Optimizing Branching on Poinsettias
Although modern poinsettia varieties branch freely, some growers experience branching difficulties with a small percentage of their crop. Occasionally, in a pinched crop, plants can be observed with only two or three main stems. There may also be smaller, under-developed shoots that don’t contribute to the final flower display. There are many different contributing factors to poor branching.
Figure 1 Example of poor branching
Contributing factors include:
- Excessively hardened cuttings. Axillary buds and stem tissue that is stressed during the end of propagation will not branch as well as buds that develop after propagation.
- Low light. If light levels are reduced following the pinch, the plant may not be able to support all the shoots that form. In the vegetative growth phase light levels should not be below 4,500 f.c.
- Tight spacing. If plants are tightly spaced for too long, the lower shoots don’t receive enough light to develop properly while the top two or three shoots become dominant.
- No growth regulators. Growth regulating can be used to even shoot growth. Spray applications can be targeted to the upper shoots which allow the lower shoots to catch up.
- Tall primary stem after pinch, and high pinches. If cuttings are allowed to stretch prior to pinch and the pinch height is high there will be several inches between the newly forming shoots and the lower lateral existing shoots.
- Hard pinch on stretched cuttings. If cuttings are allowed to stretch prior to pinch and a hard pinch is performed the tissue on the lower stem is more mature and woody. This woody tissue does not break as well resulting in fewer shoots.
- Environmental stress during propagation, establishment, and vegetative growth phases. High light levels, excessive temperatures, and reduced relative humidity can have an impact on the number of shoots formed after pinch.
- Warm finishing temperatures. Bracts can be very large when plants are grown above 68F (20C). These larger bracts crowd out smaller shoots and become the only flowering shoots easily seen. Cooler finishing temperatures create smaller bracts and a canopy where all shoots are visible.
- Loss of phytoplasm (MLO). Phytoplasm are organisms present in the phloem of the poinsettia plant. Phytoplasm reacts with a poinsettias genome and this reaction initiates a hormone responsible for “free branching”. Phytoplasm also may change the physical appearance of a poinsettia, and the response varies by variety. Heat stress conditions (high light, high heat, low moisture availability) during propagation and the establishing phase can actually eliminate phytoplasm resulting in poor branching, uneven branch development, and a physical appearance not characteristic of a particular cultivar.
Figure 2 An example of normal branching (left) and a plant (right) with poor branching, uneven branch development.
Uneven “jumping” may occur.
Even branching can be easily achieved with proper cultural practices and a controlled greenhouse environment. Proper pinching techniques and timing, growth regulating, proper timing of plant spacing, and cool finishing temperatures are all factors that will contribute to even branching and flower display.
Figure 3 Uneven bract development – a result of poor branching
Figure 4, side view of uneven development
A controlled greenhouse environment can be difficult in many areas of the country during the summer and early autumn months. As excessive temperatures and light levels contribute to poor branching it is important to manage the greenhouse environment. Here are some tips for reducing stress from heat on a poinsettia crop:
- Shade crop immediately after transplant to about 3,000 f.c. Once cuttings have established and the crop is pinched, light levels should be closer to 4,500-5,000 f.c.
- Control greenhouse temperatures as much as possible. Young plants will tolerate very warm temperatures as long as the relative humidity is maintained at a high enough level to reduce the plants water loss. It is also important to avoid excessive light levels (above 3500 f.c.) when temperatures are high.
- Mist young plants with clear water in the warmest part of the day. This is especially important before root systems are established.
- Syringe plants with clear, cool water to cool root systems or consider adjusting watering schedule to irrigate plants in the afternoon rather than first thing in the morning. An afternoon irrigation will cool the plants without the risk of over saturating the media which could be the case if the plants are watered in the morning then syringed again if the afternoon.
- Wet walk ways to allow evaporation to cool the greenhouse and raise the relative humidity.
© Paul Ecke Ranch 2008