REPORTS HORTSCIENCE 25(11):1382-1384. 1990. orchard data from four locations. The long-term (1951–89) daily weather Using a Chill Unit/Growing Degree records for the Knoxville Plant Science Farm (eastern Tennessee); the Highland Rim Ex- Hour Model to Assess Spring Freeze periment Station, Springfield (middle Ten- nessee); the Middle Tennessee Experiment Station, Spring Hill (middle Tennessee); and Risks for ‘Redhaven’ Peach Trees the West Tennessee Experiment Station. Joanne Logan 1, Dennis E. Deyton2 and David W. LockWood3 Jackson (western Tennessee), were used to estimate daily chill units and GDH. The hourly Department of Plant and Soil Science, The University of Tennessee, temperatures were estimated from the daily Knoxville, TN 37901-1071 maximum and minimum temperatures Additional index words. phenology, growing degree hours, Prunus persica (Richardson et al., 1974). The latitude of the locations ranged from 35°37'N to 36°28'N, Abstract. Peach [Prunus persica (L.) Batsch] production in Tennessee has declined the longitude from 83°57'W to 88°50'W and since 1985 due to the occurrence of freezing temperatures that kill the buds, usually the respective elevations for the four loca- in the spring. Analyses of long-term (1951-89) daily temperature data from four lo- tions were 252, 226, 213, and 122 m above cations in Tennessee were used to evaluate the freeze risks for ‘Redhaven’ peach tree sea level. buds at those sites. A model using daily accumulated chill units and growing degree The estimated daily number of chill units hours (base 4.4C air temperature) was used to estimate the dates to begin and end chill were accumulated from the day after the oc- unit accumulations and the dates of full bloom of ‘Redhaven’ peach trees for each year currence of the highest negative accumula- in the climatological record. The actual dates of freezes with air temperatures at or tion of chill units. The date of completion of below –2.2C and the estimated bud developmental stage on the date of each freeze rest for ‘Redhaven’ was estimated as the day also were determined. The model was tested using peach orchard records and was when 870 chill units had accumulated after found to be an improvement over using only freeze data. The model indicated that the start of chill unit accumulation (Richard- Spring Hill had the highest risk for peach production and Jackson the lowest. Recent son et al., 1975). Daily GDH were accu- problems with spring freezes at Knoxville and Spring Hill were due to later than normal mulated beginning on the estimated day of freeze dates rather than earlier development of the ‘Redhaven’ peach tree buds. At rest completion for ‘Redhaven’ peaches for Springfield, the recent freeze problems were due to earlier breaking of rest, earlier each year. The daily accumulation of GDH full bloom, and later freezes. was used to estimate the dates of full bloom and the stage of peach bud development on A growing concern has emerged about minimum temperatures have been deter- a day when a frost of –2.2C or below oc- peach crop losses due to late-spring freezes mined. Below these temperatures, some curred. This predictive phonological proce- in Tennessee. Years with little or no com- damage to flower buds is sustained at various dure was repeated for each location and season mercial state production have occurred as re- stages of floral development. Amount of (September-April), 1951-52 through 1988- cently as 1982, 1983, 1985, 1986, 1987, and freeze injury is also influenced by the du- 89 (38 years). 1989. (Tennessee Agricultural Statistics Ser- ration of the freezing temperatures. The av- Analysis of spring freeze dates indicated vice, 1987). erage air temperatures that result in 90% kill that five of the 10 latest spring freezes of To model the rate of peach flower bud of peach buds are –16.7, –15.0, –12.8, –2.2C or lower at Knoxville and Spring Hill development following rest, the date when –9.4, –6.1, –4.4, and –3.9C for first have occurred since 1980. At Springfield, the peach tree completes its rest period can swelling, green calyx, red calyx, first pink, four of the 10 latest spring freezes of –2.2C be estimated (Richardson et al., 1974). Many first bloom, full bloom, and postbloom stages, or lower have occurred since 1980. At Jack- developmental or phonological models relate respectively (Westwood, 1978). son, the freeze-data analysis showed that only the rate of development to the accumulation In recent years, serious problems have de- two of the 10 latest spring freezes of –2.2C of temperature above a critical threshold. This veloped with late-spring freezes and injury or lower have occurred since 1980. accumulation usually is expressed as grow- to peach tree buds. Examination of the pre- Table 