Planting Trees and Shrubs in Times of Drought and Water Restrictions

Recommendations for Responsible Planting and Care 

• Planting trees and shrubs during times of drought and water restrictions should continue with caution.  An understanding of the risks associated and consideration of proper maintenance activities to establish trees during these periods is crucial.

• Prudently planting trees can replace drought stressed and dead trees, which can help reduce the negative effects of drought on the landscape.

• Keeping trees in the landscape helps reduce soil erosion, stabilizes soils, significantly reduces storm water runoff and shades landscapes and structures to help minimize water and energy use.

• Factors to consider when planting trees and shrubs include soil conditions, available space above and below ground, exposure, moisture and light requirements.

• Planting smaller trees (2 inches or less caliper for deciduous trees and 6 feet or less height for evergreen trees) reduces the investment and risk of planting during drought periods, can establish a tree more quickly than planting a larger tree and will require less maintenance over time.

• Select from species that are hardy to the region and fit well with the Xeriscape principles of maintaining an attractive landscape with minimal water use.
  
• Proper mulching and adherence to watering guidelines for trees and shrubs will help establish newly planted trees in times of drought.  

Why you should plant trees and shrubs during times of drought and water restrictions:

Plant trees and shrubs during drought can be risky.  Watering restrictions are in place and establishing trees in a semi-arid region is difficult enough without an extended drought to contend with.  However, by eliminating all tree planting we will be missing many opportunities to keep our urban forest alive and vital in our region.  It is critical to not only have a diverse set of species in the landscape but also a diversity of age among those species in the landscape.  This means planting new and replacement trees each year, especially during times of drought, to replace trees and shrubs that will be lost to age, injury and other causes.

Plant with care and prudence during drought periods.  Using smaller trees (2 inches or less caliper for deciduous trees and 6 feet or less height for evergreen trees) minimizes the investment risk and can also lead to earlier establishment and lower maintenance of the plantings over time.  Carefully planting trees and shrubs will preserve the resources that are critical to maintain soil stability, reduce soil erosion, control and utilize storm water runoff, shade our moisture-starved lawns and reduce energy usage by shading homes in summer and blocking winds in winter.  A well-stocked urban forest also acts as an air filter and purifier, absorbing carbon dioxide and emitting oxygen to help provide cleaner air.

Finally, planting trees and shrubs during drought periods will help increase the visibility of a diverse set of species that tolerate our environmental conditions in this region and fit well with Xeriscaping (low water use landscapes).

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Other important factors to consider if planting trees at this time: 

• Selection - if the landscape calls for planting trees, buy them 2 inches or less caliper for deciduous trees and 6 feet or less height for evergreen trees. Don't invest in large trees that may die because they can't be adequately watered. Smaller trees require less water to get them established. 

• Available space - both in terms of soil rooting area and airspace - a mature tree develops a root system that extends well beyond its branch extremities. A healthy mature tree has much more biomass in its root system than its above-ground leaves, branches and twigs. Big trees need large rooting areas. For example, a honeylocust planted in a narrow parking lot planting bed is essentially doomed to a short and stressful life. Trees that grow to be large should not be planted too close to other trees, garden areas, buildings, sidewalks or to a property line, where they would encroach upon adjacent properties. Trees that grow to be large should not be planted under utility lines or within prescribed distances from them. Before planting in areas with underground utilities, contact the Utility Notification Center of Colorado at 1-800-922-1987.

• Moisture - many tree species thrive where rainfall exceeds 30" annually. The Denver area receives 14-18" annually, so supplemental water is needed for many landscape trees. Humidity is very low in the Denver area; many species prefer higher relative humidity. On a smaller scale, the wetter, low-lying areas of a property can support different species than higher and drier spots. 

• Exposure - north-facing slopes are cooler, moister and retain snow longer than south-facing slopes; east-facing slopes are cooler and moister than west-facing slopes. Similarly, the north and east sides of a house are often more conducive to certain tree species than the west or south sides. For example, maples prefer cooler, moist soils - so they grow better on north or east exposures. The reflected heat and dryness of a south exposure is better suited for other species.

