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Planting dryland plants along an irrigation ditch

Posted by ian_wa Sequim (My Page) on
Thu, Apr 29, 10 at 18:00

Along the west side of our property runs a seasonal irrigation ditch which has water in it from about May - October. Judging by the summer greenness of grass along the ditch it would seem that the water running through the ditch keeps the soil moist for only a narrow strip about 1 - 2' on either side of the ditch. Soil drainage is excellent in this area.

I'm considering installing a major planting all along this ditch over a period of time. The planting would be about 6 - 8' away from the ditch and would include many plants that generally require a summer dry period to perform well such as manzanita, Ceanothus, Arbutus and more. I'm sure they would be fine for quite a few years after planting. The question is, what happens in 10 - 20 years when the roots of these plants have grown close enough to the ditch to find this abundance of summer water? Do they grow more vigorously? Do they start dying off all of a sudden? Does the dry side of the plant's root system die off in favor of the side that has found water? Getting more to the point, what exactly are the specific causes of dryland plants dying or performing poorly with too much irrigation? Are they especially subject to fungal pathogens that thrive in moist soil? Is the problem strictly physiological for some species?

My hunch is that most of these plants will probably be fine, and I'm just going to try it anyways. But I still wanted to get your thoughts in pursuit of a better understanding of this subject.

Follow-Up Postings:

RE: Planting dryland plants along an irrigation ditch

  • Posted by bboy USDA 8 Sunset 5 WA (My Page) on
    Fri, Apr 30, 10 at 0:42

Elsewhere you see dry soil plants just above the water, perhaps the most extreme demarcation I have observed is in the Potholes Reservoir, where there is barren aridity inches from blue water - maybe like in the Middle East etc.

I'd just be sure to allow for high water episodes and leave it at that, if they never get flooded by the ditch spilling over there should be no problem with the proximity of water.

You were missed at Florabundance.

RE: Planting dryland plants along an irrigation ditch

The plants you mention specifically tend to be susceptible to root rotting pathogens when irrigated during the growing season. They have adapted themselves to soils that are dry when warm and cool when damp, conditions that are not favorable to soil fungi. They avoid infection by living in soils that don't support high populations of pathogens. When planted in soils that do support high populations of pathogens, then these plants are in trouble as they don't seem to have other methods of resisting infection.

I don't know about other dryland plants. I suspect it varies by species and by ecoregion of origin, but maybe not. I know that rare plant conservation efforts in Australia are complicated by the introduction of Phytophthora cinnamomi, and that native plants growing in well drained soils are less subject to infection than those growing in soils that are subject to waterlogging during periods of heavy rain. These plants can be wiped out by the pathogen, causing local extinctions and changes in distribution and abundance. So it could be that many dryland plants are susceptible to root rot when irrigated during their normal dry season.

Those are my thoughts on the subject. It would be very interesting to see how your plants do over the years.

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