Selecting trees and shrubs for Minnesota landscapes
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- Landscape function
- Matching plant growth requirements to site growing conditions
Successful use of trees and shrubs in landscapes is dependent on selecting a plant that will fulfill a desired landscape function. Just as important is matching the growth requirements of trees and shrubs to growing conditions at the planting site.
Kathy Zuzek, UMN Extension
Figure 1. 'John Davis' rose accenting an entry gate
Figure 2. Boxwood are often pruned into formal hedges to provide screening and enclosure or to accentuate borders in a landscape
The decision to add a plant to a garden or landscape usually starts with the recognition that there is a space waiting to be filled and that a tree or shrub planted in this space will perform a specific job or function in the landscape. Some of these functions are listed below:
Accent plants have unique form, texture, or color and are placed strategically to move the viewer's eye through the landscape and to highlight or frame features such as an entryway or a pathway (Figure 1).
Border plantings divide spaces within a landscape or are used to accent the division between adjacent properties.
Foundation plantings are located in beds surrounding the base of a building and soften building edges or provide transition from the building to patios, gardens, or other landscape features near the building.
Key plants are planted singly or in small groups. They are used to provide a sense of scale, to provide visual transition from buildings and other large structures to smaller elements or the ground, or to soften harsh lines of architectural features such as building corners, walls, and steps.
Mass plantings are comprised of many plants of the same species that fill an area. Mass plantings provide a strong unifying effect and are used as connections between other planting groups or as groundcovers. They sometimes fill an ecological role when they are used in bank stabilization and to control erosion, especially on sloped surfaces.
Screen plantings create a sense of enclosure, provide privacy, or block wind, noise, or unwanted views (Figure 2). Trees are uniquely suited to provide outdoor ceilings or canopies.
Specimen plants are very prominent in the landscape because of outstanding ornamental features that draw the eye. They provide specific seasonal interest or color through flowers, fruit, leaves, stem color, bark, or plant form. Seasonal interest refers to the season, such as spring or fall, when the plant has exceptional ornamental value (Figure 3).
Wildlife plantings are comprised of plants that provide habitat or food for birds and other wildlife.
Kathy Zuzek, UMN Extension
Figure 3. The bright stem color of 'Cardinal' red-osier dogwood adds interest to Minnesota's winter landscape
Once the function or job of a new plant is identified, the second step is to select a or tree shrub that can both fill this function and has growth requirements that match the growing conditions at the planting site. This results in long term health and vigor of plants, sustainable attractive plantings, and reduced maintenance levels.
Kathy Zuzek, UMN Extension
Figure 4. A young burning bush already restricted by being planted too close to a structure
Healthy plant growth requires adequate space for plants to reach their mature height and width. Proper levels of light are required. Soils that provide space for root growth and the correct levels of nutrition, water, and oxygen are vital. A plant must also possess adequate cold hardiness to survive Minnesota's low winter temperatures. Knowledge of disease and insect pests most commonly found on tree and shrub species used in Minnesota landscapes provides gardeners with the option of selecting species or cultivars resistant or tolerant to pests.
A tree or shrub should be planted into a space which allows it to reach its mature size without being crowded against structures, pathways, or other plants. Plant growth characteristics to consider in determining the space that a tree or shrub needs for optimal growth are mature height and width, growth rate, and the plant form.
Mature height and width describe the plant size at maturity rather than its much smaller height and width at the time of purchase. Selecting and placing shrubs and trees in the landscape without considering height and width of plants at maturity results in crowding, blocked views and access through landscapes, inappropriate placement relative to other plants in the landscape, disappointment in the appearance of a landscape, plant health issues, premature removal of plants, and excessive pruning (Figure 4). On the other hand, proper placement that allows plants to reach their mature size without crowding avoids future problems that impact plant health, landscape appearance, and maintenance.
Growth rate, which is a plant's increase in height during one growing season, is also a consideration as trees and shrubs are placed in landscapes. Growth rate provides information on how long it will take a plant to reach its mature size. Growth rate is described as slow, medium, or fast:
- Slow: 12 inches or less of growth/year
- Medium: 12-24 inches of growth/year
- Fast: 24 inches or more of growth/year
Plant form describes the overall shape of a mature plant. Plant form is an important factor to consider in determining the planting space needed and in siting a shrub or tree relative to the positions of other plants in the landscape. There is a wide variety of shrub plant forms:
- Arching plants appear umbrella-like. Stems first curve upwards, then spread out horizontally, and finally curve back downward.
- Columnar plants are cylindrical and are much taller than they are wide.
- Irregular plants appear asymmetrical and have an uneven outline.
