Planting dryland plants along an irrigation ditch
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.