Prolonged record-setting heat and drought are not comforting words if you work in the green industry. However, from March 2011 to spring 2013 a large portion of the central United States experienced just that. The historic event impacted the horticulture industry in many ways, including some surprises.
The John C. Pair Horticultural Center of Kansas State University is a research and extension facility near Wichita, Kan., with a mission of introducing and evaluating woody landscape plant material for the state and region. Seasonal drought, rapid temperature changes, scorching summer heat and perpetual drying winds make it an ideal location to stress-test woody plants. The calendar years of 2011 and 2012 were a reminder that our mission is important beyond our state’s borders.
This unusual weather event provided an opportunity to observe the effects of heat and drought on mature, established landscape plants. No greenhouse; No growth chamber; No artificially induced drought. This was real-world stress testing. The following list of 10 species survived the event with little to no observable effects.
While I am nearly certain that somewhere across the region stricken by this drought, readers will be able to point to an incidence where each of the following has failed. However, when well-established and otherwise healthy, each of the species performed admirably and deserves further consideration in areas with regular periods of extended heat and drought.
Pistacia chinensis (Chinese pistache).
This tree continues to amaze. It is perfectly hardy in USDA Hardiness Zone 6, yet is also a widely used landscape tree in Phoenix. Additionally, the species seems perfectly at home in the continental climate of the southern Great Plains where rapid temperature shifts are a common occurrence.
Fall color is a primary ornamental attribute and can range from none, to yellow, to orange, to brilliant red, and some with hues of purple. Since the vast majority of established landscape trees are of seed origin, this presents an opportunity for selecting superior specimens. There are a few named cultivars and others in the works that have reliable fall color or a cooperative growth habit.
With it comes a warning – it’s invasive. It is true that in the southern plains from Kansas through Texas you can find Chinese pistache escaping and invading fence rows, abandoned fields and empty urban lots.
Acer saccharum (Caddo sugar maple).
For 30 years this southwestern ecotype of sugar maple has repeatedly proven itself to be one of the toughest shade trees in heat and drought-prone areas.
Originating from a disjunct sugar maple population in western Oklahoma, these genes know heat and drought. As an added advantage, they also grow rather well in soil pH up to 8.0. At our facility, Caddo sugar maple is surrounded by scorched, dying and dead sugar maple cultivars. Yet Caddo sugar maple has the audacity to push a mid-summer flush of growth as if the drought had never occurred. Landscape irrigation systems are no friend to Caddo maple.
Trees surrounded by over-irrigated turf are generally in poor health. Cultivars have been selected for brilliant red fall color and are superior to seedlings for that reason. Plants have performed well as far north as Ames, Iowa, and as far south as Dallas.
Its drawback – niche market. The beautiful and abundant cultivars of sugar maple already on the market are plenty drought tolerant for most locations.
Availability can also be a limiting factor. Some growers may try to collect seed from established trees in the landscape. Unfortunately, those seeds are probably contaminated by neighboring non-Caddo pollen parents.
Acer truncatum (Shantung maple).
A beautiful tree with a purple flush of spring growth, gorgeous fall color and drought tolerance. Trees of seed origin can range in fall color from yellow to orange to red. Our experience has shown that specimen trees tend to produce a high number of progeny with fall color similar to the seed parent. In other words, collect seeds from parent trees that turn red in fall. This species continues to impress on an annual basis. There have been no observed pest problems. It has dependable cold hardiness and reliable fall color.
Size is appropriate for suburban lots and trees are not messy. However, vigorous growth going into fall can be damaged by an early freeze in more mild climates where the plants produce a late summer flush of growth and fail to properly harden off. In the southern plains region with intense summer heat, that does not seem to be a problem and plants are always prepared for the first freeze.
Chionanthus retusus (Chinese fringetree).
Widely recognized for its incredible bloom and excellent bark characteristics, we now feel confident promoting this species for its drought tolerance. In the midst of back-to-back record setting heat and drought events, not a single scorched leaf could be found, their flowering was not diminished, and their fall color was a beautiful golden yellow. Leaves remained dark emerald green throughout the growing season.
