Overwintering Container-Grown Ornamentals
Most containerized perennials and woody nursery stock held over winter need protection from cold. Roots are much less cold-hardy than shoots and unlike field grown plants, the roots of container-grown plants are not buffered from cold temperatures by the soil. In addition to root hardiness, factors such as soil moisture, temperature fluctuations, and root development in the container also affect plant survival. The type of overwintering technique used is determined by the plant species and the ability of their roots to withstand cold temperatures during winter. Plants with roots that can withstand colder temperatures may only need to be consolidated and the outside perimeter protected with bales of hay or bags of leaves. This can save time and money compared to using more significant systems.
Growers must first determine the extent of winter protection required for the plants.
Steps prior to covering plants:
- Woody plants and herbaceous perennials should be completely dormant or hardened off before covering for the winter. In zone 5 and 6, this is typically in late-November.
- Herbaceous perennials should be potted up by late September or early October to allow them to become established for several weeks before cold temperatures arrive in late November. Poorly established and pot-bound plants tend to overwinter poorly.
- Plants that die back should be trimmed back to the crown and cleaned up prior to covering. Evergreen perennials such as Phlox subulata and Iberis sempervirens should be protected from direct contact with overwintering coverings either by laying them on their sides or using a structure over them.
- Consolidate plants as close as possible one to two days before covering herbaceous or woody ornamentals.
- Water everything well to prevent desiccation, allowing foliage/branches to dry thoroughly. Moist media freezes slower and releases heat compared to dry media, offering protection to the roots.
- Check the moisture level of the media during the winter and irrigate if necessary. In an unheated polyhouse water may be needed as often as once every two weeks, while ground beds covered with thermal blankets hold moisture and probably will not need watering.
- Avoid over-watering plants which will promote root, crown and foliar diseases.
- Some growers treat plants with a broad spectrum fungicide and allow them to dry prior to covering, especially if there has been a history of disease.
- Provide rodent protection. Poly-houses (if used) can be made rodent tight by burying fine mesh screen wire such as hardware cloth around the perimeter under ground and bending it outward at a 90 angle leaving it at least 6" deep. Mow and clean vegetation around overwintering areas to eliminate protected areas for rodents. Use commercially available baits or repellents such as human hair sprinkled around or cut up deodorant soap. Begin baiting for mice about a month before covering plants to reduce their population.
Systems for covering plants
- Unheated Hoophouses covered with 4-6 ml white polyethylene reflect solar radiation and will not heat up as much as clear polyethylene. However the lack of light in the spring can cause plants to stretch. By using white poly and orienting overwintering structures north-south, cooler leaf and air temperatures inside the house are obtained thus reducing transpiration.
If possible, place cold sensitive species in the center of the house and more tolerant species around the edge. Once temperatures consistently drop to 25-30° F, pots inside can be covered with a lightweight foam blanket only for the duration of the extremely low temperatures, to further protect plants from extreme cold.
- Heated Greenhouse or Polyhouse with Roll-up Sides covered with two layers of clear poly are often used by perennial growers. Heat is used to keep the soil temperature at 25°F for nursery, and in a range of 30 to 34° F for most perennials. A soil thermometer is used to verify that plants are in this range. If the temperatures warm up for several days above 40° F then ventilation is provided.
Greenhouses are heated in the fall to 50° F until all of the plants (perennials) are well-rooted, then the temperature is slowly lowered to 35° F where that temperature is maintained throughout the winter. Clear poly allows the maximum amount of sunlight exposure to the crop, foliage stays drier and in the spring, plants break dormancy earlier allowing for early shipping. Roll-up sides maintain the same temperature inside as outside, during a winter thaw and in the spring when warm sunny days cause a rapid rise in the greenhouse temperature. Roll-up the sides are also useful for hardening off plants in the spring and protection if unexpected cold weather or snow arrives.
Many perennial growers feel it is easier to keep plants thawed than frozen due to the variability of the weather.
- Structureless systems involve laying protective covers directly over plants and securing the edges. The big disadvantage of this system is you can not check the plants as easily. Once the plants are covered, they should remain sealed for the winter. Also, towards the end of the winter when day temperatures increase, venting to inhibit plant growth is more difficult.
Plants are placed pot to pot in an upright position on the ground, tall plants and woody plants are leaned over just like laying shingles, plants with foliage are laid on their side, and larger plants are laid on their sides with the crowns towards the middle. Plants are covered with microfoam – poly cover. The blanket is pulled tight over the containers and the edges secured following the manufacturer's recommendation.
