USDA Hardiness Zone Research Articles

Results from a Multi-year Evaluation Study of Epimedium Taxa

The Chicago Botanic Garden (USDA Hardiness Zone 5b, AHS Plant Heat Zone 5) conducted an evaluation study of 40 taxa of Epimedium from 1995 to 2001. The goal of this project was to select superior taxa based on winter hardiness, ornamental merit, pest and disease resistance, and cultural adaptability. Three plants of each taxon were grown in a site that features a variety of light levels and diverse microclimates. Of the 40 taxa that began this trial, 33 survived to receive final ratings that ranged from excellent to poor. Although a wide variety of data was collected for each taxon, overall ratings were based on four criteria: flower coverage, health quality, overall habit quality and winter hardiness. Among the rated plants, three varieties stood apart from the others: Epimedium `Crimson', E. grandiflorum `Sonoyzki' and E. × versicolor `Neosulphureum'. One of the most significant characteristics that separated the best performers from the mediocre and poor performers was winter hardiness. Five taxa lost all three plants in the first winter of the trial and were not retested. Several other taxa had their overall ratings severely impacted by winter losses. In addition to winter hardiness, data on frost damage, nutrient limitation and rabbit damage were also collected. Epimedium habits varied from clumped to widespreading groundcovers. Of the 33 barrenwort taxa to receive final ratings, nine were determined to have significant potential use as groundcovers. Also, 12 taxa displayed highly ornamental, early-season foliage colors in shades of burgundy, bronze and red.

Four Cultivars of Iochroma

Iochroma Benth. (Solanaceae) is a South American genus of some 20 species of shrubs and trees (Huxley et al., 1992). Iochroma cyaneum (Lindl.) M.L. Green is a soft-stemmed, shrubby Ecuadorean endemic generally found between 1800 and 2500 m elevation. The species is described typically as having dark blue fl owers. It is used occasionally as an ornamental shrub in the United Kingdom, southern Europe, Australia, and California. In this paper, we announce the formal recognition and release of three distinct color forms of this species, and a third unidentifi ed Iochroma, all of which have shown remarkable heat tolerance in south Florida, given their native range in elevation. We believe that these fast-growing cultivars have great potential for use as landscape shrubs in USDA Hardiness Zones 9B–11 (USDA, 1990), root-hardy perennials in Zones 8B–9A, and as annuals and/or fl owering pot plants in all zones.

Phenology and Depth of Cold Acclimation in the Three Subspecies of Alnus maritima

Alnus maritima (Marsh.) Muhl. ex Nutt. is a large shrub or small tree with potential for use in managed landscapes. Because the three subspecies of A. maritima are indigenous only to areas with mild winter temperatures (USDA hardiness zones 7a and 7b), knowledge of their cold acclimation and cold hardiness is vital if they are to be used where winters are more harsh. Phenology and depth of cold hardiness were assessed by collecting stem samples seven times from 25 Sept. 2000 to 23 Apr. 2001, subjecting the samples to cold temperature ramping, and determining the lowest survival temperature (LST) via the tissue discoloration method. Samples were collected from indigenous plants of the three subspecies and from plants growing in a common garden near Ames, Iowa (USDA zone 5a). Results indicated that some plants from all three subspecies can survive midwinter extremes as low as -80 °C; that plants grown in Ames achieved a greater depth of cold hardiness for most of the winter and were more uniform in cold hardiness than plants growing in warmer native sites; and that the three subspecies did not differ in phenology or depth of cold acclimation. Results of field trials with plots of 150 plants each installed in three northern hardiness zones (USDA zones 5a, 4a, and 3a) supported these conclusions by showing survival of all 450 plants. We resolved differences among subspecies by rating the percentage of stem tissue survival for each plant in the field plots. Subspecies maritima, from the northernmost provenance (the Delmarva Peninsula), showed the least stem death across all three plots (3.9% tissue death), followed by subsp. georgiensis from northwestern Georgia (10% tissue death), and subsp. oklahomensis from southern Oklahoma (12.8% tissue death). Our results suggest that low temperatures should not limit the use of A. maritima in areas as harsh as USDA zone 3a. Selections based on cold hardiness may allow the use of A. maritima in areas with even colder winters.