• Light - most trees prefer full sunlight; many can tolerate partial shade. A few species known as "understory" trees are usually smaller trees that grow in the shade of larger trees in their native habitat. Note that some cultivars (horticultural selections) that have variegated leaves may "scorch" in the high-intensity sunlight of our mile-high area; these may fare better in partial shade or on east exposures.

• Plant Grouping - Plants in the forest and other natural settings are often found in “communities” of like plants. There is strength in numbers. This concept is applicable to landscape gardening as well. Grouping plants with similar cultural requirements and water needs makes watering and maintenance easier. Plants in groups are generally more likely to thrive than individual plants sprinkled throughout the landscape. 

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Making good choices in plant selection

Many plants are commercially available that may survive with extra care and attention, but are not necessarily well suited to a semi-arid climate. Plants appropriate to this climate will require less water, be less susceptible to pests and disease, and live longer than plants not suited to the semi-arid west. A list of regionally appropriate plants is provided below: 

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Xeric Plants and the principles of Xeriscaping 

Development of new themes in landscaping using dryland or xeric principles is radically different from the traditional approach to landscaping as it has been commonly practiced in the Front Range area. The Front Range is semi-arid, and we are just coming to grips with just how semi-arid it is.  Xeric plants are low to moderately low water use plants in the landscape.  Be aware that even low water use plants must be watered well in order to become established.  Once established, xeric plant materials need much less water and maintenance than plants not suited to semi-arid conditions. 

The traditional approach using vast swaths of bluegrass lawn in conjunction with small and peripheral shrub and perennials beds along the edges needs to be rethought.  A more practical solution will reduce water usage dramatically and will retain an attractive and vibrant landscape. One goal of revising our landscape water needs is to save existing trees and shrubs, plants that have been in the Denver landscape for years. Dryland landscape schemes provide the homeowner with options that can be used to create a landscape that is water wise from the beginning and offers an attractive alternative to expansive areas of lawn. 

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Xeriscaping principles keep a landscape attractive and water efficient 

The following are suggestions on how to keep a landscape attractive and water efficient. 

• Shrub Beds - enlarging beds under trees to the edge of their drip lines, extending shrub and perennial beds outward from along the edge of the house reduces the amount of bluegrass lawn as a percentage of the overall landscape. 

• Soil Amendments – prepare tree and shrub beds by adding compost to the soil.  This improves soil texture and adds essential nutrients that plants use. Adding organic matter to the soil helps it retain moisture as well. 

• Irrigation Systems - redesign sprinkler systems to achieve an efficient irrigation system. Drip irrigation systems apply water directly to the soil and are very efficient. 

• Plant Selection - install dryland plants in the tree and shrub beds. There are many attractive and colorful perennials and versatile groundcovers that are low water users in addition to the trees and shrubs listed above.  Grouping plants with similar cultural requirements and water needs makes watering and maintenance easier. 

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Make Your Landscape More Water Efficient

Using an organic mulch such as wood chips, bark, leaves and evergreen needles around trees and in shrub beds will provide several benefits. A four inch layer of mulch under trees and around shrubs and perennials goes a long way to keep plants alive and healthy. Mulch helps to regulate soil temperature resulting in less stress on plants between hot, dry summer days and freezing winter nights. Mulch allows for less and easier weeding of beds. Organic mulches, as they gradually break down, add nutrients to the soil.  Mulching around the base of trees also keeps the lawn mower and weed eater from damaging the bark of trees.           

Most importantly, mulching reduces water usage. A mulched area under low-water-use trees with dryland shrubs or perennials can reduce water usage by as much as 50 percent from the water needed to maintain a bluegrass lawn. 

Mulching mature trees to their drip line is beneficial as well. For a larger-sized tree this may extend a mulch circle outward from the trunk 20 feet or more, greatly reducing the amount of lawn. Having mulch to that point helps retain moisture in the root  area.

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Xeric Plant List (click on highlighted names for photos and/or more information)

Low Water Use Deciduous Trees