Water-roses’ number one requirement!

by Carolyn Elgar, Master Rosarian, Orange County Rose Society

This is a 2020 AOM winner

As much as we enjoy our roses, we cannot ignore the fact that they have needs. One of these needs is actually a major requirement for these perennial plants: water. For those who live in areas that are arid it is easy to forget the urgency of this need and how it is affected by the plant’s environment.

Plants use water in a number of metabolic processes. One of the most well-known is photosynthesis; plants combine carbon dioxide from the air and water from the soil. In a chemical reaction, the oxygen is released from the water molecules (H2O) while carbon dioxide is taken in and combined with the hydrogen from the water. Light energy from the sun interacts with this combination of hydrogen and carbon dioxide to make the sugars (glucose) the plant needs to survive. Another process, respiration occurs when the plant converts those sugars into energy.

The process in which the plant releases the oxygen in exchange for carbon dioxide from the air is called transpiration. The moisture content of the leaf is higher than the moisture in the air; through the principle of diffusion in which high concentrations of fluids flow into areas of lower concentration, much of the water vapor escapes into the atmosphere to equalize these moisture levels. In this way, most of the water (99%) absorbed by the roots of the plant is expelled from the leaves during transpiration.

How does the water in the ground get to the leaves? The same principle, diffusion, causes fluids to move into the roots and up through the cells of the plant’s stems or canes. This pull is increased when the leaves release water vapor, leaving space for more water to enter. A simple experiment with a paper towel and water demonstrates how diffusion works; a dry paper towel will draw from a pool of water until it is wet. The process of the fluids diffusing throughout the plant is called osmosis. ABOVE: This photo demonstrates osmosis; a paper towel will draw water from a plate into dry areas with no human intervention.

Water also delivers nutrients from the ground to the plant. Chemical interactions turn nutrients dissolved in fluids into a form that can be used by the plant. Another essential task water performs for a plant is turgidity. The pressure of water within the plant cells makes plant tissue rigid, supporting the plant like a skeleton supports an animal. In addition, the management of this turgor pressure regulates osmosis, providing a balance between CO2 intake and water loss.

Factors affecting absorption

A plant’s access to water can be influenced by differing factors. Competition from other plants, especially trees and large shrubs, may take water away from roses. Soil quality and temperature also have impact. Cold soils below 50 degrees inhibit water flow in roots for a number of reasons, including the permeability of the plant cells and the thickness of the fluids. Hot soils above 80 degrees can evaporate water before it reaches the roots. The ideal soil temperature for plants’ water absorption is 65 to 75 degrees.

Soil structure and texture have a large impact on water absorption. In sandy soils water leaches away quickly, before the roots can access it. Clay soils can hang onto water, impeding its flow. Soils that are friable and contain aggregates (clumps) of material, that have spaces for air and water molecules, have the best water-holding capacity. The addition of organic materials, such as compost, increases this capacity. If the area is sloped, gravity can pull water away from the plant above and below the ground.

A common occurrence in an area that has fill dirt or rocky soil is a “perched” groundwater table. Usually the groundwater table indicates the level in the ground where it is permanently saturated with water. A perched groundwater table is caused when an impermeable layer occurs above the natural water table. This can happen when there is a difference in soil structure that occurs in layers or a physical barrier of some kind in the soil. This can lead to water-logged soils or decreased water-holding capacity.

The salinity of the soil also affects water absorption. Salinity is the measure of the soluble salts in water. Salts are chemical compounds composed of dissolved minerals such as sodium, calcium, magnesium, and potassium, common ingredients of fertilizers. When they exist in high amounts in the soil, they increase the denseness of water, disturbing the osmotic balance. Water no longer flows into the root cells, causing what is referred to as physiological drought. Arid areas are prone to salt accumulation because there is little rain to leach out the salts. In mountainous areas, naturally occurring minerals in the ground set the stage for salt accumulation.

ABA - a defense mechanism

Plants have protective mechanisms to deal with lack of water. Abscisic acid (ABA), considered a “stress hormone,” is produced in greater quantities during water shortage. It responds to lack of water by signaling the plant to activate its defenses. Whereas light encourages leaf stomata to open in support of photosynthesis, abscisis acid levels cause these openings to close, protecting the plant from moisture evaporation that can’t be replaced with water. In this case the photosynthesis process does not work as it usually does. Studies have shown that over 50% of the genes regulated by ABA are controlled by the stresses of drought and salinity in the soil. In addition, research indicates that this signaling starts in the roots of a plant under water stress despite unchanged leaf water potential and lack of wilting. If water loss is not addressed, eventually ABA can cause the plant to change shoot and root growth.

Signs of stress

The first obvious sign of water stress shows up as a decrease in turgor pressure, leading to wilting. Leaves yellow as respiration and photosynthesis decrease. Transpiration stops because the stomata are closed, and water is no longer pulled through the plant and released to the air. The plant puts water retention first; anything that works against that is decreased until conditions are more favorable. Usually needed watering will revive the plant within three days. When water needs are not addressed, leaves wither, drop off, and the plant wilts and dies.

The timing of water stress affects different parts of the rose bush. The stage of growth prior to petal initiation is sensitive to water loss, affecting the quality of flower buds, petal formation, and bud size. The worst time for water stress is prior to stamen development; quantity and size of blooms and stem length are reduced. After stamen and carpel formation, water stress does not have a negative effect on blooms.

Do some roses do better with water stress than others? Perhaps you know which ones in your garden handle lack of water the best, based on your experience. Not much research has been done on this; I found one study that rated the Knock Out rose as more resistant to water stress than three other varieties.

One thing that we know for sure from studies is that Fortuniana rootstock performs better in low water situations than Dr. Huey or other rootstock plants. The roots of this variety spread out more, accessing more areas for moisture. Rose bushes on Fortuniana grow taller and produce larger blooms. Initially this rootstock gained favor in Florida where it was popular for resisting the nematodes in Florida’s sandy soils. Now many rose lovers in Texas, California, and other states that don’t require their roses to be hardy during winter freezes seek out roses on Fortuniana. Mail order or personal propagating is the only way to acquire roses on this rootstock in California.

ABOVE & BELOW: At the bottom, a water meter purchased from a big box home store. Above, a water probe that physically samples soil up to a foot under the ground.

It is very important to consistently monitor the amount of water your roses are getting. If you have a lot of roses, chances are you have a sprinkler or drip system of some type; despite the ease of automated watering, keeping an eye on your plants is essential. Emitters break, tubes are chewed, or a line may be clogged. Check the depth of the moisture in the soil by using a probe or a long stick (LEFT); push it into the ground to a depth of a foot to a foot and a half. The recommended depth of watering for roses is around 18 inches. How much water to use depends on a lot of factors, including the ones already mentioned. A good-sized plant may need four to five gallons a week in mild weather, but when the temperatures rise, the same plant may need nine gallons. Create a basin around the base of each plant or use rings of border board to keep the water near the rose until it is absorbed.

Roses in pots need special attention because the space they live in is so contained. Check them daily by probing down into the soil and evaluating its dampness. Accept the fact that both potted roses and roses in the ground may need some additional and consistent hand watering. Roses are not drought tolerant plants; they need water to work their magic. ❉

Table credit: Noble Research Institute

All photos submitted by Carolyn Elgar