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Hydrating ("Hardening Off") Fresh Cut Flowers

Hydrating is the process by which water and other ingredients are rapidly introduced into flowers to make them turgid.

Stem anatomy: Xylem, phloem, vascular bundle, vessel, tracheid, pith, sieve plate, pit and stele are some of the terms which describe the food and water conducting systems of plants. For the purpose of this manual, only the xylem will be discussed.

Xylem is the principal upward and horizontal water conducting tissue in plants. Made up mainly of tracheids and vessels, these cells are dead at maturity. Individual cells of various lengths are connected end-to-end to form continuous pipelines throughout the plant.

In simplistic terms, xylem can be viewed as thousands of tiny straws connected end-to-end. In general, the length of these straws is a few inches or less. Filters or valves are located at the points where the straws connect. These filters or valves can influence what passes from one straw to the next. Specifically, microorganisms, debris and air bubbles are restricted from moving through these filters or valves.

Role of dirt, debris, microbes and air bubbles (embolisms): In healthy cut flower species prior to harvest, the plant root system provides clean water to the xylem tissue. Once flowers are harvested, inhibitors of water movement in xylem can reduce the flower life.

Air bubbles can form immediately upon cutting. They also form later, as the flower loses water when being held dry. Dirt, debris and microbes can enter xylem tissue at any point in the marketing channel, simply from normal handling.

Accumulated dirt, debris and microbes in xylem can be greatly reduced by cutting off about one to three inches of stem tissue, either in air or underwater. Alternatively, there are four ways to rid the xylem of air bubbles to allow for the free flow of solution: cut stems underwater, place stems in warm water, use a citric acid solution or place flowers in deep holding solutions. Each of these procedures will now be explained.

  • Cutting stems in air: To reduce the levels of dirt, debris and microorganisms in xylem, remove one to three inches of stem tissue. Use any reasonably sharp instrument (i.e. scissors, knife, paper cutter, pruning shears, etc.).
  • Cutting stems underwater: When one to three inches of stem tissue is removed by cutting the stems underwater, most air bubbles are removed, too, as are the dirt, debris and microbe blockers. That's because the filters/valves located at the straw ends prevent air bubbles from traveling further up the stems. At the same time, no air is allowed to reach the newly cut stem surfaces, because the stems are underwater. Once re-cut, the flowers can be immediately removed from the water and placed in fresh-flower food or other solution(s). Florists need to cut flowers underwater only once. Retail florists should perform this procedure when processing. Once the flowers have been allowed to hydrate or harden off for about one hour at room temperature, the xylem should be full of water or solution. These same flowers should be full of water and, thus, have no room or appetite for air. Therefore, they will not need to be cut again underwater, either at time of sale or arrangement.
  • Placing stems in warm water: Normal tap water contains a certain amount of dissolved air. If it weren't for this phenomenon, fish could not survive, as they use this air to breath. When water is heated to bath temperature-about 100 to 110 degrees Fahrenheit-some of the air normally present is driven out. When this warm water is allowed to cool, it begins to reabsorb air. When flowers are cut in air and then placed into warm water-then cooled-potentially trapped air at the cut stem ends will be re-dissolved back into the water phase, thus ridding the flowers of blockage caused by air bubbles.
  • Using this information, a simple home remedy hydration solution can be easily made. Place hot water (150 - 160 degrees Fahrenheit) in a soft plastic container filled to the top. Tighten the cap and place the container in a refrigerator. As the water cools the container shrinks, air is excluded, and the remaining water is essentially air free - a remarkable scavenger of trapped air and an excellent hydration solution. Flowers cut in air and placed into this degassed water should hydrate quickly.
  • Citric acid solution: You can overcome air embolisms by placing flowers cut in air into a citric acid/water solution (about pH 3.5). See hydrating solutions below for further details.
  • Solution depth: The deeper the holding solution for flowers, the greater the head pressure created at the cut-stem ends. This greater head pressure pushes water into the stems. Thus, flowers first cut in air and then placed into deep holding solutions have a better chance to revive than those placed into shallow solutions. This procedure should be used only for severely wilted flowers.
    Cutting stems in air versus underwater: If stems are not recut, either in air or underwater, from 40 to 60 percent of the potential life can be lost. If embolisms are not a problem, end-user life will be equal for flowers cut in air versus underwater.

Cutting stems on an angle: It matters not whether you cut flower stems straight across versus on an angle. Do whatever is easiest.

