Bulletin of The Desert Legume Program of The Boyce Thompson Southwestern Arboretum and The University of Arizona Volume 16, Number 2 August 2004 Effects of the December 2003 Freeze on Plants in DELEP’s Tucson Fields Matthew B. Johnson son. Consequently, the DELEP fields 18° F. Temperature readings at the Program Manager and Curator are excellent sites to evaluate cold CAC and WCAC are taken at Ari- Desert Legume Program tolerance in legume species. zona Meteorological Network Cold tolerance is difficult to (AZMET) recording stations. The DELEP maintains fields in the quantify. May factors can influence a minimum temperature reported at the Tucson area at the University of Ari- plant’s ability to withstand freezing CAC in the December 2003 freeze zona Campus Agricultural Center temperatures including the duration was 16° F (-8.8° C). At the WCAC, (CAC) and West Campus Agricul- and intensity of the freeze, maturity of the lowest temperature recorded was tural Center (WCAC). Plants are the plant, whether the plant is actively 21° F (-6.1° C). The WCAC reading grown at these sites to increase seed growing or is dormant and hardened was recorded approximately ½ mile supplies, to provide plant material for off, protection provided by adjacent from DELEP’s field. Based on obser- research projects and to evaluate plants and buildings, and genetic char- vations of plant damage, it is likely survival and performance under local acteristics of individual plants. Usu- that temperatures in the WCAC field conditions. Initial plantings were made ally, several of these factors are in- were in the upper teens F. Due to the in 1989 and have continued as new volved in determining a plant’s re- lack of secure locations, thermom- species are acquired and propagated. sponse to freezing. eters were not present in the field The fields are located at two sites on The autumn of 2003 had plots for this freeze event so minimum the CAC and one site on the WCAC. above average rainfall with the re- low temperatures are estimates. Tem- Collectively, these fields encompass mains of three tropical storm systems peratures on both nights were below approximately 3.5 acres (1.4 ha.). As from the eastern Pacific Ocean cross- freezing for approximately 14-16 of December, 2003, there were about ing the area. Temperatures had fallen hours. 200 taxa of perennial legumes present into the low 20's F on several occa- The following report summa- in these fields. sions in late November and early De- rizes the effects of the December Freezing temperatures have cember, so most summer-growing 2003 freeze on plants in DELEP’s accounted for more plant deaths in plants had become dormant. No sig- Tucson fields. While precise informa- the Tucson fields than any other fac- nificant freeze damage was noted tion on minimum temperatures in the tor. The CAC fields are located along from these earlier freezes. On De- fields is not available, these observa- Rillito Creek and the WCAC field is cember 28th and 29th, 2003, the tions can provide a general idea of the immediately adjacent to the Santa Tucson area was subjected to a cold cold tolerance of many of the plants. Cruz River. These low-lying areas are air mass that resulted in temperatures As with past freezes, it was interesting subject to cold-air drainage at night that fell significantly below freezing at to observe the differences in the ef- and significant freezing temperatures night throughout the region. The offi- fects of the freeze between plants of occur each winter. On clear, dry, cial low temperature in Tucson fell to particular species from different ac- windless winter nights, the tempera- 19° F (-7.2° C). This is the lowest cessions and even within a single ac- ture in these fields can be over10° F temperature since December 1978 cession. colder than in warmer areas of Tuc- when the official reading dropped to 2 Aridus 16:2 2004 Notes: Damage to foliage is indicated stems at ground level were sufficiently Acacia craspedocarpa CAC (4) – only for those plants that typically protected that they escaped damage. 