4 Introduction to Plants for Difficult Planting Situations
Learning Objectives
- Describe plant tolerance for difficult planting conditions.
Few landscapes and gardens will contain the perfect planting conditions. Environmental stress from variable combinations of light and moisture levels, exposure to wind and cold, soil characteristics, site slopes and drainage can create difficult situations for planting. Some plants will be better suited to tolerate environmental stress because of morphological and physiological adaptations developed in their native habitat. For example, Berberis buxifolia (box leaf barberry), Gleditsia triacanthos f. inermis (thornless honey locust), and Ginkgo biloba (maidenhair tree, ginkgo) are able to tolerate a fairly wide range of planting conditions. When planting in difficult situations such as the examples described below, select plants from similar habitats that are naturally adapted to grow under the existing conditions.
Sunny arid conditions
Environmental stress associated with arid (xeric) conditions can severely limit plant growth. Climate characteristics include full light exposure, high summer temperatures, low and unpredictable precipitation, and low humidity with drying winds. Soils with poor structure, minimal organic matter or soil biology and low water holding capacity and nutrient availability are common in arid conditions. Where hardiness is a limiting factor for plant selection, local regional native plants adapted to the existing climate, soils, and moisture regimes are often the most suitable choice.
Shallow, extensive root systems allow species such as Rudbeckia fulgida (black-eyed Susan) to survive in drought and poor soil conditions. Plant characteristics such as small, compound, and modified leaves and stems, and light or gray colored leaves with hairy or waxy surfaces reflect sunlight, moderate the temperature at the leaf surface, and reduce water loss. Achillea filipendulina ‘Gold Plate’ (Gold Plate yarrow), Artemisia schmidtiana (silver mound), Festuca ovina glauca (blue fescue), Rosa rugosa (rugosa rose), and Abies concolor (white fir) are some examples of plants with these characteristics. Read more about plant adaptations at this link to Plant Adaptations to Arid Environments [New Tab].
Shade
Shaded areas that may seem problematic are in fact ideal for plants that occur naturally in habitats with low light, such as woodlands and ravines. There are many shrubs, trees, climbers, bulbs, ferns, and ground cover plants that either tolerate or prefer partial to full shade. For example, evergreen species and cultivars of Rhododendron spp. prefer deep to part shade while Rhododendron Northern Lights Group (azalea) prefers full sun to part shade. Characteristics of shade plants such as branched habits, two-ranked leaf arrangement, and broad, thin leaf blades are suited to capture available light. A strategy of some herbaceous plants, such as Crocus cvs. (crocus) is to emerge early, flower, set seed, and die back to resting structures before tree and shrub leaves fill in completely. Some shade tolerant trees, such as Tsuga heterophylla (western hemlock) and Acer saccharum (sugar maple) will germinate and grow as understory species until openings in the canopy allow them to grow to full size.
There is a wide array of ornamental plants suitable for planting in partial to full shade. Examples of ferns are Athyrium niponicum var. pictum (Japanese painted fern), and Matteuccia struthiopteris (ostrich fern). Shrubs for shade include Aucuba japonica (Japanese aucuba), Kalmia latifolia (mountain laurel), Kerria japonica (Japanese kerria), and Leucothoe fontanesiana ‘Rainbow’ (Rainbow leucothoe). Shade tolerant ground covers include Pachysandra terminalis (Japanese spurge) and Sarcococca hookeriana var. humilis (dwarf sweet box). The woodland understory tree, Cornus florida (Eastern flowering dogwood, pink flowering dogwood) is adapted to growing in partial shade. Learn more about shade gardening at RHS Shade Gardening [New Tab].
Dry soil
Multiple factors can contribute to dry soil conditions on a site. Soils with high sand or aggregate content that drain quickly move available water below the plant root zone, and surface slopes with rapid runoff reduce water infiltration into the soil. Overhead structures that block rainfall, such as building eaves or tree canopies with competing roots below ground can also create dry areas. While few plants will survive in permanently dry areas, drought tolerant native and garden plants can flourish in dry soil once established. Examples include ground covers, Arctostaphylos uva-ursi (bearberry, kinnikinnick) and Thymus serpyllum (mother of thyme), and herbaceous perennials, Arabis caucasica (rock cress) and Echinops bannaticus (globe thistle). A few examples of adapted deciduous shrubs and trees are Chaenomeles japonica (flowering quince), Crataegus laevigata cvs. (English hawthorn), Pyrus calleryana (ornamental pear) and Quercus robur (English oak). A conifer example, Juniperus virginiana (eastern red cedar) is tolerant of dry soil. Read more about suitable species for dry soil conditions at Drought Tolerant Plants For Your Garden [New Tab].
