Nov. 6, 2012 — Most often, the poinsettia production questions in November are about correcting something that is wrong with the crop.
These questions are also very difficult to answer with a quick fix because of the late stage in the crops. The flexibility in the poinsettia is very small in November.
So … what are some of those questions?
The plants are too tall: There is nothing you can do to reduce the plant height but micro-drenches of plant growth regulators (PGRs) can be used to prevent additional stretch.
I would suggest you consult a trusted source for specific recommendations if you have not used the technique. Also, remember the dose of chemical applied to the pots is extremely important, therefore both the rate of chemical solution as well as the amount of solution applied per pot determines dose.
DO NOT GO OVERBOARD WITH PGRs AT THIS STAGE
Over-application of a PGR at this stage will result in reduce bract size and delayed flowering.
The plants are too short: It is difficult to increase height at this stage without reducing plant quality. A positive DIF can be used to increase height but I think this should only be used on days having sunlight, which is not often the case in much of North America in November.
High day temperatures in November can result in “soft” growth, faded bract colour, and loss of cyathia, particularly under poor light conditions.
The plants are too early: Cool temperatures can be used to delay or hold many poinsettia cultivars, however not all. Make sure the cultivars you finish cool are proven for cool finish.
Some cultivars will be significantly delayed with very small bracts when night temperatures are below 67 F (19.5 C). Also, white bract poinsettia cultivars will become more “creamy” in colour at finishing temperatures below 67 F (19.5 C).
The plants are too late: Late flowering of the poinsettia crop is almost impossible to correct in November, particularly if the crop has been growing at 67 F (19.5 C) night temperatures.
BE MINDFUL OF TEMPERATURES, BOTH DAY AND NIGHT
Poinsettia bract development will be slower at night temperatures below 67 F (19.5 C), but will not be faster at night temperatures above 67 F (19.5 C).
In fact, bract development can be delayed on some cultivars when the night temperature is above 67 F (19.5 C). So don’t be quick to raise the greenhouse temperature too high, thinking you will speed flowering.
Bract development is much more time related instead of temperature related.
Watering: In November, the days are shorter, there is less sunlight, and the plants have really slowed in growth … therefore the water requirement is greatly reduced for the poinsettia plant.
Overwatering in November is likely to result in plant loss from pythium root rot. At this stage the crop is much better on the dry side than the wet side.
WATER EARLY, SO PLANTS ARE DRYING LATER IN THE DAY
Also with cool, dark weather, remember it is better to water in the early morning than late in the day so plants are drying going into the night period.
Along with watering, be sure you are controlling relative humidity by venting and heating on even those cold winter days. Allowing humidity to increase to same energy will result in reduced plant quality and increased disease pressure, particularly botrytis.
Fertilizing: Fertilizer application rate should be reduced to the 100 ppm rate, but fertilizer application should not be eliminated in finishing the poinsettia crop. Nitrogen deficiency at this stage will result in reduced plant quality from lower leaf yellowing, which will get worse with post harvest conditions.
A low rate of fertilizer application is also important for bract development and bract colour.
Temperature: Although temperature has been discussed earlier, I think it is important to highlight those points again in a separate section.
NOT ALL CULTIVARS ARE SUITABLE FOR A COOL FINISH
Cool finish can become a thought in November, but it should only be used if it was planned at the start of production in July. And it should be considered only if the specific cultivar suitable for cool finish has adequate height and timing in November.
One cannot simply drop the temperature to save energy cost in November. It is also important that the cool finish plants have excellent roots and the grower is very careful not to over-water at cool temperatures.
And that all-important question, how cool? I always suggest 60 F (15.5 C) night / cloudy days as a good cool finish temperature. On sunny days, the grower can let the greenhouse rise to 70 F (21 C), but there is no need to add heat about 60 F (15.5 C) on cloudy days.
Growers in high light areas often use 55 F (13 C) as their cool temperature, but that should only be used with great care and experience.
Insects: And lastly, there are always a few greenhouses that experience those whitefly problems in November.
My suggestion is to always scout the crop for whitefly in early November, so hot spots can be treated as early in bract development as possible.
WHITEFLY AT SHIPPING STAGE QUITE DIFFICULT TO CONTROL
Finding whitefly at shipping is almost impossible to adequately control safely for bracts and in a timely manner. There are some newer insecticides that can be used, but seek expert advice and use only as an emergency treatment.
Dr. P. Allen Hammer is involved with product development and support with Dümmen U.S.A. He is professor emeritus of floriculture, Department of Horticulture, at Purdue University. •
Poinsettias in the final stretch this month
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