Able to stand up to heat and humidity, conifers suitable for the Southern states merit serious consideration-both for landscape use and for growers’ inventories.
Photos courtesy of John M. Ruter
Not much attention has been paid to the use of conifers in the Southeast, and that’s a shame. Back in the mid-1990s, American Nurseryman ran a series of articles utilizing conifer experts from around the country, but only one of those experts was from the southeastern United States. In the Coastal Plain region, conifers are often thought of as pine trees for forestry or junipers for landscaping – that’s it.
Due to a lack of information on conifer adaptability for the lower South, in 1996 I started collecting germplasm for an evaluation project at The University of Georgia campus in Tifton, Ga. The Tifton campus is 64 miles north of the Florida border in south-central Georgia. The station is located in USDA Hardiness Zone 8a, and Tifton averages about 100 days per year at or above 90° F. My goal was to determine which conifers are best suited for this region.
The initial plantings were installed in January 1997. Individual plants were planted 12.5 feet within rows with 20 feet between rows. Composted pecan shells were applied as mulch, and drip irrigation with an emitter at each plant was installed. Fertilizer is applied at the rate of 50 lbs/N/A in the spring and 25 lbs/N/A in August using 16-4-8 soluble granular fertilizer with micronutrients. The pH of the site at time of planting was 5.5. Weeds are controlled with preemergent applications of simazine and oryzalin in February and September. Glyphosate is applied as needed as a post-emergent herbicide.
Based on observations throughout the multiyear study, I can recommend a number of conifers that deserve a greater presence in the landscapes of the Southeast.
Conifers for consideration
Abies firma – or Momi fir – from southern Japan is the only true fir that can grow across the entire Southeastern region. Momi fir has good resistance to Phytophthora root rot and is being used as an understock to evaluate other species in the South. In zones 6 and 7a, A. homolepis and A. nordmanniana perform well.
Left: Afrocarpus falcatus. Center: Araucaria angustifolia. Right: Calocedrus formosana.
For conifer enthusiasts along the Gulf Coast, Afrocarpus falcatus has finely textured foliage and makes a large shrub or small tree. New growth is damaged below 18° F, but there has been no stem dieback in Tifton.
The Parana pine, or Araucaria angustifolia, has been a pleasant surprise in Georgia, performing well in Tifton and next to the conservatory at the Atlanta Botanical Garden. Hybrids between this species and the Monkey Puzzle tree, A. araucana, are being evaluated for resistance to root rot. A. bidwillii has also grown well with minimal winter damage to shoot tips in Tifton.
In the Piedmont and mountain regions of the South (zones 6b and 7), incense cedar (Calocedrus decurrens) performs very well, but old trees are rarely seen in the Coastal Plain. Two Asian species, C. formosana from Taiwan and C. macrolepis from southeastern China, have performed exceptionally well in zone 8a. The Taiwanese incense cedar has dark green foliage that is very attractive. Unfortunately, both species have been fairly difficult to root from cuttings.
Numerous deodar cedars have been introduced in the past decade. Many growers like Cedrus deodara ‘Bushes Electra’ because it is heavily branched and looks good in a large container. The cultivar ‘Gold Cone’ has performed well in the Deep South and displays nice, yellowish foliage during much of the year.
Left: Cedrus deodara ‘Gold Cone’. Center: Chamaecyparis obtusa var. formosana. Right: C. arizonica var. glabra ‘Chaparral’.
Many plants from Taiwan perform well in south Georgia, including Chamaecyparis obtusa var. formosana. A plant in my trials reached a height of 22 feet in 10 years. Selections are needed that do not turn brown in the winter, however.
All dwarf Cryptomeria japonica selections that I have tried over the past 13 years are dead, rarely living more than seven years. The exceptions? ‘Gyokuryu’ and ‘Rein’s Dense Jade’ are two intermediate forms that have performed well. I am currently working with a selection from north Georgia that has good form and remains green through the winter.
