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Problems with Irrigation in Restoration Projects


Lack of Consistent Long-Term Benefit
The common assumption is that the extra push from irrigation will result in a long-term improvement in survival and growth. The irrigation system and schedule are designed to enhance deep root growth. The resulting large, vigorous plants will be able to thrive without irrigation. In practice, we often see the opposite. When we turn off the irrigation after the five year maintenance period, the plants often die or revert to poor, stunted growth.

We only see the long term benefit from irrigation where the plant is already in the proper soil and proper site conditions, and has little need for irrigation.

Irrigation Masks Problems
At a client’s request, we turned off the irrigation to four year old shrubs that were over three feet tall and four feet wide. From the beginning of the project, the soil conditions were a concern, because the site is compacted engineered fill with low organic mater. However, after four years of irrigation, the shrubs appeared healthy, vigorous and well established. After the irrigation was stopped, many of the plants died within months, and the rest are severely stressed. In this case, irrigation masked a problem rather than solved it. If we had restricted our use of irrigation from the beginning, we could have seen and dealt with the problem years ago, but now we are so late into the process, there is no budget left to correct it.

Irrigation cannot permanently overcome soil and site problems. It can only cover them up and stave off the inevitable, until the day you turn the irrigation off.

Poor Response to Irrigation
Although most native plants respond well to irrigation, some do not. The commercial cultivars have gone through a vetting process. One of the criteria is whether the plant will thrive with irrigation. Native plants have not gone through this process, and many of the plants decline or even die with irrigation. A prime example is Fremontodendron, which wil die if you irrigate it.

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