Cooperative Extension Fact Sheet FS528 | February 2015
Temperate climate woody plant material requires a dormant period of colder temperatures to perform well the following season. Typically, six to eight weeks of temperatures below 40°F is needed. For herbaceous perennials a process called "vernalization" (cool temperature exposure) is needed for them to flower, usually along with a dormant period.
While the primary concern for nursery plant material grown in soil is damage to the foliage, crown or stems, growers of plant material produced in containers must also be concerned about root viability. The ability of plants to adapt to cold is determined by their genetic makeup. Two factors that allow plants to acclimate (increase the plant's ability to withstand colder conditions) are decreasing day length and colder temperatures. Factors that can reduce the plant's capacity to acclimate include high fertility and physiological damage during the period of acclimation along with elevated and typically fluctuating temperatures during the normally cold winter season. Maintaining adequate media moisture throughout the winter is the single most important factor to reduce root damage.
Woody plant material: Acclimation to cold temperatures in the above ground portion of plants is initiated by shorter days followed by lower and then freezing temperatures. To become fully hardy, nursery crops require exposure to temperatures from 32°F to 40°F (0.0°C to 7.2°C) followed by temperatures slightly below freezing. While colder temperatures slow or stop root growth, they don't acclimate to the same extent as shoots. The day length doesn't seem to impact the development of hardiness in roots. Root hardiness is the most important factor affecting the winter survival of container-grown ornamentals. The temperature of growing media is the factor that determines the tolerance of a plant to different temperatures. Table 1 presents information on the root hardiness of various woody plants.
Low air temperatures significantly influence the performance of plant material when grown in containers. The primary factor that buffers the effects of low air temperature is water in the medium. Under freezing temperature conditions, the heat of fusion (the heat energy that needs to be given up in order for water to freeze) protects plant roots. It requires the loss of 79.7 calories of energy/gram to change water to ice. The media temperature will remain in the range of 28° to 32°F while it freezes (Chart 1). In comparison, it takes 100 calories or energy/gram of water to raise the temperature from 0°F to 212°F (0°C to 100°C). Following the loss of the heat of fusion, medium temperatures will drop to near ambient air temperatures.
Herbaceous plant material: There has been relatively little research conducted relating to cold hardiness of herbaceous perennials. Since the aerial portions of the plants die back during the winter, the areas of concern are for the crown and roots of the plants. Concern is also primarily directed at container produced nursery stock since in-ground material is largely buffered from the effects of cold due to soil mass. As with woody plant material grown in containers, roots are exposed to near-ambient air temperatures after being exposed to freezing temperatures for an extended period of time.
Research has been done on herbaceous perennial cold acclimation. The strongest relationship to survival was that having a well-developed root system helps plants survive better than a weak one. Work on the factor of plant age to survival and quality indicated it was not related. Based on information presented in Table 2, herbaceous perennials, in general, do not exhibit the same level of hardiness as woody plant material.
Under colder conditions, root hardiness is of primary concern when overwintering container plant material. With milder winter conditions, foliar and crown damage becomes increasingly possible. Cultural practices that stimulate late season growth can impact a plant's ability to acclimate and should be avoided. Symptoms for most of the following types of damage will not show up until the plant becomes more physiologically active.
On cold, clear days, late afternoon sunlight can cause the stem or trunk of plants to be warm and active. The temperature drop that occurs a short time after sunset can cause the fluids in the vascular tissue to freeze and expand, thus separating the bark from the wood at the cambium. Cold snaps early in the fall or late in the spring when plant are not acclimated to cold can also cause bark splitting.
Timing is everything when it comes to nursery crop protection from cold damage.