- Mounded plants have a broad elliptic plant form because they are at least twice as wide as they are tall.
- Oval plants are elliptic to egg-shaped and they are at least twice as tall as they are wide.
- Prostrate plants appear to be lying on the ground and have a horizontal and flat outline.
- Pyramidal plants are shaped like a pyramid and narrow gradually to a point at the top of plants.
- Rounded plants have a rounded circular form because their height and width are close to equal.
- Spreading plants have an erect, upright branching structure through the lower and middle parts of their canopy and then branch at a 45-degree angle in the upper crown.
- Upright plants have main branches that are stiffly vertical and diverge at a slight angle from vertical.
- Vining plants trail on the ground or when given support, climb by twining, tendrils, aerial roots, or other means.
- Weeping plants have secondary branches that hang or droop.
Mature plant size, growth rate, and plant form are found on labels attached to plants for sale in garden centers. All of these traits are largely determined by genetics. But variation in size and growth rate can occur across large geographic due to differences in climate and growing conditions. Consulting with gardening peers and horticultural professionals can provide gardeners with accurate size estimates for a particular geographic area.
Shrubs and trees have specific preferences for light and may fail to thrive when these conditions are not provided. Before selecting a plant for landscape use, observe the light level where the tree or shrub will be placed and use this information to select plants whose light requirements match these levels.
David Zlesak, University of Wisconsin
Figure 5. Trees often create patterns of part-day sun and partial shade
Light levels can be categorized as:
- Full sun: A site receiving at least 8 hours of direct sunlight each day. Most tree and shrub species perform best in full sun. Siting these plants in less than full sun can result in lack of flowering, open plant form, and weak spindly stem growth.
- Part-day sun: A site receiving distinct periods of sun and shade each day (Figure 5) as the sun's angle changes the location of shade. A pattern of morning sun and afternoon shade is an example of part-day sun and is ideal for many gold-foliaged and variegated plants that may suffer from leaf scorch in full afternoon sun.
- Partial shade or filtered shade: The amount of shade under the canopies of large trees varies depending on the density of their leaf canopy. Tree species with fairly open canopies such as honeylocust cast dappled or filtered shade so that the light below the canopy is much reduced in intensity. Species that grow in filtered shade are called shade tolerant species.
- Full shade: Trees with dense canopies such as sugar maples and Norway maples allow very little or no light to reach the ground below them. The shade cast by buildings, especially on their north sides, can create similar full shade conditions.
Light requirements of trees and shrubs are found on labels attached to plants for sale in garden centers, in garden catalogs, in reference books on woody plants, and on trusted websites.
Soils serve as reservoirs of the nutrients, water, and oxygen that plants need for growth. They also provide growing space for plant roots. As a result, they have enormous impact on plant health and vigor. Because different soil types vary in their ability to provide plants with these resources, soil analysis that provides information on soil texture, soil moisture, and soil pH is an important step in the process of plant selection.
Figure 6. The composition of a silt loam soil is often considered the "ideal" soil for growth of most plants with 45% mineral particles, 5% organic matter, and 50% pore space evenly occupied by water and air.
Much like a sponge, the soils we garden in are a blend of solids and different sized pore spaces between the solids. Soil solids include the large amounts of mineral particles (sand, silt, and clay) and a much smaller amount of organic matter (remains of plants and animals in some stage of decomposition) that are the sources of most plant nutrients. In between the solid particles are pore spaces that vary in size. Depending on their size, these pore spaces provide space for roots to grow in, contain the oxygen needed for healthy root growth, and/or contain the water and dissolved nutrients that plants absorb. The arrangement of all of these soil components – solids, space, nutrients, water, and oxygen – varies widely among soil types and determines which plant species grow and thrive in any one soil (Figure 6).
Soil texture is the term used to describe the proportion of large-sized sand, intermediate-sized silt, and small-sized clay particles in any one soil. Soils contain a mix of at least two and usually all three of the mineral particles. The soil textural triangle is a good illustration of how varied the proportion of sand, silt, and clay particles is among various soils (Figure 7). Soil structure describes the arrangement of the sand, silt, and clay particles into larger stable aggregates that are created through the binding effects of organic matter, microbial activity, and clay and iron components in soil. Together soil texture and structure create the variety of small, intermediate, and large pore spaces within soils; impact the levels of plant nutrients, oxygen, and moisture available to plants; and impact a root's ability to penetrate and grow through soil.