In areas where air temperatures are likely to fall to -10° F, or where exposure to high winds is likely, a double layer is suggested. Thermoblankets tend to trap and retain moisture and problems under the blankets are difficult to detect because the blankets are opaque. Also, venting may be needed during unexpected periods of warm winter weather.
- No covering. Some plants are root hardy enough that no protection may be necessary unless very unusual winter temperatures are experienced. If the area is not protected from winter winds, then it may be advantageous to protect the perimeter with bales of hay or bags of leaves. If this system has not been used at your nursery, it is advisable to experiment with this using only a few plants of each species.
- Other methods. A variety of other techniques have been utilized to protect container grown plants during winter, including retractable-roof greenhouses, covered cold frames, earthen pits, sunken frames, root cellars, barns and sheds, covering with evergreen boughs and deep snow.
Caring for overwintering plants
The key to overwintering is keeping the plants cold and alive but not actively growing. After the plant's chilling requirement has been satisfied, plants can respond to warm temperatures. In late winter as the temperature increases, plants can de-acclimate to cold temperatures. If the temperature decreases slowly, plants can re-acclimate to colder temperatures but fast drops in temperature can cause cold injury to plant tissue. Although white polyethylene covered houses warm up less, vent the house by opening the end doors if the inside temperature approaches 45-50°F.
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 November 15 or November 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.
|Table 1. Average killing temperatures for roots of selected species of woody ornamental plants.
|Killing Temperature °F
|Ilex crenata 'Convexa'
|Convex Japanese Holly
|Ilex crenata 'Hetzi'
|Hetz Japanese Holly
|Ilex crenata 'Stokesii'
|Stokes Japanese Holly
|Korean Spice Viburnum
|Euonymus fortunei 'Carrierei'
|Euonymus fortunei 'Argenteo-marginata'
|Hedera helix 'Baltica'
|Pieris japonica 'Compacta'
|Acer palmatum 'Atropurpureum'
|Bloodleaf Japanese Maple
|Cotoneater adpressa praecox
|Taxus media 'Nigra'
|Black Anglojap Yew
|Euonymus fortunei 'Colorata'
|Purple Leaf Wintercreeper
|Juniperus horizontalis 'Douglasii'
|Rhododendron P.J.M. hybrids
|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.
|Table 2. Growing medium temperature causing significant injury* to herbaceous perennials.
|Killing Temperature °F
|Aster lateriflorus var. horizontalis
|Digitalis x mertonensis
|Geum 'Mrs. Bradshaw'
|Hibiscus moscheutos 'Disco Belle Hybrids'
|Houttuynia cordata 'Chameleon'
|Tricyrtis formosana 'Amethystina'
|Astilbe x arendsii 'White Gloria'
|Campanula glomerata var. acaulis
|Caryopteris x clandonensis 'Longwood Blue'
|Coreopsis grandiflora 'Sunray'
|Dendranthema 'Ruby Mound'
|Erodium x variabile 'Roseum'
|Hemerocallis 'Joan Senior'
|Heuchera sanguinea 'Chatterbox'
|Leucanthemum x superbum 'Alaska'
|Phlox paniculata 'David'
|Tiarella cordifolia var. collina 'Dunvegan'
|Tiarella cordifolia var. collina 'Oakleaf'
|Tricyrtis hirta 'Miyazaki'
|Verbena canadensis 'Homestead Purple'
|Veronica 'Sunny Border Blue'
|Achillea 'Coronation Gold'
|Achillea filipendulina 'Parker's Variety'
|Dendranthema 'Baby Tears'
|Gaillardia 'Monarch Group'
|Heuchera americana 'Dale's Strain'
|Sedum spectabile 'Brilliant'
|Lythrum salicaria 'Robert'
|Monarda 'Marshall's Delight'
|Penstemon fruticosus 'Purple Haze'
|Phlox divaricata 'Chatahoochee'
|Phlox glaberrima 'Morris Berd'
|Phlox paniculata 'White Admiral'
|Physostegia virginiana 'Summer Snow'
|Rosmarinus officinalis 'Arp'
|Salvia x superba 'Stratford Blue'
|Sedum 'Autumn Joy'
|Tanacetum coccineum 'Robinson's Mix'
|Tiarella cordifolia var. collina 'Slick Rock'
|Tiarella cordifolia var. collina 'Laird of Skye'
|Tiarella cordifolia 'Running Tapestry'
* Unsalable, unacceptable regrowth
Writen by: Tina Smith
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