When compared to the outdoor temperature, white (milky) polyethylene covered overwintering structures result in decreased high temperatures and higher low temperatures, effectively offering protection to plants. Chart 2 shows actual temperature data from outside and inside a structure. The following are management suggestions to reduce the probability of cold damage.
|Highest temperature that killed more than 50% of root system and reduced top growth|
|Scientific Name||Common Name||°F||°C|
|Magnolia soulangiana||Saucer Magnolia||23||-5.0|
|Magnolia stellata||Star Magnolia||23||-5.0|
|Cornus florida||Flowering Dogwood||20||-6.7|
|Daphne cneorum||Garland Flower||20||-6.7|
|Ilex crenata 'Convexa'||Convex Japanese Holly||20||-6.7|
|Ilex crenata 'Hetzi'||Hetz Japanese Holly||20||-6.7|
|Ilex crenata 'Stokesii'||Stokes Japanese Holly||20||-6.7|
|Ilex opaca||American Holly||20||-6.7|
|Pyracantha coccinea||Fire Thorn||18||-7.8|
|Cryptomeria japonica||Japanese Cedar||16||-8.9|
|Cotoneaster horizontalis||Rock Cotoneaster||15||-9.4|
|Viburnum carlesii||Korean Spice Viburnum||15||-9.4|
|Cytisus praecox||Warminster broom||15||-9.4|
|Buxus sempervirens||Common Boxwood||15||-9.4|
|Ilex glabra||Inkberry Holly||15||-9.4|
|Euonymus fortunei 'Carrierei'||Carrier Euonymus||15||-9.4|
|Euonymus fortunei 'Argenteo-marginata'||Variegated Euonymus||15||-9.4|
|Hedera helix 'Baltica'||Baltic Ivy||15||-9.4|
|Pachysandra terminalis||Japanese pachysandra||15||-9.4|
|Vinca minor||Common Periwinkle||15||-9.4|
|Pieris japonica 'Compacta'||Compact Pieris||15||-9.4|
|Acer palmatum 'Atropurpureum'||Bloodleaf Japanese Maple||14||-10.0|
|Cotoneater adpressa praecox||Nan-Shan Cotoneaster||10||-12.2|
|Taxus x media 'Nigra'||Nigra Yew||10||-12.2|
|Rhododendron 'Gibraltar'||Gibraltar Azalea||10||-12.2|
|Rhododendron 'Hinodegiri'||Azalea hybrid||10||-12.2|
|Pieris japonica||Japanese Pieris||10||-12.2|
|Leucothoe fontanesiana||Drooping Leucothoe||5||-15.0|
|Pieris floribunda||Flowering Pieris||5||-15.0|
|Euonymus fortunei 'Colorata'||Purple Leaf Wintercreeper||5||-15.0|
|Juniperus horizontalis||Creeping Juniper||0||-17.8|
|Juniperus horizontalis 'Douglasii'||Waukegan Juniper||0||-17.8|
|Rhododendron carolinianum||Carolina Rhododendron||0||-17.8|
|Rhododendron catawbiense||Catawba Rhododendron||-10||-23.3|
|Rhododendron P.J.M. hybrids||P.J.M. Rhododendron||-10||-23.3|
|Potentilla fruticosa||Shrubby Cinquefoil||-10||-23.3|
|Picea glauca||White Spruce||-10||-23.3|
|Picea omorika||Serbian Spruce||-10||-23.3|
Source: Havis, J.R. 1964. Root hardiness of woody ornamentals. HortScience 11(4):385-386.
