Consider Soil pH Before Selecting Trees and Shrubs for Landscape Use

Landscape ornamentals perform best when grown in their preferred soil pH range. When ornamentals are planted into soils that lie outside of a plant’s preferred soil pH range, there is an increased risk of nutrient deficiencies, nutrient toxicities, and plant mortality. Prior to selecting plant materials for a residential or commercial landscape, contractors must test the soil to determine the soil pH for the site that is to be landscaped.

Most landscape contractors know ericaceous plants like rhododendrons, azaleas, and Japanese Andromeda. Ericaceous plants are acid-loving (preferred soil pH range of 4.5–6.0), yet they are frequently used as foundation plants in landscapes where the soil pH is considerably higher than what these plants typically prefer. When ericaceous plants are subjected to elevated soil pH, iron uptake by these acid-loving plants is compromised, resulting in plant stress. When stressed, plants are inherently more susceptible to pests and diseases.

When confronted by soils with a pH outside the preferred pH range for the selected plant materials, a contractor may attempt to amend the soil to adjust the soil pH to the appropriate pH range for the tree or shrub to be transplanted. In theory, this is a good workaround, but in many cases, the soil's buffering capacity and mineralogy may limit the efficacy of pH treatments.

In one Pennsylvania community I visited, the soil is underlain by limestone. The natural soil pH in this community can range as high as 7.4. Landscape contractors in this area realize that they are planting in high pH soils, yet the most common trees installed in commercial landscapes in this community are pin oaks and white pine. Pin oaks prefer to be grown within a soil pH range of 5.0 to 7.0, and white pines prefer to be grown in soils with a soil pH in the 4.5 to 6.0 range. When pin oaks and white pine are planted in soils with an elevated soil pH, they often underperform and may decline and die over time. A more prudent approach to landscape plant selection in this Pennsylvania community is to select tree species that will perform best when planted in high pH soils, like Celtis occidentalis (Hackberry) or Juniperus virginiana (Red cedar).

Institutional landscape managers who have inherited pin oaks and white pine in this Pennsylvania community are forced to wage a yearly battle to maintain the soil pH within the desired range for these pH-sensitive specimens. Failure to maintain the soil pH in the desired range for pin oak has resulted in severe iron chlorosis and even tree death on some sites. Before selecting landscape ornamentals for residential or commercial landscapes, always evaluate the soil pH at the site.

Failure to do some basic soil testing upfront may jeopardize the short-term and long-term success of the landscape planting. Landscape plant selection should not be based on profit margin, nursery inventory, or the ornamental plant preferences of the landscape designer. Plant selection should be based on plant hardiness, plant form, height, seasonal interest, pest resistance, drainage, and pH.