When considering plants in a landscape there is often an emphasis on using local or native species.  Ignoring the all important “What constitutes a native species?” .  The next consideration is “Why use native plants?”.  This is hardly a new question in the world of landscape design.  But there is a persistent myth that using native plants in landscape makes things easier and less costly.  That statement invites an interesting debate which points to the fundamental requirement for knowledge about plants and their interaction with the environment.   Planting anything without insight into the basic biology and ecology of a plant is playing “landscape roulette”.  It is also necessary to have both macro- and micro- understanding of an area

Hawaii Savannah on Lava

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Texas Savannah on Limestone

Fortunately this is relatively easy to achieve given the growing body of knowledge of plant eco-physiology and the fact that ecological patterns appear like repeating fractals at different scales.

Take for instance the standard world biomes map we see patterns connected to climate and vegetation.  It is not surprising that if we look at a map of the Texas “ Natural Regions” or “Ecoregions” we see similar patterns of vegetation connected to topography, rainfall and temperature.

Thus, learning to “read” the landscape is as important as getting to know the foibles of the local “characters” – the plants.  And we still need to answer why “locals” should get preference.

In nature where a plant is seen to grow is the result of a history of seed dispersal, germination conditions and physiology.

Fire senescent Protea sp. in South Africa

In a constructed landscape where a plant grows is the result of someone’s intent and care – and the physiology and biology of the plant.

Mammilaria cactus, Texas

Knowledge and experience of the plants, climate, ecology and landscape purpose are fundamental to success in a situation that is largely bound by the laws of nature.

So what is important in considering conditions for creating landscape are those same things that define broader picture of natural plant dispersal – rainfall and temperature, topography and soil.  [Useful documents like the soil types of Texas, describe in great detail local conditions – to the point of being overwhelming].  But these environmental variables are usually far from static, or predictable, and it is not necessarily the averages which define a plant’s distribution.  More often it is the extremes.

Consider the climate in Austin.  If you view the averages it appears to be a pleasantly warm climate, view the extremes and it is possible that a plant has to deal with freezing temperatures and days of 100+F in the same year, not to mention winds, excessive rain or no rain at all.

We are all very aware of the growing challenges associated with rainfall. Last year presented Texas with its worst drought in many years (US total precipitation 2011), and there does appear to be a shift in rainfall totals over the past decades (changing US rainfall since 1960).

It is not only rainfall that is shifting.  The USDA has recently published a new plant hardiness map minimum temperatures that reflects changes in climate.  There is finally recognition that the hottest temperatures challenge plants as well and the American Horticultural Society has produced a heat zone map.

Perhaps the most hidden, or least considered factor, is how land is actually being used.  A new vision that considers human land use in an ecological context makes the world biomes map we are familiar with seem grossly oversimplified.  Anthropogenic biomes and the impact of human land use at a local level highlight the importance of intelligent and ecologically informed decisions with regard to landscape, particularly in the urban setting.  This picture of the less than pristine biome, ever-changing land use, and pressure on fundamental natural resources, such as water, should lead us to expand the role of plants in the intelligently constructed landscape.  Plants are the ecological go-between,  the mediator between static urban structure and capricious natural events and forces.

Now having considered our landscape, we can return to the questions “why use native species?” and “what constitutes a native species?”.  Situated in context of the immediate environment the conversation may be less academic.