What is urban heat? Dark surfaces like concrete, asphalt and brick absorb and retain heat from the sun. Little spaces between buildings can create heat canyons that trap this heat, forming “islands” that are warmer than rural or suburban areas.
Urban heat can affect us in many ways, such as increasing heat stroke, heart and lung diseases, energy costs, general discomfort, hyperthermia, and poor air quality. The U.S. Environmental Protection Agency projects that by 2040 Albuquerque will see four times the current annual number of days with temperatures of over 100 degrees.
But trees can help! Trees provide shade to keep us and the ground cool, as well as cooling the air through water loss, or evapotranspiration. Increasing the urban canopy can lower city temperatures.
Let’s Plant Albuquerque is a broad community alliance dedicated to planting 100,000 new trees in the city by 2030. The alliance represents a broad range of civic, government and community organizations dedicated to this goal.
DEVELOP, a NASA Applied Sciences Program, studied Albuquerque’s urban heat. DEVELOP conducts feasibility studies that bridge the gap between Earth science information and society and works with communities and organizations to address environmental and policy concerns.
NASA DEVELOP found that a 30% increase in tree shade can help offset rising temperatures and allow neighborhoods to be cooler. Trees can also increase your body’s “thermal comfort,” how cool and comfortable you feel on hot days.
The city’s parks canopy alone provides many benefits, including energy savings, air quality improvements and increased property values. The nearly 30,000 trees in the city’s public spaces provide:
$1.7 million in carbon monetary benefit
$624,000 in stormwater monetary benefit
$3.6 million in (overall monetary benefit
The Water Authority would like to encourage you to help cool our city by planting trees. The Tree-Bate program helps offset the cost of planting and maintaining trees.
When you buy a new tree, you can receive a rebate equal to 25% of the purchase price up to $100 a year. To help customers narrow down their purchasing decisions, the Water Authority assembled a list of 20 trees that are proven to thrive in our area, are commonly available for purchase, fit a variety of situations and provide numerous environmental benefits. A broader list of more than 160 qualifying trees is in the Water Authority’s Xeriscaping Guide. They include:
Description: The vibrant bright red fall color of this maple is a showstopper in gardens. Mesa Glow®, a cultivar of the native bigtooth maple, was developed by Rolston St. Hilaire at New Mexico State University. It is an upright, oval-shaped tree with dark green leaves that has been cultivated to be drought tolerant, resistant to fungal disease and fast growing as well as to display a bigger red color than other bigtooth maple cultivars.
This plant is used as a groundcover, and its beautiful 2-inch wine-colored, cup-shaped flowers grace the garden. The flowers hang on long cord-like stems that weave into the plants around them, almost looking like they are floating around other plants in garden beds. Also known as purple poppy mallow, they are great in sandy soil, rock gardens and cascading over walls. Winecups will go dormant in winter, shrinking to just a few leaves above the ground. But beware, rabbits find this plant delicious.
As one might guess, the subtropics are a great place to grow fruit trees and a particularly good place to find plants that are heat and drought tolerant — sort of a prerequisite for plant selection in a hot, dry place like New Mexico, or at least it should be!
The term subtropical refers to a climatic region, defined by latitude, that lies between the tropical and temperate zones. This region experiences warm to hot summers and mild winters with overall annual temperature variation more significant than in tropical regions but less extreme than in temperate zones. There is also plenty of rainfall variation across the subtropics, with climates ranging from arid to humid. Subtropical regions often experience seasonal rainfall patterns such as monsoons or wet and dry seasons.
The problem with growing heat and drought tolerant subtropical plants in our high desert is that we have anything but mild winters. Much like another inhabitant of the subtropical/mediterranean climate of Greece, the hero Achilles known for his one weakness, these plants have a very specific vulnerability — cold.
I love to grow plants, fruit in particular. But as irrigation access has become harder to come by and the summer heat seems to last longer than it used to, I have become more and more attracted to plants that thrive in the hot and dry. Navigating the puzzle of a changing climate while planting trees and shrubs is challenging. Long-lived perennials need to be appropriate for today's climate as well as the climate 20 years from now and even the climate 250 years from now if you are a real optimist.
It's tempting to just plant a zone or more higher than the current USDA Plant Hardiness Zone Map (which has itself been updated to reflect higher temperatures twice since 2012), but our weakening jet stream means more chance of Arctic air slipping in and spoiling the fun as we experienced to the extreme in the winter of 2011. The way things are headed has, in many cases, led us to exchange one set of growing problems — namely, heat and drought stress — with another — cold damage.
Two fruiting shrubs from the subtropics, pomegranates and figs, have proven to have the right combination of fruiting characteristics and cold tolerance to work in our moving-target-Goldilocks-growing-zone of Central New Mexico.
The fig is extremely adaptable. I imagine this has something to do with why they have evolved and been grown by humans for thousands of years. They perform well in Albuquerque, particularly because of all the heat that gets banked into concrete, asphalt and structures during the day. This heat radiates back out at night, keeping figs at much warmer temperatures than they would otherwise experience during winter in a more exposed area. Figs are tolerant of abuse and relatively content with our intense sunlight, extended droughts and alkaline soils. They can take temperatures down to the low teens, but need to be protected, buried or even brought inside if temperatures fall below that.