Natural Selection for Survival Improves Freezing Tolerance, Forage Yield, and Persistence of Festulolium

Festulolium [Festulolium loliaceum (Hudson) P.V. Fournier] is a hybrid between meadow fescue (Festuca pratensis Huds.) and Italian ryegrass (Lolium multiflorum Lam.) or perennial ryegrass (L. perenne L.). The ryegrass parentage gives festulolium cultivars marginal winterhardiness in regions with severely cold winters. The objective of this research was to determine if natural selection for field survival resulted in a genetic improvement in freezing tolerance, forage yield, or persistence in northcentral and northeastern USA environments. The component strains of 'Spring Green' festulolium were compared with their unselected parents in a controlled-environment freezing test. The component strains of Spring Green had 186% greater plant survival and 34% greater tiller survival than their unselected parents. Spring Green was compared with its two commercially available parents in a 13-location field test. Spring Green averaged 5.0% higher in 2-yr forage yield than 'Kemal', but was similar in forage yield to 'Tandem' in a 13-location test ranging from Minnesota to Virginia. Spring Green averaged 30% more ground cover than its unselected parents at the six locations within USDA hardiness zones 2 through 4, but was generally similar to its parents outside of these severe hardiness zones. The increased freezing tolerance of Spring Green, obtained by phenotypic selection for field survival at several locations, appears to have resulted in increased adaptation to northern USA forage production environments.

Flowering Performance of 21 Hydrangea macrophylla Cultivars

Abstract The flowering performance of 21 Hydrangea macrophylla cultivars was evaluated for three years at two locations. Ten plants of each cultivar were planted during 1997 in field plots at McMinnville, TN (USDA hardiness Zone 6b), and New Franklin, MO (USDA hardiness Zone 5b). The number of inflorescences was recorded every two weeks from May through September in 1999, 2000 and 2001. Daily maximum and minimum temperatures were recorded from October 1998 through September 2001. Differences among cultivars were found all three years at McMinnville and in 1999 and 2000 in New Franklin. While rankings of cultivars for number of inflorescences differed among years and locations, ‘All Summer Beauty’ and ‘Nikko Blue’ were consistently among the best performing cultivars. ‘Blauer Prinz’, ‘Blue Wave’, ‘Mariesii’ and ‘Todi’ flowered poorly all 3 years at both locations. No cultivar flowered reliably all three years of the study. Although some cultivars flowered sporadically throughout the summer, no remontant cultivars were identified. Poor flowering performance during the study appeared to be related to damage to aboveground portions of plants from a combination of early fall freezes, low winter temperatures and late spring freezes.

Chilling Duration Affects Foliar Budbreak Of Linden Cultivars

A study evaluating the effects of varying levels of chilling on foliar budbreak of linden (Tilia spp.) culivars was initiated in 1999 in Auburn, Ala. [lat. 32°36'N, long. 85°29'W, elevation 709 ft (216m), USDA Hardiness Zone 8a]. Littleleaf linden (T. cordata) `Greenspire' and `Fairview' required the most chilling to produce measurable budbreak and exhibited the lowest budbreak percentages. Silver linden (T. tomentosa) `Sterling' and american linden (T. americana) `Redmond' needed the fewest hours of chilling to produce budbreak and exhibited the highest budbreak percentages. `Sterling' was the top performer in foliar budbreak percentage and in subsequent growth. Although `Redmond' attained high budbreak numbers, its overall growth during the following growing season was inferior to that of `Sterling', `Greenspire' and `Fairview'. This information can contribute to the development of regional planting recommendations, which can aid in the selection of lindens suitable for the area in which they will be grown. Calculated r2 values indicated the models used provided a good fit to the data for all cultivars.

Herbaceous Perennial Trials in Central Alabama, 1996-97

Fifty-seven herbaceous perennials were evaluated from July 1996 to October 1997 in USDA Hardiness Zone 8. Plants in this study generally performed better the first year after planting than the second year. Several selections did not reemerge the second year, though some natural reseeding occurred. Still other selections never fully recovered from the winter months or succumbed to stress in the summer. Plants that maintained an attractive foliage display while not in bloom and plants that had a high bloom rating during the bloom season are worth incorporating into a full sun perennial or mixed border in the southeastern United States. Performance of perennials in the landscape may vary from year to year as climatic conditions affect performance. Comparison of results from variety trials at other locations should help increase performance information reliability for perennial selection.

Performance of Red Maple Selections in Southern Georgia

In 1991, a cooperative project with the U.S. National Arboretum in Washington, D.C., was initiated in Tifton, Ga. (USDA hardiness zone 8a) to evaluate red maples (Acer rubrum L.) potentially suitable for the coastal plain region of the southeastern U.S. Greatest annual height growth across all cultivars over 6 years was for `Alapaha', a seedling selection from southern Georgia with annual height growth of 35 inches (88.0 cm), and several seedling selections from northern Florida with annual height increases in excess of 33 inches (86.0 cm). Selections showing the least average annual height growth were NA-56024 and NA-57772 (`Red Rocket'). For commercially available cultivars, the most dependable for fall color in Tifton was `October Glory'®. In addition, two new selections from the National Arboretum have also shown excellent fall color—`Somerset' and `Brandywine'.

