In general, shade trees are not very tolerant of drought. Some people have gotten the idea that since trees have big root systems, they are less vulnerable to dry conditions. Unfortunately this is not true. Truly xeric trees that are native to arid regions are usually small, more like big shrubs. Even in our semi-arid region, photos from a hundred years ago, show very open and unshaded areas where we now have our urban forests. Most shade trees come from forested areas with much more rainfall and deeper, richer soils. On top of that, our trees are often made to grow in very confined areas in heavily compacted soils. So even under normal circumstances, big trees have a hard life here in Colorado. Therefore, during a drought, we must take extra care to make sure they survive by deeply watering at least once a month. If you multiply the diameter of a tree in inches times 10, you will get the recommended gallons of water required per watering. This water should be applied, not next to the trunk, but in a wide band around the drip line.
Your best chances of establishing a tree during water restrictions are to:
1.Plant a container-grown tree with a complete root system.
2.Plant a small diameter balled-and-burlap tree 1½—2½” (B&B trees are very vulnerable to drought because they have lost 75%—90% of their root system when they were dug).
3.Plant early- April/May.
4.Mulch 3—4″ deep with a coarse mulch, not touching the trunk.
5.Install a drip system at planting.
6.Don’t plant more trees than can be cared for with hand watering.
7.Water twice a week until established (B&B may need more watering).
The native trees that make the best shade trees are Common Hackberry and Green Ash. The cottonwoods and willow in the list below are better planted near streams than close to houses where their brittle wood can be a hazard. The conifers cast less shade, but can be valuable especially on the west side of a house to protect from the late afternoon sun. The Common Hackberry, Celtis occidentalis is a very tough and adaptable tree. It is hardy to zone 2 and tolerates alkaline, rocky and clay soils. It thrives in rich moist soil but does just fine in poor, dry, windy and polluted city conditions. It is known as one of the fastest-growing hardwood trees, growing to 40—65′ tall and almost as wide. At first it is gangly, becoming pyramidal and then spreading. Its wood is very white and strong, but because it grows very densely, it is best to keep the twiggy growth thinned. This will prevent snow and wind damage.
In general, it is a very healthy tree, but the leaves can be marred by the Hackberry Nipple Gall; this is rarely significant. I have several, grown from 18″ seedlings that have been watered only a few times in 15 years. Green Ash, Fraxinus pennsylvanica, is certainly well-adapted to Colorado. It is one of the three most commonly planted landscape trees. On top of that, female trees seed thousands of seedlings which produce some good trees and many hard-to-remove “weeds”. For this reason, nurseries usually sell the seedless male clones like “Marshall’s Seedless”. They grow 50—60′ tall and wide, branch very densely with somewhat drooping branches and are best thinned to prevent breaking in our heavy wet spring snow storms.
Perhaps the most drought-tolerant conifer native to our area is the Ponderosa Pine, Pinus ponderosa. It usually grows to 50—75′ with more irregular branching than spruces and firs. The attractive bark can be dark or cinnamon-red in irregular flakes. It is found growing on the dry hillsides and valley floors of our local mountains. Its very long needles may be the instrument of its music played by the wind. In the landscape, Ponderosa Pine allows for more light penetration than most conifers and small branches may be removed without ruining its shape, to allow even more light into windows and onto flower beds. It is deep rooted, preferring good drainage, and is best planted outside the lawn area in full sun.
In the category of non-native shade trees, there are only a few that tolerate drought. One is Western Catalpa, Catalpa speciosa. It is surprising that a tree that looks so different from the other Colorado-adapted trees, is so successful. What stands out immediately are the huge leaves which can be 3″x6″ or even 6″x12″ and are heart-shaped. In June this large tree blooms with exquisite, ruffled, bell-like, white, fragrant flowers with yellow and purplish markings. And through the fall and winter, thin pods,10—18″ long, decorate the tree. This exotic-looking tree comes home from the nursery looking like a stick, generally lacking a well-developed crown, but grows rather quickly, eventually reaching 40—60′ tall and 20—40′ wide. This upright form makes it useful in narrower spaces. Its habit of leafing out very late in spring may contribute to the fact that it seems to get little storm damage. It is very adaptable as to soils and water conditions, thriving in moist, rich soils, but also doing fine in dry, lean and alkaline soils. However a monthly soaking will help it grow faster and be stronger.
