February 2025 Controlled Environment Agriculture, Production

Regulating crops in a controlled space By Heather Machovina

Roberto Lopez from Michigan State University recently joined me for a deep dive into harnessing the full potential of PGRs in CEA production. In GPN’s 2024 webinar, Lopez provided research-backed, practical techniques for enhancing rooting, managing plant growth with microdrenches and ensuring uniform ethephon spray effects in the greenhouse. 

Rooting hormones are commonly used in the industry for plants that are difficult to root, slow to root or those that have issues during transportation, Lopez said. Products labeled with hormones including IAA, IBA, AIBA and NAA help with cell elongation and adventitious root formation, increasing overall rooting percentage and uniformity, reducing crop finishing time and improving root health, he said. 

Let’s get into more about what is known about PGRs and the new MSU research for better growth control outcomes with this summary from Lopez’ educational webinar with GPN.

METHOD MATTERS 

The most familiar method of using rooting hormones is basal dip applications in liquid or powder form. This becomes labor intensive because every cutting has to be treated before sticking. There is a greater risk of spreading disease because many cuttings are touching the same rooting hormone powder or liquid. 

Foliar 

Spray drenches (or sprenches) and foliar sprays with PGRs is a newer method growers are increasingly using in the greenhouse. This method is quick and is less likely to spread diseases. A coarse spray is applied to cuttings after they are stuck and spend 24 hours under mist. Plants are turgid now, so the spray can effectively run from the leaves down the stem and rest in the substrate at the base of the cuttings. 

Not all rooting hormones are labeled for this type of sprench application. If you use the incorrect form, it may cause phytotoxicity, so a potassium salt formulation of IBA or water-soluble formulations, such as KIBA, are less likely to cause foliar damage. 

Previous research involved the foliar spray applications of common PGR products at different rates and various daily light integrals (DLI). Optimal rates for the most roots were noticed depending on PGR rate applied and crop type, and increasing DLI (i.e., from 12 moles to 16 moles) at the same PGR rates caused rooting to increase more. 

Overall, foliar sprays are as effective or more effective than basal dips at promoting rooting in difficult-to-root crops, while submerging cuttings in a rooting hormone solution is effective at promoting roots over an untreated control. 

Microdrench 

You may be quite familiar with PGR drenches — an application of a relatively large volume of a PGR solution at a low concentration to the growing substrate. It’s a much lower rate than what would be applied as a spray. 

With a PGR microdrench application, rates are further reduced compared to a traditional drench by about one-tenth to an eighth. That means more applications throughout production, usually about three to four. An advantage of microdrenching is less risk of stunting crops and delaying flowering, along with reduced residual effect for consumers. 

If you are interested in trialing microdrenches, here are a few guidelines to help along the way: 

• Start the first application anywhere from 7 to 10 days after transplant. This is generally the most costeffective time because you really want those roots to have grown since the PGR is being applied to the growing substrate. 
• If you’re using subirrigation, reduce the amount of PGR by about 25% to 50% compared to the PGR drench rate. 
• If you use bark in your substrate, slightly increase that rate because bark can bind or hold up the PGR. 
• Always conduct your own trials on a small scale to determine appropriate PGR rates for the crops that you’re growing as well as the specific conditions of your greenhouse. 

ETHEPHON EFFECTS 

Ethephon is a great product to produce more compact plants as it increases branching. Ethephon is applied as a liquid, but in the greenhouse it turns into the active form of ethylene gas that plants can absorb, as it’s a natural plant form. Ethylene is typically known as a senescence hormone, but really it suppresses stem elongation by reducing apical dominance so the plant produces more branches instead of growing upward. 

In some instances, ethephon will set the flowering clock back to zero. For example, a crop of New Guinea impatiens showed a few plants with flowers, but the flower set was too early in the production cycle. Applying ethephon now induces abscission or abortion of those flowers and buds, and sets the clock to zero so all the impatiens come into flower at the same time. 

Best practices 

A big win for ethephon is the inexpensive cost compared to other PGRs, although the biggest complaints from greenhouse growers have been the inconsistent results seen from one application to the other. 

A study took shape to determine the causes of inconsistency. Was it the water quality, varying environmental conditions in the greenhouses or application challenges? Two ethephon products from Fine Americas were used in this study: Florel and Collate. 

It’s important to note that while these two products have the same active ingredient, the concentration of ethephon varies greatly and each will require different amounts when preparing your solution. In addition, the amount of Florel or Collate that is added to the solution can change the pH, reducing its effectiveness. 

pH 

Previous research revealed the evolution of ethephon to ethylene increases as the spray solution pH increases above 4.5. Therefore, if the water you’re mixing with has a high pH, the ethylene is released right from that liquid to a gas, and the plant can’t take it in quickly enough. If the spray solution pH is too low, the plant will take a lot of that ethylene, and now there is a potential for product phytotoxicity in some species. 

Collate and Florel are acidic, so check product labels for the final recommended solution pH. This is the target number you want your solution to read before applying to plants to get the best effectiveness from your application. 

Temperature 

Ethephon product labels often recommend an application temperature range of 60° F to 95° F, but previous research has shown this is not optimal. Ethephon breaks down very slowly at low temperatures (plants take up ethylene slowly) and quickly at high temperatures (plants can’t take up all the ethylene in the time it’s available). 

THE NEW STUDY 

Developed to determine how the carrier water alkalinity and the ambient air temperature at application affected the efficacy of ethephon sprays, this study used four different ethephon rates from zero to 750 ppm. Also, three carrier water alkalinity options were studied, with five air temperatures ranging from 57° F to 79° F, in three species: petunia, geranium and verbena.

Alkalinity 

Tap water alkalinity was adjusted using sulfuric acid. Plants were grown under the same temperatures, then moved into greenhouse compartments with varying temperatures and kept there for 24 hours after spray application because it can take up to 16 hours for the plant to fully absorb the ethephon. 

High alkalinity definitely reduced efficacy, with plants growing larger than the control and flowers buds visible. 

Temperature 

Temperatures were set two hours before spray application and maintained for 24 hours after. At 79° F, ethephon was not effective at all. Results prove that temperature has a dramatic effect on ethephon, especially when it’s applied as a spray. 

Here are some pro tips for ethephon to improve efficacy: 

• If your carrier water has a high alkalinity, especially if you’re located in the Midwest, you need to add buffering solution to reduce the pH, such as sulfuric acid, or an adjuvant such as Indicate Five. 
• Only apply ethephon when your greenhouse temperatures are below 75° F. Increasing the relative humidity can keep the ethephon from drying up too quickly. Maintaining a vapor pressure deficit around 7.7 kilopascals is useful, but again, always conduct your own in-house trials. 
• Allow the solution to dry slowly over a minimum of four hours to enhance the uptake; it can take 12 to 16 hours before the plant fully absorbs the ethylene. 
• If you plan to use ethephon to remove flowers and/ or buds for timed flowering at retail, be sure to refine your timing from application to reflower so it lines up with shipping/market dates. In-house trials are a huge help here to nail down your specific production timing needs. 
• Ethylene is a senescence hormone, so never apply it to plants that are already stressed. For example, a drought-stressed plant that gets an ethephon spray may go into further senescence, losing more leaves. 
• Remember, this PGR has one of the longest REIs (restricted-entry intervals). Protective equipment such as eye protection, coveralls and apron shoes should be worn because it’s a minor eye and skin irritant.