How cold is too cold?

Understanding the tolerance level of a plant is crucial to successful overwintering.

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December 28, 2016

The key to overwintering is to maintain even temperature, keeping the plants cold and alive but not actively growing. When plants are allowed to freeze and then thaw, there is a greater risk for plant loss. The degree of winter protection should be based on the expected minimum temperatures at a particular site and the root hardiness (the lowest temperature at which roots survive) of species being grown. Root and crown hardiness have only begun to be identified for herbaceous perennials by researchers. Until root hardiness is identified, growers have no choice but to take steps to avoid losses of plants.

When to cover and uncover

It is not uncommon to see plants being covered as late as November or early December. Waiting as long as possible to cover plants will allow plants to harden off which minimizes the risk of winter injury. A general guideline for covering is to use the first frost date (F) + 30 or F + 45. For example, if the first frost date is Oct. 15 then F + 30 or F + 45 would be Nov. 15 or Nov. 30.

Plants should not be uncovered until after the danger of subfreezing temperatures. Plants are usually uncovered as soon as possible in spring and this of course will vary across the state depending on weather conditions. The goal is to prevent premature shoot growth and to ensure that unusually cold weather in late winter and early spring does not kill or injure plants. Inspect protected plants frequently for signs of shoot growth and vent to introduce cool air to slow plant development and harden plants. In early spring, some growers cut holes in poly coverings to ensure adequate ventilation while still providing adequate protection from frosts.

In summary, proper overwintering of container grown ornamentals is essential to maintain high plant quality. The system choice depends on capital as well as the amount of protection required by the plants.

To determine the extent of winter protection required for the plants, growers can refer to this UMass Extension fact sheet. The following table describes the average temperatures needed to kill the roots of woody ornamental plants.

Once you understand the tolerance level of the plant, you can develop a plan using the appropriate overwintering technique.


Average killing temperatures for roots of selected species of woody ornamental plants.

Scientific Name

Common Name

Killing Temperature °F

Magnolia soulangiana

Saucer Magnolia

23

Magnolia stellata

Star Magnolia

23

Cornus florida

Flowering Dogwood

20

Daphne cneorum

Garland Flower

20

Ilex crenata 'Convexa'

Convex Japanese Holly

20

Ilex crenata 'Hetzi'

Hetz Japanese Holly

20

Ilex crenata 'Stokesii'

Stokes Japanese Holly

20

Ilex opaca

American Holly

20

Pyracantha coccinea

Fire Thorn

18

Cryptomeria japonica

Japanese Cedar

16

Cotoneaster horizontalis

Rock Cotoneaster

15

Viburnum carlesii

Korean Spice Viburnum

15

Cytisus praecox

Warminster broom

15

Buxus sempervirens

Common Boxwood

15

Ilex glabra

Inkberry Holly

15

Euonymus fortunei 'Carrierei'

Carrier Euonymus

15

Euonymus fortunei 'Argenteo-marginata'

Variegated Euonymus

15

Hedera helix 'Baltica'

Baltic Ivy

15

Pachysandra terminalis

Japanese pachysandra

15

Vinca minor

Common Periwinkle

15

Pieris japonica 'Compacta'

Compact Pieris

15

Acer palmatum 'Atropurpureum'

Bloodleaf Japanese Maple

14

Cotoneater adpressa praecox

Nan-Shan Cotoneaster

10

Taxus media 'Nigra'

Black Anglojap Yew

10

Rhododendron 'Gibraltar'

Gibraltar Azalea

10

Rhododendron 'Hinodegiri'

Azalea hybrid

10

Pieris japonica

Japanese Pieris

10

Leucothoe fontanesiana

Drooping Leucothoe

5

Pieris floribunda

Flowering Pieris

5

Euonymus fortunei 'Colorata'

Purple Leaf Wintercreeper

5

Juniperus horizontalis

Creeping Juniper

0

Juniperus horizontalis 'Douglasii'

Waukegan Juniper

0

Rhododendron carolinianum

Carolina Rhododendron

0

Rhododendron catawbiense

Catawba Rhododendron

-10

Rhododendron P.J.M. hybrids

P.J.M. Rhododendron

-10

Potentilla fruticosa

Shrubby Cinquefoil

-10

Picea glauca

White Spruce

-10

Picea omorika

Serbian Spruce

-10

Highest temperature that killed more than 50% of root system and reduced top growth. SOURCE: Havis, J.R. 1964. Root hardiness of woody ornamentals. HortScience 11(4):385-386.

 

For more information, visit the UMass Extension online here.