Flower buds of two sweet cherry (Prunus avium L.), 12 sour cherry (Prunus cerasus L.) and one ground cherry (P. fruticosa Pall.) were collected monthly from Aug. 1990 to Mar. 1991, and subjected to freeze tests to determine the level of cold hardiness. LT50 values (temperatures at which 50% of the flower buds were killed) summed over all months were significantly correlated (r = 0.6844, P ≤ 0.01) to the flower bud low temperature exotherms (LTEs). Correlation of LTEs to LT50 values was highest, r = 0.85, P ≤ 0.01 for the acclimation and midwinter period, November to February collections. During this period the average LT50 occurred before and within 2.5 °C of the LTE, indicating tissue injury before the LTE occurrence. During deacclimation, represented by the March collection, the LT50 began within 2.0 °C, on average, of the LTE, and in 11 of 12 cultivars and seedlings preceded the LTE. In March, the correlation of LTEs to LT50 values was less, r = 0.69, P ≤ 0.05, indicating possible changes flower bud deep supercooling. LTE values were selected as a measure of flower bud hardiness in sour cherry. Exotherms were not detected in the flower buds of all germplasm tested on all evaluation dates, but were the best means of separating selections. While LTE analyses expressed significant differences in November, December, and March at P ≤ 0.01, the LT50 analyses expressed differences only in December and February at P ≤ 0.05. The relationship between ambient temperatures and floral tissue hardiness indicated that November and March are two critical times for flower bud injury. November injury would occur in years when sudden low temperatures occur without sufficient pre-exposure to freezing temperatures. March injury would occur in years when sudden freezing temperatures follow warm days. This type of injury would be most pronounced in southern genotypes. Spring freeze injury could be significantly reduced by the selection of cultivars and seedlings that have delayed deacclimation. Exotherm occurrence and bud volume were correlated (r = 0.95, P ≤ 0.05). In January, when exotherms were least prevalent, they were generally present only in the five cultivars and seedlings with large bud volumes. The LTEs in midwinter, occurred within 3 °C of the reported average annual minimum temperature for the northern range of Prunus commercial production (Zone 6). The results of the principal component analysis of flower bud LTEs indicated that other selection criteria as flowering time might have played a more significant role in the hardiness range of sour cherry than simply geographic origin. The first principal component (PC1), which accounted for 77% of the total variance was used to separate among cultivars and seedlings. Selections at the positive end of PC1 had flower buds that were more cold susceptible than selections at the negative end of PC. This concurs with other research showing that flower bud hardiness is related more to commercial range (i.e., the range of commercial production) than to geographic distribution.
Knowledge of the level of cold hardiness and how hardiness is inherited in sour cherry is essential to germplasm collection and cultivar development. Twig samples of two sweet cherries (Prunus avium L.), 12 sour cherries (P. cerasus L.), and one ground cherry (P. fruticosa Pall.) of diverse geographic origins were collected in Jan. 1990 and monthly from Aug. 1990 to Mar. 1991, preconditioned to induce maximum cold resistance, and subjected to freeze tests and differential thermal analysis. Low temperature exotherms (LTEs) were detected in all stems of P. cerasus investigated and correlated to xylem incipient injury temperatures (ITs) from December to February (r = 0.84, P ≤ 0.01). March had the best correlation of LTEs to xylem ITs with r = 0.84, P ≤ 0.01. LTEs were strongly correlated to phloem-cambium ITs in November, representing the acclimation period. The correlation coefficient (r) for the phloem-cambium ITs and the twig LTEs during November was 0.68, significant at P ≤ 0.01. Cortical tissue and vegetative bud injuries were not correlated to the stem LTEs. Xylem ITs were selected for evaluating the cold resistance of sour cherry in December to March and phloem-cambium ITs were selected for November. The degree of supercooling and hardiness of the phloem-cambium in late fall and early spring appears significant in determining the stem hardiness and commercial range of P. cerasus. Phloem-cambium tissue, expressed the most rapid deacclimation response. The average decrease in hardiness for the phloem-cambium, xylem, and cortical tissues between February and March was 4 °C, 0.32 °C, and 2.14 °C, respectively. Principal component (PC) analyses of the phloem-cambium and cortical tissues depicted gradations between minimum survival temperatures of the two presumed progenitor species of sour cherry, i.e., sweet cherry and ground cherry. The first principal component (PC1), which accounted for 61% of the total variance, was used to separate among cultivars and seedlings. Cultivars and seedlings at the negative end of PC1 exhibited hardier phloem-cambium tissue at critical injury times, October, December, January, and March than cultivars and seedlings at the positive end of the PC1 axis. Cultivars and progeny of crosses of northern origin parents showed hardiness values more comparable to ground cherry than did selections of less-cold-hardy parents suggesting that cold is a major selective force, contributing to sour cherry population variation.