Soil structure and its influence on nutrient, water, and oxygen levels can be improved through the addition of organic matter (compost, manure, grass clipping, etc.) to native soils. On the other hand, soil texture and its influence on these resources is a fixed trait that is difficult if not impossible to alter. The fixed nature of soil texture and its impact on plant growth can be seen when the soil textural triangle (Figure 7) is grouped into three broad textural groups: sandy soils, loam soils, and clay soils.
The sand group (blue) in the bottom left-hand corner includes soils whose sand particles make up 70% or more of their mineral material by weight. Sandy soils are typically low in nutrients, allow water to move too quickly out of root zones, and have low moisture-holding capacity (Table 1); plant growth on sandy soils can be limited during the growing season by low levels of both water and nutrients.
Table 1. Soil traits of sandy, loam, and clay soils
|Soil trait||Sandy soils||Loam soils||Clay soils|
|Nutrient holding capacity||Low||High||High|
|Moisture holding capacity||Low||High||High|
Loam soils (orange) in the center and bottom right portions of the triangle are the largest group of soils in the textural triangle. Loamy soils are considered the "ideal" soil type for many plants because they provide the best combination of nutrient, water, and oxygen availability. The intermediate soil density of loam soils also allows for easy penetration and growth of roots.
Clay soils (green) at the top of the triangle have the highest proportion of clay particles. Nutrient availability is usually high in clay soils but water movement in these soils is slow. These soils often have poor drainage and often retain too much water within pore spaces at the expense of oxygen availability. Plant growth on clay soils can be limited by poor nutrient uptake and poor root growth that occurs when soil oxygen levels are low. Densities of some clay soils are also high enough to make root penetration and growth through the soil difficult or impossible.
Soil moisture is usually described in terms of available water capacity and drainage. Available water capacity, often referred to as moisture-holding capacity in gardening references, is the amount of water that soils can hold for plant use. This water is easily absorbed by plant roots and is the supply of water for plants between rainfall or irrigation events. Informal terms describing soils with increasing available water capacities are dry or droughty soils, slightly moist soils, average or moist soils, and wet soils.
Drainage refers to the length and frequency of soil saturation and is largely determined by soil texture (Table 1). Terms describing soil drainage include excessively drained soils, well drained soils, and poorly drained soils. Water in excessively drained soils is removed so thoroughly from soil that plant growth is usually inhibited by lack of water. The water in well drained soils is removed easily but not rapidly. Plant growth is not restricted by too much or too little water on well-drained soils. Poorly drained soils are waterlogged for long periods of time. Plant growth on these soils is often inhibited by lack of oxygen because water occupies the spaces that oxygen would normally occupy.
Available water capacity and drainage are soil traits that can be difficult to change and improve. Improving soil structure through the addition of organic matter can result in small increases in available water capacity and small improvements in drainage.
Figure 8. The effect of soil pH on nutrient availability; the wider the band, the greater the nutrient availability
Soil pH indicates a soil's acidity or alkalinity and has enormous impact on the availability of soil nutrients to plants. Soil pH values range from 1 to 14. At pH 7, soils are neutral. A soil with a pH less than 7 is considered acid and soils with pH levels above 7 are alkaline. Nutrients are taken up by plants as dissolved ions in soil water. Soil pH has enormous effects on the chemical form, the solubility, and the availability of soil nutrients for plant uptake (Figure 8). As with soil texture, soil pH is very difficult to change or to maintain after modification.
Shrubs and trees have specific preferences for soil pH that result in optimal growth. Most woody plants prefer to grow in a soil whose pH is slightly acidic or neutral (pH = 5.5-7.0) because at these levels, all of the essential elements that plants require for growth are readily available. Some species are more adaptable and will grow across a wider range of soil pH ranging from slightly acidic to alkaline. A very small subset of woody plant species are called acid-loving because they perform well in strongly acidic to moderately acidic soils with pH levels between 4.0 and 5.5. Planting a tree or shrub other than those adapted to the pH of a native soil often results in poor growth or plant mortality when nutrient toxicities or deficiencies occur.
Soil Testing: All plant species have preferred soil textures, soil moisture conditions, and soil pH values that provide for optimal growth. Some plant species have a very narrow set of soil environments that they grow and thrive in. Other species are more adaptable and grow over a broad range of soil conditions. A species' soil preferences can be found in garden catalogs, in reference books on woody plants, and on trusted websites.
A routine soil test provides information on soil texture, organic matter, and soil pH that gardeners can use as they select plants. Because soil texture and drainage are closely related, soil texture can be used to as an indication of drainage (Table 1). Information on how to collect and submit a soil sample for analysis is available at the University of Minnesota Soil Testing Lab website.
Armed with information on soil preferences of plant species and soil test results, gardeners and landscapers are well equipped to select trees and shrubs that will have long and healthy lives in the landscape.
