Most green thumbs who propagate plants by cuttings are concerned with relative humidity, temperature, rooting hormones and media type. While all of these factors play an integral role in the success or failure of rooting, there is one factor that growers may be overlooking: water status.
It is so simple and ambiguous that I would argue it is rarely, if ever, addressed by the propagator. So, why do we care about how much water is retained in cuttings? In the case of softwoods, and to a lesser extent semi-hardwood cuttings, proper hydration will directly impact turgor pressure.
When the turgor pressure of a softwood cutting is high (in hydrated plants), the cuttings will feel rigid. Cuttings with low turgor pressure (as a result of water stress) will have a soft feel and will be noticeably wilted.
A cutting experiencing water stress or lower hydration will have a smaller chance of root formation relative to one that is fully hydrated. Let’s explore some reasons why proper hydration is so important to the success of root formation on cuttings.
No roots and leaves? We may have a problem
Roots are a plant’s organs that take up water and nutrients. A combination of water and nutrients is pulled up the plant through the evaporation of water from the leaves. Therefore, a plant’s cuttings will pump water out and become desiccated quickly. Water stress will lead to biochemical signaling events that can partition energy away from reserves that would go toward root formation. In addition, the stress response can lead to the abscission or dropping of leaves in an effort to reduce water loss. Collectively, both factors will significantly reduce the odds of successful root formation.
Leaf abscission (drop)
If the leaves are the source of water loss, why should it matter if they drop off? Won’t that help reduce water stress? Plants are certainly adaptable to certain stressors, and dropping leaves is a natural response to water stress. But things are not so black and white when we are dealing with a section of a plant.
As you might already know, a plant’s main energy source (sugar) is produced in the leaves by using light energy, carbon dioxide and water. Eliminate the energy producing factories and the cutting will not have sufficient reserves to manufacture roots. Total failure will result. While saturated photosynthesis running on all cylinders is disadvantageous due to nutrient requirements (no roots to take them up), you do not want a complete shutdown either.
Stress is costly
To ensure the highest probability of successful rooting of plant cuttings, a grower must minimize energy sinks and water loss while promoting factors leading to root formation. Water is a lifeline to all levels of proper plant health, and water stress can be viewed as a decline in health.
Think of the times when you are fighting a cold. How energetic do you feel, or how well do you perform other tasks? In this example, your body is expending considerable resources trying to fight off these invaders. Water stress will cause a slow down or cessation (during prolonged and extreme drought conditions) of a plant’s metabolism or the biochemical reactions needed to sustain normal functioning. Maintaining adequate hydration throughout the rooting process will reduce these negative events. Fortunately, there are ways to level the playing field and make sure all of the cuttings start off on the right foot.
How we can rehydrate cuttings
Is simply placing the cuttings in a beaker of water enough? That may be adequate, but let’s look to the world of the florist to understand what factors will help promote fast, efficient water uptake. Cut flowers are often treated with hydration solutions.
A hydration solution may contain any or all of the following chemicals: water, an acid, a germicide and a surfactant. The role of water is obvious, while acids have a two-fold purpose. Lowering the pH level to between 3 and 5 facilitates an opening of the conductive tissue (xylem/phloem), as blockages to the xylem will greatly slow down or completely block water uptake.
To this end, make sure you re-cut each plant cutting underwater so that an air embolism or bubble does not form. Lowering the pH also helps to reduce microbial growth, which may or may not be paramount in this application. Next, a germicide will help sanitize microbes that have colonized the cut end or stem of the cutting.
Using a germicide may provide dividends down the road in preventing fungal or bacterial infection that can rot the cutting. Lastly, a surfactant may accelerate the speed of water uptake by reducing surface tension. While this may help, I personally don’t believe it is necessary for re-hydrating plant cuttings.
While there are significant anatomical differences across plant species, the total length of the cutting will be the largest determining factor in determining how long rehydration will take. As a general reference, a 2 to 3-in. cutting should fully rehydrate within 15 to 30 minutes.
Proper hydration is paramount to an individual’s success rate with cuttings. Softwood cuttings are especially sensitive, but semi-hardwood cuttings should be regarded as well. The purpose of this article is to stress the importance of maintaining proper hydration of plant cuttings.
If a plant is fully hydrated, subsequent rehydration will be of little use. Remember, the role of water in cuttings is multidimensional. Water accumulation within plant cells exerts pressure against the cell wall, which creates the rigidity that keeps a plant upright.
Loss of water pressure due to dehydration leads to wilting. Since cuttings do not have roots to uptake water, and a callus will eventually block uptake from the cut portion, the leaves will quickly become a major source of water loss, resulting in wilting and failure of the cuttings to root.
Stressors like dehydration can promote undesirable consequences like leaf abscissions or drop that will also decrease root formation. Stress is metabolically costly in that partitioning of resources to alleviate the stress will ultimately be taken away from the energy pool needed for rooting.
While some cuttings will respond favorably to simply being immersed in water for rehydration, others will benefit from the addition of an acid or a surfactant to keep the conductive tissue clear and to accelerate water uptake.
Even though it is not commercially available, rehydration solutions for cuttings could offer a potential vehicle, reinforcing leaves and thereby reducing leaf drop as well as delivering water soluble compounds promoting root initiation.
While the traditional factors of relative humidity, temperature, rooting hormones and media type are quite important when it comes to growing using cuttings, make sure those thirsty cuttings are fully hydrated, and I know you will be happy with the results!