Irrigation Scheduling

Phytophthora diseases are more prevalent in crops that are frequently irrigated.  If you irrigate nursery crops on a regular schedule, it’s likely that you are applying too little or too much water.

A better way to irrigate is according to the amount of water stored in the plant’s root zone compared to the amount of water removed by evaporation and plant uptake (also called evapotranspiration).

While soils can hold a large amount of water, only a limited amount can be stored in a container. This is why container-grown nursery crops are irrigated frequently, sometimes twice a day. For both field-grown and container-grown nursery crops, plant health is best maintained if the amount of water depleted by evapotranspiration from the soil or media is about 20% to 35% of the total amount that can be stored.


The amount of water a specific plant uses will change during the production cycle and according to the weather. In the production cycle, for example, a seedling uses less than a rapidly-growing 1-year-old plant. And, an irrigation system set to deliver the same amount each time usually applies too much water on cool or humid days.

Grouping plants that use water at similar rates within the same irrigation zone can help you to manage Phytophthora. When a plant that uses little water is irrigated as much as a plant that uses a lot of water, the root zones of the low-use plants are often water-logged. Phytophthora can thrive under this condition.

Table 2.  Relative water use of some common container-grown nursery crops in the Willamette Valley.*

High Water Use Moderate Water Use Low Water Use
Hydrangea macrophylla 'Nikko Blue' Berberis thunbergii 'Atropurpurea' Daphne X burkwoodii 'Carol Mackie'
Juniperus virginiana 'Skyrocket' Juniperus horizontalis 'Blue Chip' Juniperus squamata 'Blue Star'
Rhododendron 'Nova Zembla' Rhododendron 'PJM' Rhododendron 'Ramapo'

* Data collected from studies at North Willamette Research & Extension Center, Aurora, Oregon, 1989-1995.

Nurseries use several methods to determine when and how much water to apply, including:

  • Soil moisture monitoring (field)
  • Evaluating container media weight (container)
  • ET - based irrigation (field and container)
  • Determining leaching fraction (container)

For details, see Strategies for Improving Irrigation Application Efficiency by R. Regan.