Postharvest is an Art and a Science

Choosing the best treatments for top quality begins with knowing your water. Water affects so many pre-harvest and postharvest procedures that it’s important to know what elements you’re dealing with. Of course, maximizing the vase life of cut flowers is dependent on more than just water. Long before harvest, cultivar selection must be considered for postharvest longevity. Weather conditions are also at play. Impending storm conditions sometimes force harvesting at tight cut stage just to get flowers out of the field. A heavy rainfall can splash soil onto plant stems ramping up the risk of microbial contamination and resulting in blocked stems after harvest. Even the simple procedure of watering prior to cutting is important to ensure cell walls are turgid to mitigate water-stress associated with harvest.

No chemistry PhD required, but knowing the levels and measurements of various elements in your water reduces trial and error of postharvest success. A basic water test provides valuable information on pH, EC and alkalinity. Additional elements affecting cut flower success include levels of Ca (calcium), Mg (magnesium), Cl (chlorine), fluoride, Fe (iron) and B (boron). Once you receive the analysis, interpreting results can be confusing (especially if you weren’t listening in chemistry class!) so I’ve included a chart developed by the Chrysal laboratory in Holland showing the impact of various elements and pH on cut flower quality.

Cleaning up your initial water may require installing filters, or the use of a flocculant to precipitate impurities. Iron can be reduced through aeration. A quick online search mentioned something as simple as using a nozzle to aerate water. The method is explained like this: “Aeration introduces oxygen to the water source to convert soluble iron to its insoluble form so it precipitates out of the water…”. Seems worth investigating, if high iron levels are an issue. Reverse osmosis is another very effective way to clean up water and works great, but is expensive, requiring a series of special membranes to filter unwanted minerals.

Another consideration when deciding on postharvest solutions is logistics. Do you plan to harvest into one solution and deliver in another? How many days will blooms rotate in the cooler? Do you sell to grocery stores that offer consumers a vase-life guarantee? Do you provide customers with a food packet? Does your product mix include ethylene-sensitive cultivars? If so, please note that STS, the systemic anti-ethylene treatment, is registered for use in Washington, California, Florida, Minnesota, and Texas.

Let’s Drill Down
    
Two questions frequently asked at the Chrysal trade show table during the Grand Rapids ASCFG conference were “Is the gerbera pill a hydration solution” and “What’s the difference between the various hydrators?”

A bit of solution background info is helpful in discovering the answers. Arguably, the two most important aspects of hydration center around lowering the pH to dissolve air bubbles, and controlling bacteria. Both bacteria and air bubbles clog the delicate vascular system of a flower, inhibiting stem flow. Research proves that it is impossible to kill bacteria once inside stems. In fact, once a flower drinks polluted water, whatever is inside the stem remains unchanged—bacteria stay there regardless of what solution is introduced later (one hour later or 24 hours later). And bacteria travel up stems four to six inches (even higher in gerberas), which is why a fresh cut to remove the bacterial plug is important every time flowers go into fresh solution.

Because minimizing the explosion of bacteria colonies is significant to postharvest success, sanitation measures are super important. Everyone knows the story of clean buckets, clean work tables, etc., but what about tools? Dipping cutters into a disinfectant as you harvest helps prevent cross-contamination.

The story behind Floralife’s Quick Dip centers directly on sanitation too. QD was developed in the 1970s, when postharvest interest (and research dollars) were focused primarily on roses and carnations. It was developed as a stem dip for field flowers. The purpose was to rinse away surface grime, as well as provide a liquid band aid (citric acid) to the fresh wound prior to plunging bunches into plain water. Keep in mind, bacteria thrive on the juices bled out by cut stems, so it takes only hours for untreated water to become contaminated.

As transit times lengthened between grower and wholesaler, interest in refining postharvest care ramped up. New products entered the market. Aluminum sulfate became the darling of hydration because it’s so stable, more so than either citric acid or chlorine. Some quaternary ammonium compounds were found to be effective as first drink flower solutions and a few other chemicals that EPA no longer allows. Formulas vary based on the “active” ingredient. Chlorine, citric acid, aluminum sulfate and quat compounds are the primary “actives” used in hydration products.

Back to the question of “What’s the difference between product X and Y?” Hydration solutions (first drink after harvest) basically perform two functions: keep the bacteria count low, and reduce the pH of water to dissolve air bubbles. Of course, there are a few exceptions. Some hydration solutions don’t acidify the water, some focus on rebalancing cell chemistry. Sometimes a hydration solution doesn’t give as good of results as using a holding solution as the first drink. Holding solutions fall into the flower food category and contain a source of sugar, generally glucose.

So which solution is best?* It depends on the starting water and flower type. Some growers prefer the KISS, one-size-fits-all mentality, but why not treat the weakest issue determining longevity success? A solution like Professional 1 is based on aluminum sulfate and gives great results with hydrangeas. Why? Hydrangeas LOVE aluminum. Zinnias and gerberas are sensitive to stem blockage (usually a bacteria issue) so Professional Gerbera pills (the active is slow-release chlorine) work great. Cotinus is tough to hydrate. It responds well to Chrysal OVB (“actives” are quat compounds). Tuberoses love sugar so placing these beauties into a holding solution (containing glucose) gives better results than a hydration solution (which contains no sugar). Different crops respond differently to distinctive “actives” which makes  comparison testing so important. Find which solutions are best suited to your water, crop list and logistics then plan accordingly.

Table 1 shows a summary of the water characteristics and external factors and the importance in relation with vase life of flowers.

Acidity (pH)

Usually tap water is a neutral solution with a pH of +/7 (between 6 and 9). Generally, water with a low pH (4 – 5) is much better for cut flowers. Many postharvest treatment products (e.g. Chrysal Professional 1, Professional 2, Bulb T-bag), lower the pH.

Water uptake at lower pH is improved and the bacterial growth is limited. The biocides used in Chrysal products more effective at lower pH’s.

If the pH of the water is too low (lower than 4) there are chances of stem discoloration with flowers with soft stems (e.g. Gerbera and Chrysanthemum).

*Disclosure: As the technical manager for Chrysal, I am more familiar with Chrysal products than other brands, which is why only Chrysal product specifics are provided. I recommend comparing brands to find your best fit.

Gay Smith

Technical Consulting Manager

Gay Smith is the Technical Consulting Manager for Chrysal USA. Contact her at [email protected]