2 accessions: No freeze damage was remain evergreen through the winter in The plant is recovering. observed on these plants. Tucson. Where more than one acces- Acacia borleae CAC (2): A few Acacia crassifolia CAC (1): The sion is indicated, these are usually, stems froze back to 6" (15 cm) from leaves were killed and many stems though not always located in separate the tips. froze back to 9" (23 cm) from tips. fields. Acacia brachybotrya CAC (1): Acacia davyi CAC (2): These This plant sustained some foliage plants froze back 6' (1.8 m) to the Acacia acuminata CAC (3) / damage only. base with stems 1" (2.5 cm) thick WCAC (1): None of these plants Acacia brachystachya CAC (1) / killed. They are recovering. exhibited freeze damage. WCAC (1) – 2 accessions: No freeze Acacia deanii CAC (1): This plant Acacia adsurgens CAC (1): The damage was observed on these did not exhibit any freeze damage. foliage was killed and stems froze to plants. Acacia difformis CAC (2): No 1" (2.5 cm) thick. Acacia brandegeeana WCAC (2): freeze damage was observed on these Acacia ampliceps CAC (2): This These plants did not exhibit any freeze plants. species freezes to the base at about damage. Acacia ehrenbergiana WCAC (1): 25° F (-3.8° C). A few root sprouts Acacia brevispica CAC (1): This This plant froze back 2.5' (0.75 m) to continue to grow despite removal of plant froze back 3.5' (1 m) to the the base with stems ¼" thick killed. It the parent plants several years ago. base with stems ½" thick killed. It is is recovering. Acacia aneura CAC (2) / WCAC recovering. Acacia erioloba WCAC (2): No (2): No freeze damage has been ob- Acacia cambagei WCAC (2): freeze damage has been observed on served on this species. These plants were planted in summer this species. Acacia angustissima var. 2003. They froze to the base with Acacia farnesiana CAC (5) / angustissima CAC (several plants): stems 1/8" thick killed. They are re- WCAC (2) – 2 accessions: These Most stems were frozen back to the covering. plants did not suffer any apparent base from 7' (2.1 m) tall. The plants Acacia cana CAC (1): This plant freeze damage. exhibit vigorous recovery. did not exhibit any freeze damage. Acacia fasciculifera CAC (2) / Acacia angustissima var. Acacia caven CAC (1) / WCAC WCAC (2): These plants froze to suffrutescens CAC (6): As is typi- (2) – 2 accessions: These plants did stems 1" (2.5 cm) thick and the foli- cal in most winters, these plants froze not suffer any apparent damage. age was killed. The plants are recov- back to the base. The plants exhibit Acacia citrinoviridis CAC (2): ering. vigorous recovery in the spring to a One plant froze back 6' (1.8 m) to the Acacia fleckii WCAC (1): Minor height of 3' (0.9 m). base with stems to 1" (2.5 cm) thick stem die-back to 3" (7.5 cm) from the Acacia aroma CAC (2) / WCAC killed, and is recovering. The other tips. (2) – 2 accessions: A few stems of plant had severe foliage damage but Acacia furcatispina CAC (3) – 2 CAC plants were frozen to 1' (30 did not sustain stem damage. accessions: A few stems died back to cm) from the tips. WCAC plants ex- Acacia constricta CAC (7) – 2 6" (15 cm) from the tips. hibited no damage. accessions: No freeze damage has Acacia galpinii CAC (2): Stems to Acacia ashbyae WCAC (2): No been observed on this species. ½" thick were killed. These plants are damage was observed on these Acacia coolgardiensis WCAC (1): 20' (6.1 m) tall and continue to grow plants. This plant had been planted in sum- larger despite sustaining freeze dam- Acacia atramentaria CAC (2) / mer 2003 and was killed by the age each year. WCAC (2): None of these plants freeze. Acacia gerrardii subsp. negevensis were damaged. Acacia coriacea CAC (2): The CAC (3) / WCAC (2): None of these Acacia berlandieri CAC (5) – 2 slender stems of this species froze plants exhibited any freeze damage. accessions: Some stems of some back to the base. These plants regu- Acacia grandicorniculata CAC plants suffered minor tip damage only. larly freeze at this site but recover (2): These plants froze back 6' (1.8 Acacia bivenosa WCAC (1): This vigorously and grow to 7' (2.1 m) tall m) to the base with stems 1.5" (3.75 plant froze to the base with stems to the following summer. cm) thick killed. They are recovering. 2" (5 cm) thick being killed. A few Aridus 16:2 2004 3 Acacia grasbyi WCAC (1): Severe Acacia millefolia CAC (9) – 2 2.5' (0.75 m) to near the base and is damage to foliage and some stem tips accessions: Some stems on each plant recovering. were killed. froze back to 1' (30 cm) from the tips Acacia pravissima WCAC (1): Acacia greggii CAC (3) / WCAC while other stems were unaffected. Severe foliage damage and minor die- (2): No freeze damage has been ob- Acacia montana CAC (1): This back of some stem tips. served on this species. plant sustained minor foliage damage Acacia pruinocarpa CAC (3) / Acacia haematoxylon CAC (2): only. WCAC (2): The foliage of all plants No freeze damage was observed on Acacia montis-usti WCAC (2): was frozen. The WCAC plants suf- these plants. Both plants were killed by the freeze. fered no stem damage while the CAC Acacia harpophylla WCAC (2): Acacia nebrownii CAC (2): Froze plants had surprisingly variable dam- No freeze damage was observed on to the base from a height of 9' (2.7 age with stems killed back several these plants.