American Nurseryman -Jan 2012

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 effi ciently 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 fi eld 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 fi rst 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 1000 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 conifers worst enemy since high temperatures and low oxygen availability lead to increased respiratory demands If suffi cient 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 suffi ciently 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 suffi cient 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 infl uence 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 suffi cient 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 defi ciency offer occurs on conifers such as Keteleeria evelyniana and Nageia nagi Epsom salts magnesium sulphate can be used to correct defi ciencies of magnesium Iron defi ciency 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 Wicked weather 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 fi eld grown plants and spider mites Wooly adelgids are now problems in the upper South on fi rs and hemlocks 18 JANUARY 2012 American Nurseryman www amerinursery com