Chlorotic Poinsettias in the Fall and Suggested Strategies
This week, Nick Flax is preempting some later-season poinsettia issues related to chlorosis … and provide some strategies to deal with the issue if it arises in production.
PROBLEM: Later this month and into November, we expect to hear from growers struggling to keep mid- and later-season poinsettias green. Not the bracts, of course—leaf color in various parts of the canopy is the issue here.
NICK’S TIP: While the emphasis of poinsettias is their showy bracts and golden cyathia, no one wants to see a poinsettia with chlorotic foliage in the mid and upper canopy. If you tend to struggle with anemic-looking poinsettias, this week’s tip will help you when the crop is ready to ship.
Potential Causes for Late-Season Yellowing
Different factors can cause chlorosis in your poinsettia crop toward the end of the production cycle. Some of the most common reasons include:
Too-low fertilizer rates throughout production. Poinsettias are regarded as heavy feeders, but some growers opt to use a lower fertilizer rate to check excessive growth and reduce need for growth regulators. However, plants’ demand for mineral nutrients increases as they develop—especially after pinching. If you’ve been riding the line between “just enough” and “not enough” fertilizer throughout the crop cycle, deficiency symptoms may start to manifest in the last several weeks before shipping.
Fertilizer was cut off too early. While it’s generally regarded as best practice to irrigate poinsettias with clear water toward the end of production, the last week (or two) before ship is the timeframe to do this. Switching to clear water too early will flush mineral nutrients out of the media while the crop is still growing and cause general chlorosis.
Poor water quality is catching up to the crop. If your irrigation water has moderate to high alkalinity and it’s not being managed appropriately, chlorosis is most likely in the upper canopy; this is the typical iron (Fe)/manganese (Mn) deficiency symptoms. If you normally use an ammonia-based fertilizer during warmer months, the acidity of your feed may be enough to prevent high pH issues. However, when you switch to a more nitrate-based fertilizer as the weather cools down, your growing media pH will increase over time and limit uptake of Fe/Mn by plants later in the growing cycle.
What Is “Too Yellow” to Ship?
Poinsettias are usually your last major crop of the year, so it’s easy to get anxious about crop quality this close to shipping. Unless you recently applied a PGR containing chlormequat chloride (Cycocel, Citadel) at too high a rate, chances are that the yellowing you are observing is due to nutrient deficiency of some kind. However, before you take corrective action, make sure any adjustments to crop culture you make are in proportion to the scale of the problem.
Level I: If only the very bottom of the canopy is showing some chlorosis, it may not be worth doing anything at all. Leaves on the bottom nodes will most likely be obscured by a decorative pot cover, so changes to crop culture may only cause problems at this point.
Level II: If leaves in the mid and lower canopy are chlorotic and visible with a pot cover on, a last-minute green-up treatment is in order.
Level III: If leaves throughout the canopy are chlorotic and lower leaves are (likely) senescing, identifying the exact cause and quick action to correct nutrient deficiency symptoms is critical. Also, be sure to enact preventative disease control measures, as your crop will be more susceptible to attack from pathogens in its weakened state.
Chlorotic Poinsettias—Corrections
Level I Correction: If only the lower-most leaves are yellowing, the main concern is whether they will impact shelf life. Senescing leaves often become infected with Botrytis, and fungus-infected leaves hanging out in the pot will serve as the source for a secondary infection. Since poinsettias are often watered heavily before shipping and pot covers further restrict air flow in the lower canopy, the combination of high moisture and humidity, darkness and a source of fungal spores creates the perfect storm for a bigger disease outbreak. Carefully remove and clean up yellowing lower leaves during the pulling and sleeving process to keep chlorotic, senescing foliage from becoming a liability.
Level II Correction: Increasing fertilizer rates for mid- to lower-canopy yellowing may seem like a logical solution, but increasing feed close to shipping can cause plants to become soft and more prone to lodging and breaking. Whether low feed throughout the growing cycle or cutting off fertilizer too early is the cause, applying an Epsom salt (magnesium sulfate; Mg2SO4) drench can help re-green the mid to lower canopy without making plants too brittle. One or two applications of Mg2SO4 at 1 to 2 lbs./100 gal. will help you achieve green-up.
Level III Correction: When plants are exhibiting chlorosis throughout, more drastic measures need to be taken to make your crop shippable. There’s also a good chance that overall growth and development has been impacted, so this is likely not the prettiest crop you’ve ever grown, too … But if you have a few weeks before they need to be ready, take the following steps if you want to try and save your crop:
• Make sure your fertilizer injector is working properly. If this is the reason your crop is so malnourished, attempts to treat symptoms with fertilizer won’t work until it is repaired or replaced. Also, check your crop’s root zone for signs of disease. Root rot hampers nutrient uptake, so apply a fungicide ASAP if you see disease symptoms.
• If/when roots are healthy, feed ASAP with your fertilizer of choice at 300+ ppm N once you are certain your injector works; repeat two times if possible, allowing for some dry-down between applications.
• Increase your air temperature slightly to speed up uptake and distribution of nutrients throughout the plants. After a couple of fertilizer “hot-shots,” you can follow-up with one or two Mg2SO4 drenches at 2 lbs./100 gal. to get the last bit of green-up, if necessary. (Note: Warmer temps and increased feed WILL make your crop softer. However, you must weigh the risks of shipping a softer-than-ideal crop over taking a 100% loss on your poinsettias.)
• Once you crop has regained sufficient color, reduce air temperatures to help tone the crop as much as you can before ship. Also, if possible, try to provide at least two clear-water irrigations to flush excess salts out of the media. A light drench with paclobutrazol (Bonzi, Piccolo, Paczol) at about 0.25 ppm can also help tone plants more quickly and suppress excessive stretching due to the sudden feed increase.
Level II and III (especially) corrections are last-ditch efforts to make your crop salable, rather than having to eat a major loss on your poinsettias. By NO means should these be considered best practices.