Prevent Standing Water in Production Areas and Improve Root-zone Drainage

You can discourage Phytophthora disease in both field-grown and container-grown crops by eliminating standing water in the nursery and by improving root-zone drainage. Use gravity to move excess water away from the plant and its root system. The approach you take will be different if you grow plants in the field or in containers.

Improving Drainage in Field Production Areas

Standing water is found on soils with low infiltration rates and/or poor drainage. Under these conditions, water from rain or irrigation will not enter the soil rapidly and therefore accumulates on the soil surface. Even a gentle, 2% slope moves water downhill, and it tends to accumulate in low areas of the field.

Crusting of the soil surface reduces the rate at which water and air can move into it. The soil surface has open pore spaces that are created by the soil structure. Repeated, shallow cultivation for weed control weakens this soil structure. At this weakened stage, the impact of water droplets from irrigation or rain will easily destroy the soil structure. Thus, the open pore space is gone and a crust forms that can seal off the soil surface.

Soil compaction is another problem for field nurseries in Oregon. Improving soil structure will help reduce compaction and increase surface water infiltration rates.

Improve soil structure in field nurseries by:

  • Minimizing traffic in fields when they are wet.
  • Using track-type versus wheel-type vehicles.
  • Reducing the number of times a field is cultivated.
  • Planting cover crops whenever possible.
  • Matching the irrigation application rate to the soil infiltration rate.
  • Incorporating organic matter to increase microbial activity.
  • Avoiding irrigation systems that deliver large drops of water.
  • Installing tile drains to help remove excess water.

Tile drainage systems are effective in removing water from saturated root zones and in reducing the amount of standing water.

Many of the silt loam soils used to grow nursery crops in Oregon develop perched water tables. This condition holds ground water closer to nursery crop roots and encourages Phytophthora diseases. These soils will benefit from a well-designed drainage system, especially when tile lines are less than 20 feet apart. To work efficiently, the tile drain system must discharge water at the lowest elevation in the field.  Sometimes, the system also conveys drainage water to a storage reservoir to be reused for irrigation.

Improving Drainage in Container Nurseries

A container nursery's surface allows water to collect in depressions — ideal conditions for Phytophthora. Be sure that plant beds slope at least 2% to provide sufficient drainage. Beds must be carefully graded to eliminate depressions or low areas where water will collect. Use a combination of drainage ditches and tile lines to move water away from production areas and, possibly, to a storage reservoir.

When designing a container nursery drainage system:

  • Use slightly crowned plant beds , or keep flat beds short to drain water more quickly.
  • Use gravel to break the water column between the container and any temporary standing water.
  • Do not use plastic to cover beds; plastic can create standing water.
  • Woven fabric ground cover can become plugged with algae and organic matter also creating standing water. Inspect these covers regularly and clean or replace when needed.
  • Regularly check tile drain lines which can become plugged due to vehicle traffic and accumulated organic matter.
  • Plan for routine maintenance of the plant bed surface and the drainage system.