Miscanthus Anderss. Produces Viable Seed in Four USDA Hardiness Zones

Abstract Forty-one taxa of Miscanthus grown in USDA Hardiness Zones 4, 5, 6, and 7 were examined in 1996 and 1997 for seed set and viability. Although laboratory results varied widely between years, climatic zones, and cultivars, many plants set viable seed. Eleven types had <18% viable seed, including ‘Morning Light’, ‘Variegatus’ and ‘Zebrinus’, and appear to represent the least risk for becoming invasive plants, especially in northern climates. Other characteristics of Miscanthus are discussed in terms of invasive potential in the United States.

Root membrane thermostability of Cornus florida L. provenances

Flowering dogwood seed collected from native trees in 3 USDA Hardiness Zones (6b, 7a, and 8a; American Horticultural Society (AHS) Heat-Zones 7, 7, and 8, respectively) were germinated and grown in containers prior to laboratory procedures to determine root cell membrane thermostability. Unsuberized, current season, fine root tissues were subjected to temperatures ranging from 20 to 60°C for 30 min and analyzed for cellular electrolyte leakage. Electrolyte leakage from root tissue exhibited a sigmoidal response to temperature for trees from each location. Critical midpoint temperature ( T m) was greater for seedlings native to USDA Hardiness Zone 6b (AHS Heat-Zone 7), 52.4±0.6°C, than T m for seedlings originating from USDA Zone 7a (AHS Zone 7), 51.2±0.5°C. However, seedlings from USDA Zone 8a (AHS Zone 8), at 51.5±0.4°C, were similar to those collected in USDA Zones 6b (AHS Zone7) and 7a (AHS Zone 7). The results of this study find little genetic variability across this part of the native range of flowering dogwood regarding root thermotolerance. Although differences between two provenances were found, these differences may be of little ecological significance.

457 Woody Plant Selections and Introductions from North Dakota State University

Woody plant improvement research has been an active program at North Dakota State Univ. (NDSU) for many years. Eighteen cultivars have been introduced since 1986, and ≈10 additional releases are anticipated over the next 6 years. These superior, winterhardy cultivars have increased the inventory of adapted woody plants for landscape use in the northern plains, particularly USDA hardiness zones 3 and 4. Emphasis will be placed on recent introductions and promising plants for potential future release. An extensive, statewide cooperative evaluation program is conducted at seven sites throughout North Dakota and also at the NDSU Research Arboretum near Absaraka.

264 Identification and Characterization of Overwintering Methods for Container-grown Herbaceous Perennials

A survey was conducted to identify and characterize the effectiveness of overwintering methods used to protect container-grown herbaceous perennials in USDA hardiness zones 3 through 8. Survey questionnaires were sent by first-class mail on 20 Aug. 1996 to 634 firms involved in growing and/or selling container-grown herbaceous perennials identified from the Perennial Plant Association Membership Directory. Completed questionnaires were received from 293 individuals (46.2% response rate) in 38 states, the District of Columbia, and six Canadian provinces. Survey participants reported using several overwintering methods: structureless systems (71.0%), polyhouses (52.9%), polyhouses with inflated double polyethylene covers (30.7%), and low-profile polyhuts (12.3%). Over three-fourths of the respondents (78.8%) said their winter protection methods resulted in minimal to no plant loss (0-10%). Only 53 respondents (18.1%) reported losses >10%. The most frequently cited reason for plant loss across all hardiness zones was excessive moisture inside the overwintering environment (50.2%). Equal percentages (33.4%) indicated low temperatures and damage from animals as the next most likely factors responsible for plant loss. Respondents identified, in descending order, Iris, Delphinium, Lavandula, Papaver, and Lupinus as the five genera most difficult to overwinter.

