Woody Ornamental Plant Hardiness Trials
Lyle Littlefield, University of Maine
Reprinted from Cooperative Extension Service, University of Maine, Bulletin #599.
Introduction
During the past eighteen years, various woody ornamental plants have been planted at a test and demonstration site on the Orono campus of the University of Maine. The objectives of these plantings are to observe plant hardiness, adaptability for landscape purposes and provide a collection of ornamental plants in a relatively small area for public observation.
The test area is very exposed. Therefore, these plantings are probably more severely subjected to wind than they would be in most home or commercial plantings. According to the USDA Plant Hardiness Zone Map, Orono is on the borderline between plant hardiness zone 4 and 5.
The soil is classified as Marlow, a fine sandy loam with many stones. Irrigation is used when necessary. The shrubs are planted in beds and mulched with several inches of old bark from a local pulp mill. Grass growth is controlled around tree plantings with herbicides.
Most of the plants have been secured from commercial nurseries. Some have been obtained from the U.S. Plant Introduction Station, Glenn Dale, MD; the Northeast Regional Plant Introduction Station, Geneva, NY: the U.S. National Arboretum, Washington, DC and the Arnold Arboretum, Jamaica Plain, MA. In a few instances plants have been started from seed or scions secured from various sources.
Many of the plants are small when received and are grown in a greenhouse or nursery for a few years before being planted in permanent locations. A few plants have died in the nursery before they could be set in a permanent site, and these are included in List No. 2, "Plants that Failed to Survive."
Wherever possible, a recognized common name follows the scientific name in the listing. Plants with a P.I. number, such as
Cornus mas
P.I. 293771, are those plants received from the U.S. Plant Introduction Station. Numbers preceded by "G" are those from Geneva, NY, and those with "NA" are releases from the National Arboretum.
The number in parentheses ( ) following the scientific name is the year the plant was set in a permanent location and observations were started. This will give some idea as to how reliable the plant is over a period of years.
Hardiness Rating
The plants have been observed each spring for winter injury. They have been rated for hardiness on a scale of 1 to 4 based upon these observations. See List No. 1, "Plants that Survived in Trials." Plants with little or no winter injury to vegetative growth are rated 1, and plants that have had severe injury, resulting in dieback to near the ground line are rated 4. Plants that have not been observed during winter in the permanent planting are designated by the symbol "*". These ratings include the winter of 1981 -82.
There has been no attempt to rate the plants for flower bud kill. A few plants, such as forsythia and flowering quince, normally have flower bud kill during extreme winters and only flower on the lower branches, but the vegetative part of the plant is not injured.
A few plants, such as boxwood and Alberta spruce, have had protection from winter sun and wind by burlap screens or snow fencing.
Several plants have died due to lack of winter hardiness. Rather than including them in the general listing and giving them a rating of 5, they are listed separately in List No. 2, "Plants that Failed to Survive," which follows the general list. In several instances more than one planting was tested. A few of these plants do grow on the Orono campus and are identified with the symbol "A" before the name. The harsh winter 1980-81 killed several plants that had showed tenderness in previous seasons.