Shrubs and trees used in Minnesota landscapes must be able to survive Minnesota's low winter temperatures and to grow and thrive in the subsequent growing season. In order to rate the hardiness of landscape plants, the United States Department of Agriculture developed a Plant Hardiness Zone Map which shows the average annual minimum temperatures (aamt) across different areas of the United States (Figure 9). Thirteen hardiness zones differentiated by 10° F differences in aamt are designated. Zone 1 (with aamt between -60 and -55° F) is the coldest and Zone 13 (with aamt between 65 and 70° F) is the warmest. The zones are further divided into sections "a" and "b" by 5° F increments, with "a" being the colder section.
State hardiness maps have also been created. A look at Minnesota's map shows that plants must be hardy to Zone 3, 4, or 5 depending on where they grow in the state (Figure 10). Shrubs selected for landscape use in the northern half of Minnesota should be able to survive minimum temperatures of -40 to -35° F in Zone 3a or -35 to -30°F in Zone 3b. Shrubs selected for use in the southern half of Minnesota should be able to survive minimum temperatures of -30 to -25°F in zone 4a or -25 to -20°F in zone 4b. A small area in south central Minnesota is Zone 5a where average minimum temperatures between -20 and -15° F can be expected.
Cold hardiness information can be found on labels attached to plants for sale in nursery centers, in woody plant references, and on trusted websites. Information will be listed by hardiness zone or by temperature that plants are hardy too.
Another consideration in the selection of woody plants is the prevalence of disease and insect pests common to trees or shrub species grown in Minnesota. Knowledge of pests common in Minnesota landscapes gives gardeners the option of selecting species or cultivars that 1) are not hosts to these pests or 2) have resistance or tolerance to these pests. Insects and diseases most commonly found on tree and shrub species used in Minnesota landscapes can be found on the What's wrong with my plant? online diagnostic tool.
Deciduous trees and shrubs drop their foliage each autumn and grow new leaves in spring. As deciduous plants prepare for leaf drop, the green pigment chlorophyll is lost from leaves. This often results in attractive fall color in tones of yellow, orange, or red depending on the species and cultivar.
Evergreen trees and shrubs retain their foliage throughout multiple growing seasons. Most evergreens in Minnesota are narrow-leafed and have needle- or scale-like leaves. Junipers, arborvitae, pines, spruce, and yews are examples of evergreen. There are a few broadleaved evergreen trees and shrubs that grow in Minnesota. Boxwoods and rhododendrons are examples. Although fall color is lacking in evergreen shrubs and trees, their green foliage adds interest to winter landscapes.
Many gardeners are interested in landscaping with plants native to the United States or to Minnesota. A plant's native range is the geographic area that a plant species originated in as a result of natural processes and without human involvement. Native species have evolved over many thousands of years and are adapted to the existing conditions - climate, soils, competing species, predators, diseases and insects – of their native range.
Plants native to Minnesota are the plants indigenous to Minnesota. In contrast, non-native plants in Minnesota are those native to other states or countries that are now grown in Minnesota, usually because they were introduced by humans for horticultural or other agricultural purposes. Many non-native tree and shrub species are used in Minnesota landscapes with great success because these plants were introduced from areas with growing conditions similar to those in Minnesota.
Problems can occur though when non-native plants become invasive. Invasive plants are non-native plant species that are now growing in Minnesota and have become harmful in some way to Minnesota's native plant ecosystems, to Minnesota's economy, or are harmful to human health. Escape of these plants into native ecosystems often occurs when wind moves seed into native areas or when birds and other wildlife eat the fruit produced on these plants. New plants establish as seeds are passed through the digestive tracts of these animals, become widely dispersed into new areas, and germinate. If these non-native plants win the competition with native plants for space, light, nutrients, water, and oxygen, they become invasive. Examples of invasive woody species in Minnesota are Amur maples (Acer ginnala), black locust (Robinia pseudoacacia), common buckthorn (Rhamnus cathartica), glossy buckthorn (Frangula alnus), exotic honeysuckles (Lonicera tatarica, L. morrowii, and L. x bella), Japanese barberry (Berberis thunbergii), multiflora rose (Rosa multiflora), Norway maple (Acer platanoides), Russian Olive (Elaeagnus angustifolia), Siberian elm (Ulmus pumila), and Siberian peashrub (Caragana arborescens). All of these species were introduced into Minnesota for use as landscape plants but are now growing in native ecosystems and replacing native plants within these ecosystems. Planting of these species is not recommended even though some of them are still available to gardeners at garden centers.