|Growing medium temperature causing significant injury (Unsalable, unacceptable regrowth)|
|Scientific Name||Common Name||°F||°C|
|Aster lateriflorus var. horizontalis||Calico Aster||28||-2.2|
|Digitalis x mertonensis||Strawberry Foxglove||24||-4.4|
|Geum quellyon 'Mrs. Bradshaw'||Chilean Avens||24||-4.4|
|Hibiscus moscheutos 'Disco Belle Hybrids'||Disco Belle Hibiscus||>38||>3.3|
|Houttuynia cordata 'Chameleon'||Chameleon Plant||28||-2.2|
|Kniphofia uvaria 'Pfitzei'||Torchlily||27||-2.8|
|Polystichum tsussimense||Korean Rock Fern||> 38||>3.3|
|Thelyptris kunthii||Kunth's Maiden Fern||28||-2.2|
|Tricyrtis formosana 'Amethystina'||Japanese Toad Lily||28||-2.2|
|Astilbe x arendsii 'White Gloria'||False Spirea||15||-9.4|
|Campanula glomerata var. acaulis||Clustered Bellflower||15||-9.4|
|Caryopteris x clandonensis 'Longwood Blue'||Bluebeard||21||-6.1|
|Chrysanthemum coccineum||Painted Daisy||21||-6.1|
|Coreopsis grandiflora 'Sunray'||Tickseed||18||-7.8|
|Dendranthema x grandiflora 'Emily'||Garden Chrysanthemum||> 10||>-12.2|
|Dendranthema x grandiflora 'Megan'||Garden Chrysanthemum||> 10||>-12.2|
|Dendranthema x grnadiflora 'Ruby Mound'||Garden Chrysanthemum||> 10||>-12.2|
|Dendranthema x grandiflora 'Triumph'||Garden Chrysanthemum||> 10||>-12.2|
|Erodium x variabile 'Roseum'||Heron's Bill||18||-7.8|
|Gaillardia x grandiflora 'Goblin'||Goblin Blanket Flower||14||-10.0|
|Hebe macrocarpa 'Margret'||Hebe 'Margret'||18||-7.8|
|Hemerocallis 'Joan Senior'||Daylily||15||-9.4|
|Heuchera sanguinea 'Chatterbox'||Coral Bells||14||-10.0|
|Leucanthemum x superbum 'Alaska'||Shasta Daisy||15||-9.4|
|Phlox paniculata 'David'||Garden Phlox||21||-6.1|
|Tiarella cordifolia var. collina 'Dunvegan'||Heartleaf Foamflower||15||-9.4|
|Tiarella cordifolia var. collina 'Oakleaf'||Heartleaf Foamflower||21||-6.1|
|Tricyrtis hirta 'Miyazaki'||Toad Lily||21||-6.1|
|Verbena canadensis 'Homestead Purple'||'Homestead Purple' Verbena||18||-7.8|
|Veronica 'Sunny Border Blue'||Speedwell||21||-6.1|
|Achillea 'Coronation Gold'||Yarrow, Sneezewort||8||-13.3|
|Achillea filipendulina 'Parker's Variety'||Fernleaf Yarrow||12||-11.1|
|Campanula takesimana||Korean Bellflower||2||-11.1|
|Dendranthema x grandiflora 'Baby Tears'||Garden Chrysanthemum||<10||<-12.2|
|Dendranthema x grandiflora 'Debonair'||Garden Chrysanthemum||<10||<-12.2|
|Gaillardia x grandiflora 'Monarch Group'||Monarch Blanket Flower||12||-11.1|
|Heuchera americana 'Dale's Strain'||Coral Bells||12||-11.1|
|Hylotelephium (Sedum) spectabile 'Brilliant'||Showy Stonecrop||-6||-21.1|
|Lythrum salicaria 'Robert'||Purple Loosestrife||12||-11.1|
|Monarda 'Marshall's Delight'||Bee Balm||8||13.3|
|Penstemon fruticosus 'Purple Haze'||Shrubby Penstemon||12||-11.1|
|Phlox divaricata subsp. Laphamii 'Chattahoochee'||Wild Sweet William||7||-13.9|
|Phlox glaberrima 'Morris Berd'||Smooth Phlox||7||-13.9|
|Phlox paniculata 'White Admiral'||Garden Phlox||8||-13.3|
|Physostegia virginiana 'Summer Snow'||Obedient Plant, False Dragonhead||7||-13.9|
|Rosmarinus officinalis 'Arp'||Rosemary||12||-11.1|
|Salvia x superba 'Stratford Blue'||Hybrid Sage||10||-12.2|
|Sedum 'Autumn Joy'||Stonecrop||-11||-23.9|
|Tanacetum coccineum 'Robinson's Mix'||Pyrethrum Daisy||10||-12.2|
|Tiarella cordifolia var. collina 'Slick Rock'||Heartleaf Foamflower||12||-11.1|
|Tiarella cordifolia var. collina 'Laird of Skye'||Heartleaf Foamflower||8||-13.3|
|Tiarella cordifolia 'Running Tapestry'||Heartleaf Foamflower||12||-11.1|
Source: Perry, L. P. 1998. Herbaceous Perennials Production: A Guide from Propagation to Marketing. NRAES-93. pp 56.
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