Role of sugar: Sugar retards water uptake, which is one reason hydrating solutions don't contain sugar (see below). If flowers arrive very dry, don't place them immediately into a fresh-flower food solution. Properly hydrate them first. However, once hydrated, flowers must be transferred to a fresh-flower food solution, which, by definition, contains sugar (food).

Hydrating solutions: Commercially available products can be used as hydrating agents. Many are either citric acid or aluminum based. In addition, while not specifically available in a commercial floral product, chlorine from household bleach is used, too.

  • Citric acid versus aluminum based: Researchers in the mid 1970s demonstrated the benefits of low pH solutions (about pH 3.5) for increasing water uptake in roses and other flowers. After testing many acidifiers, citric acid proved to be the best. Citric acid is known to have a number of positive effects on flowers such
    • May act as a germistat to control germ growth because of the pH
    • Can not only reduce solution pH but can act as a buffer
    • Can act as a chelator to hold and transport certain nutrients
    • Improves solution uptake
    • May act as a food source
    Recommendations about making up and using citric acid solutions, especially on roses, soon followed. However, industry professionals found out that proper sanitation procedures were absolutely essential to avoiding microorganisms. In addition, self-manufacture of citric acid solution had regulatory implications, mostly from the Environmental Protection Agency. Floral companies may have difficulty obtaining these regulations.

    Many of the earliest fresh-flower foods contained aluminum, mostly in the form of aluminum sulfate. That is still true today and researchers have demonstrated over the years the benefits of this compound. Aluminum-based solutions:

    • Can act as a modest germistat to help control microbe growth
    • Can reduce solution pH
    • Can complex certain chemicals, ions, dirt, etc. out of solution
    • May influence stomata function and hence transpiration
    • Can influence flower color
    • Can influence water uptake
    Aluminum- versus citrate-based products play different roles. Specifically, aluminum-based ones retard flower development by slowing, but also maintaining, solution uptake. Citrate-based solutions enhance solution uptake, so as to prevent bent-neck symptoms. Unfortunately, these differences are not always obvious or consistent. Research and experience findings presented below demonstrate some of the differences.
    • Citric acid-based products perform better, while aluminum containing solutions are less consistent when used in rose-hydration, especially when stems have not been re-cut later in the marketing channel. In short, if aluminum-based hydrators are used at grower/bouquet manufacturer level, the stems must be re-cut once the customer receives the product or reduced vase life is almost assured (aluminum flocculation probably causes a physical blockage to solution uptake at the base of the stems).
    • Aluminum-based products are more susceptible to water quality changes because the aluminum solubility is very pH dependent.
    • Citric acid-based products can be toxic to some flower cultivars at high concentrations.
    • If flowers are placed in citric acid solutions for too long or under the wrong temperature conditions or if the flowers are old or have been improperly handled prior to treatment, rapidly opening flowers may result. While this has a downside, it also offers a good measurement of old or improperly handled flowers at the time of treatment. No chemical treatment can overcome aged flowers.
    • The responses of roses to citrate- and aluminum-based products is often confounded by the cultivar being tested. For example, water uptake characteristics of three common rose cultivars-'Visa', 'Royalty', and 'Madame Delbard'-are quite different. Therefore, responses to these products can be very different, depending on the physiology of the flowers at the time of treatment.
    • If your goal is to make sure flowers look fine when they leave your business-and you don't care about end-user life-aluminum-based products are better because flower opening is delayed. In the same vein, if you are dealing with old roses, you most likely will prefer the aluminum-based products. However, in both cases, flower performance at consumer level can be unsatisfactory. Unfortunately, short-term goals sometimes outweigh the benefits of end-user flower performance.
  • Chlorine based hydrating solutions: For decades, growers, wholesalers and retailers have been adding various amounts of bleach to flower buckets to control microorganism growth and improve flower hydration and flower life. Until recently, no product was labeled for this use. Now a leading manufacturer of bleach has stated that their product can be used at one teaspoon per gallon of water to help control microorganisms and extend flower life: a finding confirmed by research for some crops like baby's-breath and roses. After using a bleach solution for an initial hydration step at grower, wholesale and/or retail level, make sure the flowers are transferred to a fresh-flower food solution. Only add bleach to water: do not add bleach to hydrating, STS or fresh-flower food solutions, as the chemicals may not be compatible.

Information reprinted from SAF Flower and Plant Care Manual, (Society of American Florists: Alexandria, VA, 1994), pp. 147-150.