The genus Cupressus is in taxonomic turmoil at the moment. Until a final determination can be made, I will just call them all Cupressus. Leyland cypress can be destroyed by several pathogens, including Cercosporidium blight (now known as Passalora), which requires spray treatments every two weeks from June to Thanksgiving to prevent the problem. I have an unnamed clone that has shown excellent disease resistance that may be useful for the Christmas tree industry. The selection ‘Gold Rider’ is the best yellow-foliaged form, but it is the poster child for Phomopsis tip blight in south Georgia. C. funebris is an attractive, upright conifer with pendulous foliage that deserves more use, while the Arizona cypress selection C. arizonica var. glabra ‘Chaparral’ receives positive comments from all who see it.
Numerous junipers thrive throughout the Southeast. Juniperus formosana is an upright grower with weeping branches that does well in zones 7 to 8a. Keteleerias are fir relatives from southern China and northern Vietnam. K. davidiana is rare in the Southeast, but it is well-worth the effort if you want a tree that looks like a fir and thrives in the lower South. Another nice species is K. evelyniana, which has performed well in south Georgia and north Florida.
Spruces are not good plants for the lower South. The only spruce to survive in my trials has been Picea chihuahuana from Mexico. At Cox Arboretum in Canton, Ga. (USDA Zone 7a), P. morrison-icola, P. omorika and P. orientalis have all grown well.
Pinus pseudostrobus from Mexico is an excellent, five-needled pine that has grown well in south Georgia. I have also been pleasantly surprised by the growth of certain P. armandii and P. wallichiana selections.
Taiwania cryptomeroides, while not a plant with great commercial potential, makes a fantastic specimen conifer for large gardens and public spaces. On the commercial side, Thuja ‘Green Giant’ performs exceptionally well throughout the South and holds up well in the heat of south Georgia with none of the disease problems destroying the Leyland cypresses.
Interest in conifers for Southern landscapes is increasing. Research is being conducted at The University of Georgia Tifton Campus and the Center for Applied Nursery Research in Dearing, Ga., to bring new introductions to market in the near future.
Information in this article was pre-sented at the 2010 meeting of the IPPS Southern Region at Raleigh, N.C.
Conifer growing factors to consider
During the study of conifers conducted at the University of Georgia campus located in Tifton, Ga., the lowest temperature in the winter has been 14° F. Tifton is located in an area where the temperatures never spend 24 hours below freezing. Even though temperatures may plummet into the upper teens every winter, temperatures the following day are usually in the low to upper 30s. If protection from drying winds is provided, certain conifers from more tropical areas can be grown in the Coastal Plain region of the Southeast, because temperatures never linger below the freezing mark for days on end.
Fun in the sun
Photoinhibition can also cause damage or winter browning of foliage under conditions of high light intensity and low temperatures. The response only occurs in leaves exposed to the sun, as those on the north side of the plant often remain green. In plants such as Chamaecyparis, Cryptomeria, Platycladus, Taiwania and Thuja, foliar browning is due to the production of a pigment known as rhodoxanthin. This pigment is produced to prevent damage to the chloroplasts when light levels are high and temperatures are low or when photosynthesis cannot run efficiently enough to dissipate excessive light energy. In Japan it was discovered that natural polyploids of Cryptomeria remained green in the winter due increased levels of antioxidant enzymes.
In 2008 Dr. Ryan Contreras and I initiated a program to develop polyploid Japanese cedars to see if we could create non-browning “evergreen evergreens.” After several attempts, we successfully produced more than 100 polyploid seedlings. These plants are being grown out and were planted to the field in Watkinsville, Ga. (USDA 7b), in 2011 for further evaluation.
Beating the heat
High temperatures also create problems for conifers in the South. The three main problems are the duration of high temperatures, high nighttime temperatures and elevated soil temperatures. In south Georgia, we average 90 to 100 days per year with temperatures at or above 90° F, and summer lasts from early May until mid-October.