Be aware that clonal propagation can be frustrating, but it can be done. Reports of biannual blooming exist but this has not been widely viewed as a criticism of the species.
Heptacodium miconioides (seven-son flower).
The bark is eye-catching when the plant is limbed up for viewing. The flowers light up the landscape the first week of September (Wichita) and are visited by more species of pollinators than can be counted. The sepals keep the show going for two to three more weeks when they change to a rosy pink.
Perhaps most surprising was the way its dark emerald-green leaves persisted through exceptional drought completely unscathed. Even flowering was perfectly normal despite coming at the end of the growing season. But the plant suffers from a lack of diversity. It’s an amazing plant, but the gene pool is relatively small which limits the potential for cultivar selection. A species this good, yearns for improved cultivars.
Quercus shumardii (shumard oak).
This widely adapted species is native to moist bottomlands, yet handles drought extremely well. A fibrous root system lends its self well to B&B production and makes transplanting fairly successful. Its fall color can be outstanding burnt orange to red in some selections. Some individual trees push a red flush of growth in the spring that holds color into early summer. This could be an interesting character to select for. I have never seen this species suffer the effects of an extended drought.
The only drawback I can provide is that clonal propagation is difficult. A moderately successful means of asexual propagation would simplify and speed the introduction of new selections. Acorns can be messy in heavy fruit set years.
Quercus muehlenbergii (chinkapin oak).
No bells and whistles here – it’s just a really tough tree. Often found growing on elevated limestone outcroppings, this species is right at home in well drained, high pH soil. Chinkapin oak is a large tree with attractive bark, clean green leaves, and acorns that are highly prized by all forms of wildlife. Keep watch for cultivars with improved foliage quality and growth habit. They are on the way. Transplanting is tricky. If careful attention is not taken to ensure a well-branched fibrous root system, then B&B trees are likely to contain few roots in the ball, which correlates to few successes in the landscape. Acorns can be objectionable in years with heavy fruit load.
Ulmus americana(American elm)
There is a reason it used to be the most common landscape and urban shade tree — adaptability. New cultivars with disease resistance are once again finding their way into our landscapes. As part of the multistate National Elm Trial, we were fortunate to observe several cultivars of elm over the past few years, and drought tolerance appears to be a trait distributed throughout the 18 taxa in our evaluation. The species transplants well, establishes quickly and grows rapidly. It has a few negatives — elm leaf beetle feeding, grasshopper feeding, rank growth requiring frequent pruning, and cultivars with unattractive growth habits. However, when a fast-growing, broad-spreading shade tree with wide adaptability and impressive drought tolerance is required, the list of available species gets pretty short. American elm will be on that list.
Maclura pomifera (Osage orange)
Seriously, Osage orange. This tree, more than any other, played a significant role in the westward expansion of agriculture into the Great Plains region. As a windbreak and living fence this species has no competition. Improved cultivars are available. Extreme weather event after extreme weather event, these trees never fail. No drought has made them defoliate, no spring freeze has caught them in leaf, and no ice event has broken their branches. Attractive bark and clean green foliage combined with impressive stress tolerance makes it a good shade tree and a good urban tree. But those thorns and fruit are undesirable, to say the least. Several cultivars exist that are thornless and fruitless (male). Availability is also an issue because demand is low. However, a couple more years like the last few, and we may see increased demand for truly drought tolerant trees.
Cupressus arizonica(Arizona cypress)
With a name like that it has to be drought tolerant. The species will easily handle the heat and drought of the southern Great Plains. In our studies, a one-year seedling transplanted to the field increased its root biomass by 5,000 percent in the following 12 months and roots were excavated to a depth of 4 feet. In regions routinely plagued by extended high summer temperatures and drought, this underutilized species should be a regular component in the conifer mix. Hardiness will limit use of this species in colder regions. If specimens are crowded, they tend to not age well and cause thinning and defoliation on the lower portion of the tree. New cultivars with blue foliage, improved cold hardiness, pyramidal growth and that root well from stem cuttings would be desirable.
The author is director of the John C. Pair Horticultural Center, Kansas State University.