Thursday, December 16, 2010

Weed corridors

Weeds don't just spontaneously generate in your garden or landscape, they are transported there by some means.  Understanding the means of transportation of weeds into your landscape or garden takes careful observation but can save you a lot of pain in the future.

Seeds move through the landscape in a variety of different ways, the most important are likely wind, water, and animal.  When placing a garden, designing a landscape, or trying to manage or understand natural areas this is extremely important to factor in.  Understanding how seeds move through the landscape can mean both good and bad things.  For example, understanding how a stream floods can aid where willow or cottonwood seeds will be deposited for habitat restoration.  Or if you place your garden in a low laying area or a place that catches wind deposited seeds you could have a huge weed problem.  Unfortunately, we don't always have control of how we design our landscapes or where we place our gardens, we simply have to deal with the problems that come a long with the site.  Also, it is often hard to determine seed corridors until it is too late.

For example, the college garden area below:
Note the large gap between the two walls.  This appears to be a major source of weed seeds that are deposited in the garden.  Seeds blow in through this gap and are deposited in the garden.  The first beds immediately adjacent to this gap can have pretty bad weed problems compared to the rest of the garden area.  One solution to this would be to close the gap, but that really isn't a possibility so I'll just have to deal with it.  Another solution if we were in the design phase of this garden might be raised beds.  Often, far fewer seeds will be deposited in beds slightly elevated above ground.  However, in the case for these gardens we wanted sunken beds in order to conserve water, so we'll just have to deal with the weeds.

The dry wash below is another example of a weed corridor:

Any type of waterway, whether it is perrenially wet or dry for most of the year transports huge amounts of seeds.  If you want to identify what invasive species will be invading an area in the near future you need to look along waterways.  Water transports seeds and provides a moist habitat with higher quality soil in which all kinds of plants can get a foot hold in the landscape.  This includes ditches a long roads.  Before designing a landscape or placing a garden it is very important for you to know how water flows through the land and where it concentrates.  This is much easier then identifying where wind deposits seeds.  Simply observe water movements in the landscape during and after large rain events, then adjust your landscape design accordingly.  Riparian areas, whether dry most of the year or wet, are also major corridors for the movement of animals which carry sometimes large amounts of seeds.

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