Dry shade
A combination of shade and dry soil can create a difficult planting situation. Dry shade is typically found under tree canopies where dense fibrous roots close to the surface compete with other plants for water. While plants will not survive extended periods of drought without some watering, there are some such as Berberis spp. that will tolerate dry shade once they are properly established. Alchemilla mollis (lady’s mantle), Epimedium hybrid cvs. (hybrid barrenwort), and Pachysandra terminalis (Japanese spurge) are suitable herbaceous ground covers for planting in dry shade. Learn about some practical approaches to planting in dry shade at RHS The Garden Dry Shade [New Tab].
Wetlands
Natural wetlands with soil that is permanently or seasonally saturated often have anaerobic (low oxygen) conditions. Wetlands are typically vegetated with hydrophytic plants that are adapted to grow wholly or partially in water. Some hydrophytic species float on the surface of water, while others are completely submerged. Emergent species that root in soil underwater and grow shoots up and out of the water are usually found along the shoreline or margin of a wetland.
While the roots of many garden plants would rot when deprived of oxygen, hydrophytic plants are suitable choices for sites with water features as well as low areas with seasonal poor drainage or a high water table. Examples of herbaceous perennials suitable for wetland planting include Acorus gramineus ‘Variegatus’ (variegated sweet flag), and Matteuccia struthiopteris (ostrich fern). Aronia melanocarpa (black chokeberry), and Sambucus nigra (elderberry) are adaptable deciduous shrubs for wet conditions as are the deciduous trees Liquidambar styraciflua (American sweetgum), and Salix x sepulcralis var. chrysocoma (weeping willow). Depending on the available space, the large conifer Metasequoia glyptostroboides (dawn redwood) may be a suitable choice. Learn more at RHS Gardening on Wet soils [New Tab].
Compacted soils
Compacted soils are common in urban areas that undergo construction damage, or repeated machinery use and foot traffic. Damage to soil structure from tilling or working heavy clay and loam soils when they are too wet or frozen, and crusting of bare soils from the impact of rainfall contribute to compaction. As soil particles become densely packed together pore space is reduced and the movement of air, water, organisms, and plant roots is impeded. Once compacted, poor soil drainage, water logging, low oxygen, and hard surface conditions inhibit plant root growth. Plants symptoms may include poorly formed or rotted roots, stunted growth, discolored leaves, and drought stress.
While the addition of compost is a long term solution for compacted garden soils, there are a number of species that are able to tolerate compacted soils reasonably well. For example, Catalpa speciosa (western catalpa) is a tough tree that tolerates poor soils and compaction as well as dry and wet soils. Acer saccharhinum (silver maple), Juglans nigra (black walnut), and Ulmus americana (American elm) tolerate some compaction as do Amelanchier canadensis (serviceberry), Juniperus communis ‘Green Carpet’ (Green Carpet juniper), and Matteuccia struthiopteris (ostrich fern). Read more about adapted species available at Plants for Compacted soils [New Tab].
Slopes
Sloped embankments and hillsides can be difficult planting situations. Successful plant growth will be influenced by soil type, the north to south aspect, the amount of rainfall, and the degree of incline and length of the slope. Steeper slopes increase the risk of erosion and soil loss that exposes roots or buries small plants. In addition, the run off of sediment from eroded slopes can adversely affect drainage systems and waterways that connect to fish habitat.
Planting slopes with grasses and shrubs is an effective way to protect soil and prevent erosion. Fast-growing, adaptable species with dense fine roots that hold the soil together and take up water help stabilize slopes and keep soil in place. Complete vegetation coverage will reduce the impact of rainfall and the potential for soil disturbance and erosion. Methods such as planting pockets and terraced steps will slow surface run off and facilitate the infiltration of irrigation for plant establishment.
Plants for slopes typically include native and ornamental grasses and low, spreading shrubs and ground covers that leave no areas of bare soil exposed to the elements. On hot, dry southern aspects, drought-tolerant shrubs and grasses such as Juniperus sabina ‘Tamariscifolia’ (tamarix juniper), Rosa rugosa (rugosa rose), and Festuca ovina glauca (blue fescue) are suitable options. Cooler, moister northern aspects are better suited for shade-tolerant understory shrubs and ground covers such as Gaultheria shallon (salal), and Pachysandra terminalis (Japanese spurge). Read more about slope gardening at Pacific Horticulture Society Dry Slope Gardening in Seattle [New Tab].