`Chickasaw', `Kiowa', and `Pocomoke' Lagerstroemia

Additional index words. shrub breeding, crapemyrtle, Lagerstroemia indica, Lagerstroemia fauriei, Lythraceae, plant introduction, powdery mildew, Erysiphe lagerstroemiae previous Lagerstroemia introductions from the U.S. National Arboretum, these cultivars were named after Native American tribes. ‘Kiowa’. This selection originated from a shipment of nine cuttings of unnamed L. fauriei received from Dr. Y. Tachibana, Botanical Garden of Osaka, City Univ., Osaka Prefecture, Japan, in 1968 (PI 326424). It was evaluated initially on the grounds of the U.S. National Arboretum in Washington, D.C. (USDA hardiness zone 7b, U.S. Dept. of Agriculture Plant Hardiness Zone Map, 1990), with further evaluation by cooperators in California, Georgia, Maryland, and Tennessee. The cultivar name ‘Kiowa’ was registered in 1994 with the International Registration Authority for Lagerstroemia, in accordance with the International Code of Nomenclature for Cultivated Plants—1980 (Brickell, 1980).

Root membrane thermostability of red maple cultivars

Nine red maple cultivars originating in different USDA hardiness zones were grown in containers in 1995 and 1996 prior to laboratory procedures to determine root cell membrane thermostability. Electrolyte leakage from excised root tissue exposed for 30 min to temperatures ranging from 20 to 63°C, was used to assess cellular injury of unsuberized, current season, fine roots. Electrolyte leakage was an effective means for measuring cell membrane thermostability in root tissue. The critical killing temperatures of root tissue of cultivars evaluated ranged from 52.0±0.8°C to 53.3±0.5°C, indicating minimal differences in root membrane thermostability. Critical temperatures for cultivars selected from the northern part of the native range did not differ from cultivars originating elsewhere.

Differences in Growth of Container-Grown Red Maple Cultivars in Different Hardiness Zones

Abstract Growth of eight container-grown red maple cultivars was evaluated at four locations under dissimilar environmental conditions/hardiness zones in Georgia and Alabama for two years. Rooted cuttings and tissue cultured plantlets were planted in #3 (9.1 liter) containers in one location in the same substrate in April 1995. Trees were transported the second week of June to Blairsville, GA; Muscle Shoals, AL; Auburn, AL; and Tifton, GA in USDA hardiness zones 6b, 7a, 8a, and 8a, respectively. Dormant trees were transplanted to #10 (38 liter) containers the second week of December 1995. At the conclusion of the study, December 1996, growth differed by location and cultivar. Height and stem diameter increase, and root and stem dry weights were greatest at the two Alabama locations for most cultivars and were least at the two Georgia locations. Root:shoot ratios were greatest at the two Georgia locations, an indication of increased carbon partitioning of photoassimilates to roots over stems under dryer growing conditions. Irrigation rates and different environmental conditions among locations have a significant impact on growth response of container-grown red maple cultivars.

Freezing Tolerance in Rhododendron and Its Association with Dehydrin Expression

This study examines whether dehydrin expression in leaves is associated with varying levels of cold hardiness among evergreen rhododendrons. Initially, differences in leaf freezing tolerance (LFT) were determined within three groups of plants: 1) a cultivar group; `Chionoides' (CND), `Grumpy Yellow' (GY), and `Vulcan's Flame' (VF) 2) a segregating F2 population derived from a super coldhardy (R. catawbiense) × less-hardy (R. fortunei) cross, and 3) juvenile seedlings and mature plants of wild-collected R. maximum. LFTs in fully acclimated cultivars corresponded with their USDA hardiness zone ratings—CND (zone 4, –32 °C) GY (zone 7, –16 °C), and VF (zone 6, –19 °C). F2 segregation was characterized by a continuous, normal distribution of LFT values, with groups of progeny at the “tails” differing in their mean LFT by 20 °C. Juvenile seedlings of R. maximum exhibited LFTs that were 12 °C lower than LFTs from mature plants. Western blots of leaf proteins revealed a common 50-kDa dehydrin that accumulated during cold acclimation in all three cultivars and appeared to be quantitatively associated with LFTs. Isoelectric focusing of the 50-kDa Rhododendron dehydrin revealed two isoforms (pI 5.5 and 6.5). The more acidic isoform was detected only in the hardiest (CND) cultivar. Experiments are underway to examine qualitative and quantitative expression of dehydrins and its association with LFT in the segregating F2 population and in the group of R. maximum plants differing in physiological age.

