Ottawa Forests & Greenspace

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Planting in Sensitive Clay Soils

The attached draft was presented to the Chair and Vice-Chair of the Planning Committee, Councillor Hume and Councillor Harder, for consideration. At their request a copy was submitted to the City's GM of Public Works, J. Manconi,  and to the Manager of Forestry Services, D. Barkley, for review and comments.

OFGAC Planting in sensitive clay soils.pdf [500 KB]

 

Here is one excerpt from the document:

Tree Selection

This section includes a list of some potential candidates for tree selection in sensitive clay areas. While some of the listed trees may not be suitable or available for various reasons, it should also be recognized that this is not a complete list, but intended merely as an example to illustrate additional possibilities to those already listed in the city’s tree planting guidelines.

If trees are to be prescribed for sensitive clay conditions, what are the criteria for this selection? How are these criteria measured? Even for unmodified problematic soils, there should be a bigger list of acceptable tree species for planting, and a gradient of soils to which these apply. We offer the following examples of some trees and large shrubs (with average heights attained at maturity, which is typically more than 50 years) which may be used in their tree-form that tolerate poor drainage, clay, drought and salt to different extents:

Cedar [Thuja occidentalis or orientalis]: provides a screen, thrives in wet clay soil. 40’ / 12 m (without pruning)

River birch [Betula nigra]: fast-growing, tolerates salt and poor drainage. . 40’ / 12 m

Buckeye [Aesculus glabra]: can handle salt, poor drainage, drought. 25’ / 7.5 m

Catalpa [Catalpa speciosa]: tolerates salt, poor drainage, drought. 60’ / 18 m

Black cherry, or choke or pin cherry [Prunus serotina, P. virginiana, P. pensylvanica]: tolerates clay soils and drought, although sensitive to salt and poor drainage. 60’ / 18 m, 25’ / 7.5 m, 25’ / 7.5 m

Eastern wahoo [Euonymus atropurpureus]: tolerates drought and poor clay soils; shade tolerant. 20’ / 6 m

Tamarisk, salt cedar [Tamarix sp.]: very tolerant of salt, drought and clay soils. 25’ / 7.5 m

Red elder [Sambucus racemosa]: tolerates moist heavy clay soils. 20’ / 6 m

Hazelnut [Corylus avellana]: tolerates drought and poor heavy clay soils. 20’ / 6 m

Smoketree [Cotinus sp.]: tolerates moderate drought and heavy clay soils. 25’ / 7.5 m

Wayfaring tree [Viburnum lantana]: tolerates heavy clay soils, shade tolerant, however not drought tolerant when planted in full sun. 10’ / 3 m

Witchhazel [Hamamelis virginiana]: tolerates heavy clay soils, shade tolerant, however not drought tolerant when planted in full sun. 15’ / 4.5 m

Smooth sumac [Rhus glabra]: tolerates poor saline soils, moderately drought tolerant, and tolerates clay soils. 15’ / 4.5 m

Kentucky coffee tree [Gymnocladus dioicus]: tolerates drought, poor drainage, clay and salt. 60’ / 18 m

Ginkgo [Gingko biloba]: tolerates drought, poor drainage, clay soil, and salt. 60’ / 18 m

Hackberry [Celtis occidentalis]: tolerates drought, poor drainage, clay soil, but not salt. 40’ / 12 m

Bitternut hickory [Carya cordiformis]: can tolerate drought, poor drainage, clay soil, and salt. 60’ / 18 m

Honey locust [Gleditsia triacanthos var. inermis]: tolerates drought, poor drainage, clay soil, and salt. 40’ / 12 m

Red maple (Acer rubrum): tolerant of urban conditions, drought and poor drainage tolerant. 60’ / 18 m

Freeman maple [Acer saccharinum x Acer rubrum]: tolerant of urban conditions, fast growth, less invasive roots than red maple. 60’ / 18 m

Basswood, or linden [Tilia americana or cordata]: tolerates drought, poor drainage, clay soil, but not salt. 60’ / 18 m

Bur oak [Quercus macrocarpa]: tolerates drought, poor drainage, clay soil and salt.60’ / 18 m

Pin oak [Quercus palustris]: tolerant of clay, salt and poor drainage. 0’ / 18 m

Black walnut [Juglans nigra]: tolerant of salt, clay, poor drainage and drought. 60’ / 18 m

Various columnar trees [Pyrus, Malus, Quercus, Cupressus, Fagus, Chamaecyparis, Juniperus, Carpinus]: limited roots, tolerant of poor drainage.

We further recommend that in extreme cases where trees are considered inadvisable under any conditions, a list of suitable shrubs and groundcovers be developed. The current standard of primarily using grass lawns leads to a lower water infiltration rate, increased evaporation of stored soil moisture, and lower water-interception rate than more suitable shrubs and groundcovers. These problems all contribute to the drying out of sensitive clay soils, in turn creating and exacerbating foundation issues. There is a need to be more proactive in developing landscapes that mitigate and prevent these potential problems, and that are better for people and the environment.