One of the most successful oaks for Colorado is the Bur Oak, Quercus macrocarpa. It is adaptable to our clay soils and tolerates our alkaline conditions better than most oaks. It grows 50—70′ high and the powerful horizontal branches can extend the width equal to the height. The overall effect can be quite grand and sculptural, sometimes like a Chinese painting. The short taproot is surrounded by a massive root system which is strongly competitive. The acorns are distinctive with a mossy fringe around the cap. Bur Oak is hardy to zone 2, withstands strong wind, city pollution and drought; and is a good choice to plant on the west side of the house in a windy location. Fall color is usually yellow turning to brown. Is slow growing and can live for 300 years or more.
I should say a few words about Silver Maple, Acer saccharinum. Isn’t it interesting that this tree that is so successful in our area, gets discouraging reports and is not allowed to be planted as a street tree in Boulder, for example. Yes, it is true that Silver Maple has somewhat brittle wood and can develop hollow cavities, and it can have lanky growth. However I believe this is a case where “the use or uselessness depends on qualities the user lends.” Because it is our fastest-growing maple, it needs to pruned every 3-5 years. Tipping the leggy branches by 20% or so will keep them much stronger, and clean-cutting branches broken during storms will allow the wounds to close instead of developing decay and hollows. Silver Maple is not extremely drought tolerant, but I would certainly call it a water-wise tree, and it is graceful and charming when well maintained.
In Boulder, Mapleton Hill was made famous by this tree, and the Norway Maples that were planted there as preferable replacements, did not fare nearly so well in the heat and drought of 2002. Water wise trees in the 15—40′ height range are more common. One of the natives, Big Tooth Maple, Acer grandidentatum is a beautiful tree for Colorado. It is not well known, perhaps because it does not establish well in Ball and Burlap, and few nurseries are growing it in containers or in root-control bags. It is native to thin and rocky soils, but would not be a good choice for an exposed, very dry site. However if it gets some water and some protection from all day sun, especially from late afternoon sun, it can be very successful. It can take dry summers, heat and alkaline soils, growing usually to 25—35′. Its orange-red fall color is outstanding.
Quercus gambelii, Gambel Oak, is a small native tree growing 8—25′. It has very irregularly lobed leaves and rough and scaly bark. It is tolerant of both dry and alkaline soils and can sucker to make little groves. The fall color is golden yellow and sometimes orange or red. It is hardy to at least 8000′. Normally we don’t think of Arizona Cypress as being hardy in Colorado, but there are selections from high elevations in New Mexico which have survived –25F. Several beautiful large specimens can be seen at Denver Botanic Gardens. Cupressus arizonica looks at first glance like an upright juniper, columnar in form with similar leaves; but the leaves are not prickly and can be a blue-green or a silvery blue. Some are very blue. It can grow to 20—35′ high and 8—20′ wide or bigger. The foliage can winter burn and some people recommend winter shade. Mine had some winter burn for 3 winters, but appears to have outgrown that. It is very beautiful.
A non-native ornamental tree that is drought tolerant is Acer ginnala, Ginnala Maple. Whereas its leaves can yellow from chlorosis in very alkaline soils, it is generally well-adapted and very tough, growing to 15—25′ tall and wide. The leaves are three-lobed and not as lacy as most Japanese Maples, but the effect is similar, with far more likelihood of success in our area. It can be grown multi-stemmed or single trunk, but I think the multi-stem type are more inclined to lean out and therefore more susceptible to storm damage. In fact because Ginnala Maple has a tendency for weak branch crotches, and can split badly, it is good to keep its form compacted to be stronger. Its dark green leaves contrast nicely with the smooth gray bark and its fall color is the intense red so many of us wish for. Russian Hawthorn, Crataegus ambigua is a great xeric tree, growing 15—25′ high and wide. It is dense, with a beautiful branching structure, profuse white spring flowers and profuse dark red berries that hang on for months. I have two that are watered only once or twice a year. Like all trees, they will grow faster with more frequent watering.
Another non-native that is well-adapted to Colorado is Golden Rain Tree, Koelreuteria paniculata. It grows 25&mdash30′ tall and wide and has large, attractive compound leaves emerging red, then turning green. It is somewhat susceptible to weak branch crotches, which can be pruned to be strong. In the summer, panicles of fragrant yellow flowers can cover the tree. Later, seeds form in Chinese Lantern shaped papery pods and the fall color is an attractive golden-orange. The seeds often start seedlings which should be removed the first year before they get big. Since it is marginally hardy, try to get one that has been grown from hardy stock. Many, like mine, have been hardy for over 15 years. All trees, like all people, have their pluses and their minuses. Here in Colorado, if we want tolerance to drought in our trees, we have to accept a few minuses. Even these trees that stand drought, will grow faster and flower better with some water. With moderate care, they can provide the shade and beauty that makes the difference between the desert and the oasis.