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Drought Avoidance VS Drought Tolerance

Spring is certainly in the air and with the landscaping season fast approaching we are all eager to get started out in the garden. This is the perfect time of year for people to think about planning your landscape designs. The first step when building any landscape is having a well thought out design, if you want to create a healthy garden, landscape design is crucial. This means choosing the right plants for the right conditions.

As people have become increasingly mindful of their water consumption over the years, more individuals have began designing landscapes to be Xeriscape. One of the key aspects of a Xeriscape garden is using a variety of “Drought Tolerant” plants that can survive in extremely arid conditions, such as the Okanagan. Today we are going to discuss some often confused terminology and what you need to know regarding the differences between drought avoidance and drought tolerance.

Photo by Matthias Cooper from Pexels

Drought avoidance is visually much easier to identify when compared with drought tolerance. Plants have evolved a multitude of mechanisms for retaining water while going through periods of drought. Shrubs such as Oregon Grape (Mahonia aquifolium) have thick waxy leaves which work to reflect or filter direct sunlight to prevent epidermal loss of water. Leaf pubescence, where leaves are fuzzy or hairy, is another common method seen in plants such as Lambs Ears (Stachys bysantina). Other common plant adaptations involve plants dropping their leaves during the day and re-sprout smaller, thicker leaves which have a sunken stomata. (small holes in the leaf surface)

This is another means of lowering the risk of desiccation by wind or heat. These are all very easily identifiable ways in which plants have adapted to AVOID drought stress. Another less visually obvious example of drought avoidance would be plants with particularly deep or wide roots.(ie. Ponderosa Pines) This is of course helpful during drought as deeper or wider root systems allows the plants to have access to water deeper within the Earth.

The term drought tolerance refers to more of an internal physiology and is therefore more difficult to identify visually. However, drought tolerance can be further broken down into two categories, Isohydric and Anisohydric. A good way to remember the difference between the two is that Isohydric plants are more conservative and Anisohydric are more risky.

Anisohydric plants have a more variable Leaf Water Potential (LWP) and will keep their stomata open and essentially go full tilt until the drop in Turgor pressure (Turgor pressure, also called hydrostatic pressure, is the water pressure within a plant which allow it to remain plump and standing tall.) causes the stomata to close and the plant to wilt. Certainly a much riskier method of tolerating drought. However the benefit is that having the stomata open means Anisohydric plants can fix more carbon.

Isohydric plants maintain a constant midday Leaf Water Potential (LWP) regardless of drought by controlling the stomata. The roots will send a signal to shut the stomata as the soil drys out and the LWP will remain essentially the same. The problem with having the stomata closed during the day is that it greatly decreases the plants ability to fix carbon and increases the likelihood of Photorespiration.

Photorespiration occurs when plants are exposed to very hot and dry conditions and the stomata closes. Essentially plants can’t breathe and having less CO2 and more O2 inside the leaf results in the plants fixing O2 instead of CO2. This flaw wastes energy and time while in no way benefiting the plant.

Why does any of this matter? When creating a Xeriscape garden there are certain requirements that must be met beyond the visual aesthetics in order to ensure you’re getting the most water efficient design possible. Xeriscape gardens on the surface often appear to be drip irrigation buried under landscape fabric and covered in rocks. The reality is that to build a high quality Xeriscape garden the designer must put serious thought into exactly what plants they’re installing and where they’re being placed. Knowing which plants can tolerate drought and which simply avoid it will make your designs infinitely better and in turn an average landscaper or designer into a well educated gardener.


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