stunted. Keeping the humidity high and reducing light and heat stress can help all of the branches develop normally.

7. Mistake No. 7: Spacing on time

   As crops are constantly competing for available labor and space, spacing the plants on time is a very common problem. If plants are left pot tight too long, the lower branches or outside bracts can become stretched and weakened. When the plants are finally spaced the bracts spread out. The weakened outer branches often fail when the full weight of the mature bracts comes to bear, often failing during the selection process.

   Leaving plants spaced pot-to-pot can also lead to increased height. Some growers successfully use this technique to get compact varieties to size up.

8. Mistake No. 8: Managing the fertility level

   As poinsettias grow, and go through different developmental stages, their rate of nutrient uptake changes. With careful monitoring of the growing medium electrical conductivity, a grower can identify the times when the plants are using more or less fertilizer, and adjust the feed levels accordingly.

   After poinsettias are planted and pinched, they begin a phase of rapid growth and higher feed utilization. Shortly after flower initiation, the plants go through a brief period of lower fertilizer usage. This is the stage where growers most commonly allow a dangerous increase in the growing medium electrical conductivity.

   It is easy to carry over this higher electrical conductivity level into November and the finishing phase, where the nutrient uptake in the plants drops dramatically. This rise in electrical conductivity, combined with shorter day lengths and lower light levels, can trigger outbreaks of Pythium. Excessive nutrients in the growing medium at the end of the crop cycle have been shown to reduce the shelf life.

9. Mistake No. 9: Pythium

   As indicated in the mistakes made with managing fungus gnats and with excessive fertilizer levels, the big loss in poinsettias comes from Pythium. Failure to treat for Pythium is a huge risk and can result in very large crop losses. Growers should be cautious when applying metalaxyl drenches, as there are many strains of Pythium that are resistant to this active ingredient.

   Picking the right variety for product form and market date is one of the easiest ways to simplify poinsettia growing.

10. Mistake No. 10: Variety selection

    Picking the right variety for product form and market date is one of the easiest ways to simplify poinsettia production. The one-poinsettia-fits-all way of growing may seem simple at first, but results in excessive manipulation and often reduced quality. Unless a grower is going to use black cloth or lighting, matching variety response times to the market date is an easy way to deliver fresh poinsettias to consumers.

    I often see growers producing early season varieties and holding them with cold temperatures until mid-December. These plants decline rapidly in the retail environment and offer short shelf life for consumers.

Other considerations
Other production mistakes include: not managing whiteflies, Botrytis and powdery mildew. As a grower, poinsettias are definitely one of the more challenging crops to produce with a long crop time and many variables. By eliminating some of the key mistakes, you can reduce the variables and make poinsettia production easier to manage. 

Gary Vollmer is technical support product manager, Selecta Poinsettia, gvollmer@ballhort.com; www.selectanorthamerica.com.