There are numerous times during a poinsettia crop that things can go wrong, but perhaps the most overlooked time is the period from when a cutting has rooted until the plant is established in the final container. During this time, roots can be lost and then the plant never grows at the normal rate in September and October.
This article focuses on the critical establishment of a healthy root system during August, so your poinsettias will be uniform and vigorous during the vegetative phase of growth prior to flower initiation in mid to late September. Our primary goal is to help you establish healthy roots, because root loss shortly after transplant is common with poinsettias. Diagnosing the cause of root loss can be difficult. There are three main factors that contribute to healthy roots: 1) fungus gnat management; 2) pythium prevention; and 3) proper watering.
Fungus Gnat Management
Fungus gnat control in propagation begins on Day 0 (preparation phase), because Day 1 (sticking) may be too late. The propagation area must be clean and the fungus gnats need to be under control before the arrival of the first poinsettia cuttings. Carryover of pests from a preceding crop can be difficult to overcome once propagation starts and the environment is perfect for fungus gnat survival.
Once the cuttings are stuck, the pest control recipe needs to be in place and operating like clockwork. Pesticides that appear to be working well include Citation sprays and Safari drenches. Biological controls such as the beneficial nematode products (Nemasys) can also work. Multiple applications of Citation or Nemasys may be needed for excellent control, while Safari drenches can be used only once per crop. Distance is effective for fungus gnats, but phytotoxicity is commonly observed on poinsettias so this application is not recommended.
Treating the floors under the benches can also be beneficial. Growers have reported success with the use of Talstar granules or lime spread on the floors in propagation greenhouses.
Pythium control should be approached preventatively following transplant. The most common chemical options are Truban or Terrazole rotated with Subdue MAXX. Fenstop is a newer product that we do not have experience with yet, but it would be useful to have an additional option in the fungicide rotation because resistance to Subdue is a concern. Some growers have observed favorable results with biological controls such as RootShield.
Perhaps the bigger issue is water management. Our observations suggest severe drought stress is the major cause of root damage, which then allows pythium to attack the plants. So, this leads us to our next focus area.
It is common to observe poinsettia rooted cuttings that fail to root into the surrounding growing media in the first 10 days after transplant. When the rooted cutting is popped out of the surrounding growing media, the roots are severely damaged and have frequently failed to grow out of the propagation media (Figure 1, page 38). Pythium or fungus gnats are frequently thought to be the cause, but our work suggests water management is the real key.
We have compared the root growth of cuttings propagated in Oasis foam and peat-based propagation media following transplant. We have observed healthy cuttings that are properly watered will have roots that elongate 1 centimeter within 48 hours after transplant, regardless of the propagation media (Figure 2).
In experiments, we were able to simulate the root loss commonly observed by growers at the time of transplant by failing to properly water the propagation media prior to and following transplant. The water management challenge begins when cuttings need to be held before transplant. During this holding period, the rooted cuttings can use a lot of water and irrigating the propagation media is not trivial because the leaves can deflect the water away from the media. It is critical that the person watering examines the actual media to make sure it is receiving the proper amount of water during each irrigation event. Daily irrigation may be needed on well-rooted poinsettia cuttings in the heat of July.
Similarly, roots can be lost when rooted cuttings sit for an extended period of time on racks waiting to be transported or sent to the transplanting lines. It is critical the propagation media be properly watered prior to being placed on racks. After the cuttings are transplanted to their final container and set back into the greenhouses, they should be irrigated once again. If done properly, this should provide enough water for the roots to become established in the final container.
Questions often arise about whether or not the surface of the propagation media, such as Oasis foam, needs to be covered in order to prevent wicking of moisture out of the foam. While wicking of exposed media will occur, our experimentation suggests that if the above watering guidelines are followed, the roots will emerge out of the propagation media and into the surrounding media before the Oasis foam has a chance to dry out (Figure 3).
Root loss the week before and after transplant has a detrimental effect on many poinsettia crops every year. While it is easy to neglect this critical phase, proper attention must be drawn to irrigation management near the time of transplant, as well as fungus gnat control.