1 shows that the predicted mean full- ing degree days (GDD) or growing degree viously published studies of spring freeze dates bloom date at Knoxville (25 Mar.) was 3 hours (GDH). The growing degree hour (base at given probability levels (Fribourg and In- days earlier than the actual mean full-bloom 4.4C air temperature) requirements for crit- gram, 1977; National Oceanic and Atmos- date (28 Mar.). At Jackson, the predicted ical reproductive stages of ‘Redhaven’ peach pheric Administration, 1985; Pickett and mean full-bloom date (27 Mar. ) was 2 days trees have been estimated (Richardson et al., Bailey, 1964) does not provide sufficient in- earlier than the actual mean full-bloom date 1975). The GDH requirements are 1981, formation to assess the spring freeze risks (29 Mar.). There was no significant differ- 2643,2963,3667,4204, 4923, and 5863 for for the peach crop in Tennessee. Whether ence between predicted and actual mean full- first swelling, green calyx, red calyx, first the severe damage occurs because the spring bloom dates at either location. pink, first bloom, full bloom, and postbloom freezes have been later than normal or be- The average dates and standard errors (in stages, respectively. GDH are considered to cause the peach buds have ended rest earlier days) for the estimated beginning of chill begin accumulation after rest is completed. than normal is unknown. Also, faster bud accumulation, end of rest, and full bloom of For any particular crop, such as peach, development after the breaking of rest could ‘Redhaven’ peaches at the four locations are result in a more-advanced and freeze-sus- presented in Table 2. The average bloom pe- ceptible stage of development when the freeze riods (mean full-bloom date ± 1 week) were Received for publication 1 Sept. 1989. The cost occurs. 16-30 Mar. at Knoxville, 25 Mar.–8 Apr. of publishing this paper was defrayed in part by Our objective was to use a simple phen- at Springfield, 21 Mar.4 Apr. at Spring Hill, the payment of page charges. Under postal regu- lations, this paper therefore must be hereby marked ological model involving the chill unit and and 19 Mar.-3 Apr. at Jackson, advertisement solely to indicate this fact. GDH requirements of the common peach A simulated freeze-damage history for 1Assistant Professor. cultivar Redhaven to study the relationships ‘Redhaven’ peach trees at Knoxville is shown 2Associate Professor. between peach bud development and spring in Table 3. Only those years in which a prob- 3Professor. freezes. The model was evaluated using peach able 90% kill of the peach buds occurred are 1382 HORTSCIENCE, VOL. 25(11), NOVEMBER 1990 significant risk to peach production in the Knoxville area. For the 6 years of predicted freezes since 1980, the average date of the completion of rest was 6 days later than the mean (23 Jan.), the average full-bloom date was 1 day earlier than the mean (23 Mar.), and the average date of the last freeze of –2.2C or lower was 19 days later than- the mean (27 Mar.). These results indicate that the recent problems with freeze damage at Knoxville have been due to later than normal freezes. Some local temperature modification can be a viable option to influence the frequency of freezing temperatures in the spring. An irrigation application rate of 4 mm·hr-1 combined with a low dewpoint protects blos- soms down to –6.7C (Childers, 1973). With the minimum temperatures for Knoxville (Table 3), overhead irrigation might have prevented peach bud injury 10 years out of the total 14 freeze years predicted by the model. At Springfield, (1972-85) freeze-outs oc- curred four times (1973, 1974, 1978, and 1983). The model predicted damaging freezes in all but one (1978) of these years (data not shown). Freezes of –2.2C or lower never occurred during the average bloom period in 1973 and 1978. The model also correctly predicted the absence of freeze damage in all but one (1982) of the remaining 10 years. Four of those years had freezes of –2.2C or lower during the average bloom period. Since 1952, the model predicted that at least one damaging freeze occurred in 11 out of a possible 38 years (29%). This high per- centage of damaging freezes represents a significant risk to peach production in the Springfield area. For the 4 years of predicted freezes since 1980, the average date of the completion of rest was 17 days earlier than the mean (6 Feb.), the average full-bloom date was 6 days earlier than the mean (1 Apr.), and the average date of the last freeze of –2.2C or lower was 14 days later than the mean (30 Mar.). These results indicate that the recent problems with freeze damage at Springfield have been due to three factors: earlier breaking of rest, earlier blooming, and later than normal freezes.