Soil drainage and plant health in the pacific northwest

CONSIDERATIONS WHEN PLANTING

Soil drainage is important for long term plant health, at least as important as pruning, mulching, and fertilizing. Locating a plant that likes well-drained soil in a soggy area is an invitation to disaster.

Plant roots must be able to breath air as well as absorb nutrients from the soil. They thrive on oxygen, which they get from air filling the spaces between soil particles. In poorly-drained soils, water displaces air, and roots suffocate as a result. Each plant has a different capacity to deal with lack of oxygen, but as a general rule, poor drainage kills plants. In addition, disease-causing fungi grow well in soggy soils, making plants with wet feet far more susceptible to fungal root diseases. “Poor drainage” is excess water staying in the soil, causing sogginess or puddling

SOIL STRUCTURE AND ORGANIC AMENDMENTS

Depending on soil structure, different amendments are recommended to improve drainage.

In clay soils, soil particles are extremely close together with few air spaces. Drainage is poor and very slow. Adding organic matter like fine fir or hemlock bark, which do not retain water, will increase air spaces, allowing a greater variety of plants to be grown. We recommend adding organic amendments up to 30% by volume of the native soil.

In sandy loam soils, air spaces are plentiful. Drainage is excellent. Minor additions of water-retentive organics such as compost, manure, or peat are required.

In highly sandy soils, there are many large air pockets between particles. Drainage is fast. Adding large amounts of organic matter helps hold water around plant roots, which is necessary for plants that require it. 

ALTERNATIVE STRATEGIES

One can also compensate for poor drainage with alternative planting techniques. Dig a hole 3–5 times the width of the rootball, but no deeper, instead of the normal 2–3 times. This will allow roots to survive in the amended area before they must become established in the poorer soil.

Raised beds provide drier, better-drained soils by creating well-drained areas atop the poorer ones. In all raised beds, it is essential to amend the soil under the bed to a depth of 6–8 inches to insure full drainage out and away from the bed. These beds may be natural mounds or shapes held by rock, brick, or timbers.

For information about the drainage requirements of specific plants, please consult one of our nursery professionals.