LIST #1 PLANTS THAT SURVIVED IN TRIALS
Rhododendron calendulaceum (74) Flame Azalea .................... 1
R. calendulaceum x roseum (56) Azalea Hybrid ....................... 2
R. Exbury Hybrid (66) Exbury Hybrid Azalea .......................... 4
R. impeditum (73) Cloudland Azalea ........................................ 1
R. japonicum (66) Japanese Azalea .......................................... 2
R. japonicum aureum x luteum hybrid (56) Azalea Hybrid ......... 1
R. kaempferi-tall (obtusum) (74) Tall Torch Azalea................... 3
R. kiusianum P.I. 330366 (82) Kyusho Azalea ..........................*
R. 'Louise Hunnewell' x luteum (65) Azalea Hybrid .................. 4
R. Mollis Hybrid (66) Mollis Hybrid Azalea ............................. 4
R. 'Northern Lights' (82) Northern Lights Azalea ......................*
R. prinophyllum (roseum) (155) Roseshell Azalea .................... 3
R. rupicola (71) Cliffplum Rhododendron ................................ 1
R. schlippenbachii (66) Royal Azalea ....................................... 2
R. vaseyi (156) Pinkshell Azalea............................................... 1
R. viscosum (74) Swamp Azalea ............................................. 1
R. viscosum x japonicum (65) Azalea Hybrid ........................... 1
Rhus aromatica (71) Fragrant Sumac........................................ 1
R. copallina (76) Shining Sumac................................................ 3
R. typhina Staghorn Sumac ...................................................... 1
R. typhina 'Lacinata' (68) Cutleaf Staghorn Sumac .................... 1
Rosa acicularis sayiana P.I. 236942 (66) Prickly Rose .............. 1
R. arkansana P.I. 236944 (66) Arkansas Rose ......................... 1
R. blanda P.I. 236945 (67) Meadow Rose ............................... 2
R. 'Carefree Beauty' (79) ............................................................... 4
R. centifolia muscosa 'Mossman' (69) ............................................. 4
R. damascena 'Madame Hardy' (73) ............................................... 4
R. eglanteria (69) Sweetbrier Rose ............................................ 4
R. foetida 'Austrian Copper' (78) .................................................... 2
R. gallica 'Charles De Mills' (69)...................................................... 2
R. gallica officinalis (69) Apothecary Rose ................................. 4
R. gallica 'President De Seze' (71)................................................... 4
R. harisoni (69) Harison's Yellow Rose ...................................... 1
R. hemisphaerica (sulphurea) P.I. 205218 (64) Sulfur Rose ......... 3
R. hugonis (67) Father Hugo Rose ............................................. 2
R. macrantha 'Duesterlohe'G-13215(66) ......................................... 2
R. multiflora Multiflora Rose ...................................................... 2
R. multiflora Platyphylla' (70) Seven Sister Rose ........................ 2
R. mundi (73) ................................................................................. 3
R. 'Rhode Island Red' (71) ............................................................. 4
R. rugosa P.I. 227432 (66) Rugosa Rose .................................. 1
R. rugosa P.I. 384453 (74) Rugosa Rose .................................. 1
R. rugosa (65) Rugosa Rose ...................................................... 1
R. rugosa 'Agnes' (70) .................................................................... 3
LIST #2 PLANTS THAT DIDN'T SURVIVE
Philadelphus coronarius 'Aureus' Golden Sweet Mockorange
P. 'Galahad' Galahad Mockorange
P. lemoinei 'Innocence' Innocence Mockorange
P. lemoinei 'Silver Showers' Silver Showers Mockorange
P. microphyllus P.I. 233562 Littleleaf Mockorange
P. purpurea-maculatus 'Ophelia' Ophelia Mockorange
P. 'Snow White' Snow White Mockorange
Pieris japonica 'Pygmaea' Pygmy Japanese Andromeda
Pinus cembroides Mexican Stone Pine
P. thunbergi Japanese Black Pine
Prinsepia sinensis Cherry Prinsepia
P. uniflora P.I. 309638; G-13150
Prunus blireiana 'Newport' Newport Plum (A)
P. ceresifera Thundercloud' Thundercloud Redleaf Plum
P. 'Crimson Rocket'
P. maacki P.I. 26271 6 Amur Chokecherry
P. 'North Star' North Star Cherry
P. subhirtella 'Pendula' Weeping Higan Cherry
P. tenella 'Fire Hill' Fire Hill Russian Almond
P. triloba Flowering Almond
Pseudolarix amabilis Golden Larch
Pyracantha crenulata P.I. 285401
P. calleryana 'Bradford' Bradford Callery Pear
Quercus coccinea Scarlet Oak
Q. velutina Black Oak
Rhododendron aberconwayi
R. barbatum P.I. 307331
R. campanulatum P.I. 307332
R. keiskei
R. lepidotum P.I. 307344
R. 'Mars' X 'Elizabeth'
R. 'Mars' X (yakushimanum X 'Mars')
R. metternichii Leatherleaf Rhododendron NA 45155
R. mucronulatum X racemosum 'Gable's Pioneer' Gable's Pioneer Rhodo.
R. mucronulatum 'Cornell Pink' Cornell Pink Azalea (A)
R. molle P.I. 159034 Mollis Azalea (A)
R. poukhanense Korean Azalea
R. 'Raphael De Smet' Raphael De Smet Rhododendron
R. dauricum 'Hokkaido'
R. serpyllifolium P.I. 274546
R. tschonoskii NA 44806
R. weyrichii P.I. 317273
Rosa alba semiplena G-13199
R. 'Alchymist' Alchymist Rose