H.M. Mathers and C. Stushnoff
Twelve-week-old Malus seedlings were induced to cold harden by exposure to low temperature and freezing environments. The effectiveness of induced acclimation by exposure to stimuli such as low temperature (3 to 5 °C), frequency of exposure to freezing temperatures (-3 °C), storage time before and after induction and the effects of different screening temperatures (-20, -30, and -40 °C) were investigated with seedlings grown in a greenhouse from open-pollinated `Golden Delicious' apple (Malus pumila (Mill.), `Antonovka' apple (M. baccata (L.) Borkh. × (M. pumila) and `Rescue' apple (M. baccata) × (M. pumila). Differentiation of the seedling populations with respect to cold hardiness was not achieved until after acclimation at cool temperatures (3 to 5 °C) for 6 weeks. Further population differentiation was achieved by exposure to one or more frosts (-3 °C). Once the acclimation response had been initiated the seedlings could be held for up to 11 days, under the same conditions, with no significant decrease in hardiness. Hardiness levels of acclimated and nonacclimated open pollinated seedlings coincided with known inherent hardiness responses for all three maternal cultivars evaluated. A binomial form of regrowth data collection, percent seedling survival, was determined to be the most efficient and most precise measure of evaluation. Induction of cold hardiness in very young seedlings and the use of a controlled freeze testing protocol should facilitate rapid screening of large progenies and improve the rate of progress in breeding for cold hardiness.
Hannah M. Mathers, Luke T. Case, and Thomas H. Yeager
As limitations on water used by container nurseries become commonplace, nurseries will have to improve irrigation management. Several ways to conserve water and improve on the management of irrigation water applied to container plants are discussed in this review. They include 1) uniform application, 2) proper scheduling of irrigation water, 3) substrate amendments that retain water, 4) reducing heat load or evaporative loss from containers, and 5) recycling runoff water.
L.T. Case, H.M. Mathers, and A.F. Senesac
Container production has increased rapidly in many parts of the U.S. over the past 15 years. Container production has been the fastest growing sector in the nursery industry and the growth is expected to continue. Weed growth in container-grown nursery stock is a particularly serious problem, because the nutrients, air, and water available are limited to the volume of the container. The extent of damage caused by weeds is often underestimated and effective control is essential. Various researchers have found that as little as one weed in a small (1 gal) pot affects the growth of a crop. However, even if weeds did not reduce growth, a container plant with weeds is a less marketable product than a weed-free product. Managing weeds in a container nursery involves eliminating weeds and preventing their spread in the nursery, and this usually requires chemical controls. However, chemical controls should never be the only management tools implemented. Maximizing cultural and mechanical controls through proper sanitation and hand weeding are two important means to prevent the spread and regeneration of troublesome weeds. Cultural controls include mulching, irrigation methods (subirrigation), and mix type. Nursery growers estimate that they spend $500 to $4000/acre of containers for manual removal of weeds, depending on weed species being removed. Economic losses due to weed infestations have been estimated at approximately $7000/acre. Reduction of this expense with improved weed control methodologies and understanding weed control would have a significant impact on the industry. Problems associated with herbicide use in container production include proper calibration, herbicide runoff concerns from plastic or gravel (especially when chemicals fall between containers) and the need for multiple applications. As with other crops, off-site movement of pesticides through herbicide leaching, runoff, spray drift, and non-uniformity of application are concerns facing nursery growers. This article reviews some current weed control methods, problems associated with these methods, and possible strategies that could be useful for container nursery growers.
H.M. Mathers, S.B. Lowe, C. Scagel, D.K. Struve, and L.T. Case
Container production has many advantages over traditional in-ground (field) production, including less damage occurring to the root system when transplanted, better establishment after transplanting, decreased labor and land acquisition costs for production, and increased product availability and longevity in the retail market. Growing plants in containers, however, alters root growth and function and can change root morphology. Numerous factors influence root growth in containers. Roots of container-grown plants are subjected to temperature and moisture extremes not normally found in field production. The effects of substrate aeration (Ea) as well as water holding capacity (Pv) interact with different pot characteristics, resulting in changes to root morphology. Successful plant establishment after transplanting is often linked to root health. This review focuses on the roles of substrate physical and chemical properties, container characteristics, and temperature in altering root growth in container-grown woody nursery crops. Root circling, planting too deeply or “too-deep syndrome” (TDS), and the use of composts as container substrates will also be examined.
Hannah M. Mathers, Alejandra A. Acuña, Donna R. Long, Bridget K. Behe, Alan W. Hodges, John J. Haydu, Ursula K. Schuch, Susan S. Barton, Jennifer H. Dennis, Brian K. Maynard, Charles R. Hall, Robert McNeil, and Thomas Archer
The U.S. nursery and landscape industry generates 1.9 million jobs and had an annual payroll of greater than $3 billion in 2002, yet little is known about nursery and landscape workers. This lack of information is even more pressing considering that labor generally accounts for greater than 40% of production costs and 31% of gross sales. Labor shortages, immigration reform, and legal status of employees are widely reported as the industry's most critical issues. We hypothesized that relevant data regarding the nursery industry workforce may raise an appreciation of the industry's diversity, increase political power and public awareness, and help stakeholders evaluate policy decisions and plan corrective strategies in a more informed manner. A total of 4466 self-administered questionnaires were sent in 2006, attempting to reach 30 nurseries in each of nine states with 1561 returned (35% response rate). Hispanics constituted 70% of the average nursery workforce, including general laborers (76%), crew leaders (61%), and sales/managers (others) (21%). Across firms, labor retention was less than 51% after 5 years and only 22% of employees understood English, raising questions regarding availability and access to training. Sixty percent of nursery employees had not received work-related training, although 81% of men and 72% of women were interested, and an association between training and employee retention existed. The highest rated training topic of interest was English/Spanish (respective of Spanish/English primary language respondents). There was a positive correlation between developing fluency and worker turnover, making the laborer attrition rate even more unfavorable for employers who not only lost employees with acquired experience, but also with acquired English skills.