Depending on their origin, most conifers have an optimum photosynthetic temperature range of 60 to 85° F. Add in high nighttime temperatures that increase plant respiration, and many conifers begin to run a negative carbohydrate balance as they burn up all the sugars made by photosynthesis during the day.
Because carbohydrates are needed for plant growth, little to no growth occurs. One of the hottest summers on record in Georgia occurred in 2010 – particularly nighttime temperatures. It has been fairly common over the past 20 years for temperatures to still be in the upper 80s at 10:00 p.m., but this was the first summer where temperatures were in the 90s that late into the evening. Many conifers grow at elevations much higher than those found in the Southeast. Due to the adiabatic lapse rate, which dictates that temperatures usually decrease about 3° F for every 1,000 feet increase in elevation, many conifers grow in habitats with much cooler night temperatures.
The dirt on soil
High soil temperatures increase respiratory demands of the root system as well. Throughout much of the Southeast, clay soils dominate the landscape. Wet, heavy soils can be a conifer’s worst enemy, since high temperatures and low oxygen availability lead to increased respiratory demands. If sufficient oxygen is not available, this can lead to the death of roots and eventually a decline of the entire plant. Many conifers die during periods of heavy rainfall late in summer because the air pockets in the soil are saturated with water, and oxygen is not sufficiently available to meet the demands of the roots. Heavy soils should be amended or raised beds can be utilized.
Low oxygen availability is less of a concern on the sandy soils of the Coastal Plain region where there is sufficient drainage. Supplemental irrigation is essential for plant establishment and often helps with survival during the growing season during periods of drought. Plants such a Japanese cedar grow best in coarse, mountain soils in areas that receive upwards of 100 inches of rain per year. Much of the Southeast gets less than 50 inches of rainfall in a normal year.
Soil type (sandy vs. clay), soil pH and alkalinity of irrigation water all influence nutritional aspects of conifer growth. On our sandy loam soils in south Georgia, it is necessary to fertilize most conifers to optimize growth and keep foliage color looking good throughout the winter. On clay soils in the northern part of the state, many collectors only fertilize their collections at planting. The difference – clay soils have sufficient cation exchange capacity to hang onto soil nutrients, whereas they tend to leach from sandy soils.
Nitrogen seems to be the biggest limiting factor for good growth. Many conifers from Vietnam and southern China grow on acidic limestone karst soils, indicating the need for calcium. Gypsum can be used to increase soil calcium levels without increasing soil pH, otherwise dolomitic limestone can be used. In the highly acidic, leached sands of the Coastal Plain, magnesium deficiency offer occurs on conifers such as Keteleeria evelyniana and Nageia nagi. Epsom salts (magnesium sulphate) can be used to correct deficiencies of magnesium. Iron deficiency can also occur on alkaline sites or when alkaline irrigation water from limestone aquifers is used for a number of years, particularly with drip irrigation systems.
Hurricanes can wreak havoc on a conifer collection. Many pines snap off at the top or may blow over in shallow soils. In 2004, four hurricanes with winds in excess of 40 mph came through south Georgia in a matter of months. Many conifers in the collection were not damaged, but all the ones that blew over were in the genus Cupressus. In many cases it appears that top growth outgrows the supporting root system.
Pests and pestilence
There are a number of pests that attack conifers in the Southeast. In my experience, the main pests have been foliar diseases (Cercosporidium, Phomopsis tip blight); cankers (Bot canker and Seridium); pine tip moths on two and three-needled pines; root rot in container and field-grown plants; and spider mites. Wooly adelgids are now problems in the upper South on firs and hemlocks.
John M. Ruter, Ph.D., is Allan Armitage Professor of Horticulture at the University of Georgia, Tifton. His co-authored book on conifers for Southeastern landscapes is slated to be published in Spring 2013 by the University Press of Florida. He can be reached at email@example.com.