Evaluation of Lonicera Taxa for Honeysuckle Aphid Susceptibility, Winter Hardiness and Use

Abstract One-hundred-thirty-five honeysuckle (Lonicera L.) taxa were evaluated in North Dakota, Manitoba and/or reviewed in the literature for relative honeysuckle aphid [Hyadaphis tataricae (Aizenberg)] susceptibility, winter hardiness and landscape characteristics. Sixty taxa are rated susceptible to highly susceptible, 13 lightly susceptible and 55 with apparent resistance to aphid attack. Lonicera tatarica L.,L. morrowii Gray. and L. ruprechtiana Reg., including cultivars and hybrids derived from these species, were particularly susceptible. Taxa with apparent aphid resistance were evaluated for hardiness zone assignment and landscape qualities. Only 3 taxa are rated in the high recommendation category for landscape planting in USDA hardiness zones 2 through 5. These include L. x brownii (Reg.) Carr. ‘Dropmore Scarlet’, L. x xylosteoides Tausch. ‘Miniglobe’ and L. xylosteum L. ‘Emerald Mound’. Sixteen taxa received an average recommendation and 27 taxa a low recommendation. The 60 taxa which proved susceptible or highly susceptible to honeysuckle aphid, plus 17 additional taxa, are placed in the non-recommendation category. These 77 taxa represent 57% of the taxa evaluated in this study. Eleven taxa are recommended for potential use in shelterbelt or conservation plantings as replacements for aphid susceptible honeysuckles.

Seed Viability in Miscanthus Grown in Different Hardiness Zones

Miscanthus is one of the most popular ornamental grasses. Reports of self-seeding however, have occurred in the Central Atlantic states, making it a possible weed threat. Ascertaining whether Miscanthus self-seeds or not may determine its continued use as an ornamental, decorative plant. With more than 50 named cultivars of Miscanthus sinensis and several other Miscanthus species available in the trade, wide morphological variation appears to exist within this genus. Because Miscanthus is a warm-season grass requiring a relatively long growing season, self-seeding may vary depending on the USDA Hardiness Zone in which the plant is grown. Mature inflorescences from 35 different cultivars or species of Miscanthus were collected or acquired from nurseries or arboreta in USDA Zones 4, 5, 6, and 7 in the fall or early winter of 1996. Inflorescences were examined for seed set by hand cleaning. The percentage of viability seed and seed germination was determined by germination in laboratory conditions. Results varied by cultivar or species and as well as by source. A comparison of results will presented and the implications of Miscanthus self-seeding or becoming a potential weed threat will be discussed.

Cold Hardiness of Flowering Crabapple Cultivars

Abstract Stem cold hardiness of 38 crabapple taxa (Malus L.) was evaluated on 6 dates throughout the dormant period of 1992–93. Differences in cold hardiness among taxa were detected on all sampling dates, with no single cultivar being consistently most hardy. Midwinter hardiness levels ranged from −32C (−26F) for ‘Jewelberry’ to −44C (−47F) for M. baccata ‘Jackii’. ‘Dolgo’, ‘Red Jade’, ‘Red Splendor’, ‘Selkirk’, and M. baccata ‘Jackii’ were all hardy to at least −40C (−40F) in midwinter and also exhibited excellent early-and late-season hardiness. Large differences in hardiness among crabapple taxa were observed on November 9 and April 26 sampling dates [14C (25F) and 17C (31F), respectively], indicating that taxa vary substantially in timing and/or rates of acclimation and deacclimation. Although most cultivars demonstrated adequate hardiness for use in USDA hardiness zone 4a, the acclimation and deacclimation characteristics of some taxa may predispose them to injury from extreme low temperatures in the fall and early spring.

Classification of Cephalotaxus Species Based on rbcL Sequences

Cephalotaxus species are needle evergreens offering the aesthetic qualities of Taxus, yew, yet are heat- and drought-tolerant, sun- and shade-adaptable, and resist deer browsing. They are adaptable to nursery and garden cultivation in USDA hardiness zones (5)6–9. Unfortunately, the various species are frequently confused in the American nursery trade due to their extreme similarity in morphology. Recently, molecular data have been widely applied in the taxonomic studies, especially DNA sequencing. The chloroplast gene rbcL of Cephalotaxus has been sequenced for determining species relationships. The preliminary results show that C. oliveri Mast. has 10 base changes from C. drupacea Sieb. et Zucc., while only one base difference occurred between C. drupacea and C. harringtonia (Forbes) Koch. There are between one and 10 base substitutions among C. fortunei Hooker, C. koreana Nakai, and C. sinensis (Rehd. et Wils.) Li. Compared with other closely related conifers, Cephalotaxus has a substantial number of differences among species except between C. drupacea and C. harringtonia, which may not be distinct species. Detailed data relative to gene sequencing, growth morphology, and horticultural characteristics should lead to correct identification of species and great horticultural uses. Furthermore, the method of rbcL sequence can be applied to distinguish other morphologically homogeneous ornamental plants.