We are all faced with a difficult year ahead of us due to current energy costs and availability. Growers are looking at opportunities to turn down the thermostat, save money and conserve their oil supply; however, doing so at this stage of a poinsettia crop carries risks that should be explored before taking such actions. Below are some factors to consider regarding the impact of using less-than-optimum temperature regimes on poinsettia crops from mid-October through finish.
Temperature plays an important role in the time required for a poinsettia to mature and bracts to expand (see Figure 1). Lower greenhouse temperatures slow development, resulting in additional bench time, and increase the risk of damage from disease and other problems (see Figure 2). While early flowering varieties are the most tolerant to finishing at cooler-than-optimum temperatures, they will still suffer delayed flowering, which means they will not be ready until late November.
Not all early flowering poinsettias are easily adaptable to cool-temperature production. Varieties that are already compact in size or have smallish bracts would not be good candidates for cool-temperature production. Factors to consider:
Separate early flowering varieties from mid- to late-season varieties to avoid impacting the timing of these crops. Mid- to late-season varieties do not require high temperatures; they just need to be kept within normal temperatures later into development for normal bract expansion and color.
Use plastic curtains to isolate crops based on temperature requirements. This will reduce dilution of heat between adjacent plants with different response timing.
Bract size is impacted by cool temperatures. Fortunately, most cultivars grown today have naturally large bracts. Avoid cool temperatures on varieties with small bracts.
Root Zone Management
Cold greenhouse temperatures affect more than just upper plant growth and development; they directly affect root systems. Growers with bottom heat systems that maintain adequate soil/root temperatures are more able to protect the crop than growers without. Keeping roots in the optimal temperature range of 65-70° F will help facilitate normal water/fertilizer uptake and minimize risk of diseases favored by cool, wet media (Pythium, Thielaviopsis, etc.). Factors to consider:
Avoid growing pots directly on the ground where cold temperatures are transferred to the root zone. Growers with crops already sitting on the ground should evaluate options to mitigate this problem.
Adjust irrigation to begin at 10 a.m., and finish by noon because irrigation water applied too early in the day will further cool media. This is extremely important for growers using cold-water sources.
Cold media slows the uptake of water and nutrients. Check the crop closely to determine frequency and volume of irrigation solution applied.
Less frequent irrigation reduces the opportunity to supply nutrients from fertilizers. Monitor EC and pH closely, and adjust concentration of fertilizer to maintain crops within acceptable levels for plant growth.
Preventive fungicides are recommended, as the risk of attack by root rot pathogens will increase with cool media temperatures.
Plant Development Management
Lower temperatures slow plant development, with a distinct impact on timing and bract expansion. If you are not tracking plant size and rate of bract color development it is highly likely specs and delivery dates will be missed. Can you afford to lose these sales or to discount the crop because of these factors? If the crop is not on track and extra heat is required to recover size and timing, any savings gained earlier by using less heat will be lost, as will crop quality. Factors to consider:
If PGR applications made earlier in the season were not modified for cooler temperatures the effect may last longer and have greater impact on size and timing than normal. Do not use the same frequency or rates as previous years due to slower plant development at cold temperatures. If using the new, early, low-rate Bonzi drench program or a late drench program, both programs are affected by temperature so adjust accordingly.
Do not use cool morning temperature drop (DIF) if already growing at cool night temperatures. Further reduction of temperatures will result in delay of flowering.
Do not use elevated day temperatures to compensate for cold nights. Large differences between day and night temperatures may encourage stretch, forcing use of PGRs that further impact the crop. Night temperatures below 60° F impact chemical processes required by the plant during the night (respiration, red and far red phytochrome conversion, etc.). Colder temperatures can change or stop poinsettias flower development. Reduced temperatures are acceptable once plants have matured but not until that time.
Cool finishing temperatures can prevent bracts on some varieties from lifting above foliage for optimum presentation and expansion.
Manage relative humidity levels through ventilation, heating and HAF fans to prevent condensation in the greenhouse that results in water on bracts and leaves and encourages disease.
Preventive fungicide treatments targeted towards Botrytis and powdery mildew are recommended, as cool greenhouse conditions provide a favorable environment for those organisms.