There are plenty of examples around town of figs without protection that rarely, if ever, freeze to the ground. Even so, freezing to the ground does not mean death for a fig — a winter-killed fig of an early ripening variety can easily end up fruiting by the end of the summer. That's because figs often come up from the roots and fruit their main crop on the current season's growth. Given an early enough ripening variety, that once-frozen fig can still yield fruit in the same year!
Pomegranates, I have found to be a bit more cold-hardy than figs — although they, too, benefit from a protected, south-facing location to improve winter survival. Optimal conditions for the wild pomegranate exist in high sunlight, hot climates with minimum temperatures not lower than 10 degrees. Although, with the protective care of a thoughtful gardener, lower temperatures are negotiable. There is also significant variation in cold-hardiness among pomegranate varieties, with dwarf varieties experiencing damage at 19 degrees, many soft-seeded varieties at 10-12 degrees and hard-seeded varieties at 0-3 degrees.
For both pomegranates and figs, young plants are more frost sensitive to cold than mature plants. So if you have trouble the first year or two with losing branches to cold, remember that these plants get more frost resistant with time. These are both wonderful subtropical fruiting shrubs that are worth the extra trouble during our high desert winter to experience their summertime beauty and bounty.
Learn more about specific types of gardening here:
Author: Graeme Davis is an ISA certified arborist and the owner of Flora Fauna Farm, a nursery that focuses on plants that grow well in the high desert. Flora Fauna Farm grows a diversity of edible trees and shrubs, useful native plants and unique landscape plants curiously underrepresented in the urban canopy of Albuquerque. You can see its offerings at www.florafauna.farm. Have a question about the article? AskAnExpert@abcwua.org
Passive rainwater harvesting allows you to collect rainwater runoff from roofs, patios and driveways as well as the overflow from rain barrels and cisterns (active rainwater harvesting sources) and direct it into the landscape. The water then gets stored in the soil where plants can take advantage of the moisture.
Several rainwater harvesting techniques can turn potential problems (i.e. erosion, flooding or excess moisture near foundations) into water for your plants. Rainwater contains extra nutrients not always found in potable water, so the plants receive additional benefits. Passive rainwater harvesting also helps filter out pollutants, increase infiltration to the aquifer and provide healthier ecosystem habitat.
Start by calculating the amount of water that flows from your potential runoff sources. For every 1,000 square feet of hard surface, 1 inch of rain will generate approximately 600 gallons of water. It’s easy to see why rain barrels or cisterns can quickly fill during medium to larger rain events, so a passive collection system for overflow is always a good idea. You want to make sure whatever system you create will be able to handle a
good-sized rain event (refer to the link above). It’s very important to create a directed overflow so that when there is a large rainstorm, the excess water will flow into a basin or other feature onsite or into the street (if your capacity is limited) without creating erosion.
There are many different techniques, and some can be used together to enhance the benefits. These techniques include rain gardens, swales, basins, soil sponges, curb cuts, French drains and pumice wicks, check dams and gabions. Study your site and these techniques to determine the best option for your garden. Watch your site during a rain to see where the runoff comes from and where it goes. You may want to take advantage of existing patterns.
Rain gardens are depressions to catch surface runoff from slopes and are best planted with native plants that can take occasional flooding.
Swales are sloped depressions that convey water into basins or rain gardens from canales, downspouts or other drains. Make sure the slopes consistently move the water downhill and are large enough to not get overwhelmed. Swales are usually lined with large gravel or cobble to prevent erosion. If you have extra material onsite, this is a great recycling solution. Make sure that swales are designed to catch the slow drip of a light rain as well as the powerful flow of a heavy downpour.
Basins are catchment ponds designed to handle large rain events and provide a great place to plant trees and larger shrubs or heavy water use plants that can benefit from the extra water. Calculating the size to match your runoff is vital here. The area where the runoff from your swale(s) enters the basin will need gravel to dissipate the erosive forces. Shredded wood mulch works best in the bottom of the basin. The capacity of the basin and the health of the plants can be further enhanced by installing soil sponges (see below) in the low points. In larger yards, you may want to create a series of basins to increase capacity and opportunities for plantings.
Soil sponges are post holes dug and filled with a soil mix containing compost to increase infiltration and introduce organic matter and beneficial microbes to the soil sponge and surrounding soil. These also provide an avenue for plant roots to quickly grow deeper, improving health and drought tolerance.
Curb cuts are made with a concrete saw to channel runoff into a basin. This can be done on your own property, but permission is needed to cut into a public curb to direct excess water into the street.
French drains and pumice wicks are progressively deeper trenches lined with geotextile fabric (to prevent silting up) and filled with gravel or pumice. These are best used when you have existing trees or large shrubs and the base of those plants is on the same level or higher than where your runoff originates. Aim them directly at the tree or shrub to minimize root damage while still providing additional water to the plants’ root zone. The open space in pumice provides additional water capacity.
Check dams and gabions are small dams used in areas where existing runoff has started to erode and cut deeply into the soil. These small dams slow down the water so it infiltrates and lessens erosion. Gabions are a type of check dam constructed of riprap (oversize gravel/cobble) held together with a wire basket. Generally, both have an area of cobble below to prevent erosion.
Passive rainwater harvesting is a great way to lessen your use of potable water, reduce erosion and provide a supplemental water source for your garden. Remember, you still will need to water your plants (through irrigation or hand watering) to establish them and, in most cases, to maintain their health.
