Some plants in the landscape need a little extra protection to make it through the winter months. Overwintering is the process of protecting a plant over the winter season.
It is often used for those tropical and semi-tropical plants that beautify the garden over the summer months but will not survive the cold winter months without some sort of protection.
It is also beneficial for newly-planted, containerized, or otherwise vulnerable plants that are typically hardy but may need a little help while small or not yet established.
Here is what you need to know to successfully overwinter plants in Iowa.
Reasons & Benefits | How to Overwinter Plants | Why Damage Occurs Over Winter | Advice for Specific Species | More Information
Reasons & Benefits of Winter Protection
Not Adapted for Cold Temperatures
For tropical and semi-tropical plants overwintering is necessary as they are not adapted to survive cold temperatures.
Marginally Winter Hardy
For plants such as Japanese maple (Acer palmatum), overwinter protection is needed because they are marginally hardy in Iowa. Plants may survive some winter but will suffer extensive damage or die from cold temperatures in extreme winters without extra protection.
Protect Flowers & Buds
Plants, such as strawberries, are winter hardy but need overwinter protection to prevent damage to flowers or above-ground stems. Cold damage to unprotected plants would inhibit flowering (and fruiting) the subsequent year.
Newly Planted
Newly planted perennials, trees, and shrubs have not yet established extensive root systems and are more likely to see winter damage without some protection for the first year or two.
Protection from Winter Winds
Some conifers or broadleaf evergreens, such as boxwood (Buxus), may need protection from drying harsh winter winds since their leaves are present and able to lose water over the winter months.
Prevent Animal Damage
Plants, like arborvitae (Thuja) or roses (Rosa), are more likely to see browsing damage from hungry rabbits, mice, voles, or deer when the ground is covered with snow so protection is needed to prevent damage.
Keep Unique Plants
Some non-hardy plants can be hard to find and if not overwintered would only be in your garden one growing season.
Grow Large Plants
Plants kept from year to year can grow to impressive size and be trained or pruned to feature a unique habit or feature. If marginal or non-hardy plants are not protected they never reach a large size.
Save Money
When saved from year to year, you can avoid buying the same plants every spring or paying a premium price for a larger, more mature specimen.
How To Overwinter Plants
There are several approaches to overwintering plants to consider.
Straw, Mulch, or Soil | Structures & Coverings | Protected Locations | Moving Live Plants Indoors | Taking Cuttings | Dormant Plants
Utilize Straw, Mulch, and Soil
Place a layer of straw, mulch, or soil to insulate plants from the cold. This is especially useful for newly planted perennials or those plants that are marginally hardy, such as roses.
Wait until plants are fully dormant but before the ground (or your mulch pile) has frozen. This is typically early to mid-November in much of Iowa. By putting mulch down at this time, you will help stabilize the temperature of the soil. Applying mulch too soon may prevent plants from going fully dormant.
Layer a loose mulch material like straw or pine needles 6 to 18 inches deep, depending on the size of the plants. Avoid using leaves as they often compact too much and can smother perennials and other pants.
To keep these loose mulch materials in place, surround the plant with chicken wire or other types of garden fencing and secure it to stakes and/or the ground.
A protective blanket can also be created by layering topsoil or a heavier mulch material, such as shredded bark or wood chips, 3-to-4 inches thick over the crowns of perennials or at the base of woody shrubs.
Remove the straw, mulch, and topsoil in early spring after the extreme temperatures of winter have passed but before the plant breaks dormancy - typically around mid-March in Iowa.
Removal may need to happen over several days as lower layers may still be frozen. After it is removed, the mulch can be spread in the garden around plants to add organic matter and suppress weeds during the growing season.
Utilize Structures and Coverings
Build structures and coverings to protect plants from extreme and fluctuating cold temperatures. Place plants in structures or under coverings after they have gone fully dormant but before the ground freezes.
Structures can be simple such as burlap or row cover material stretched over a simple frame constructed from rebar, PVC piping, wooden stakes, or other materials. Firmly secure coverings with twine, landscape staples, or other fasteners to prevent them from blowing around.
More complex structures can be built from a wide range of framing materials and covered with plastic sheeting, Styrofoam, bubble wrap, and other insulating materials. You are only limited by your imagination and ingenuity!
The structure should be large enough to accommodate the plant with minimal pruning (you want as much plant mass as possible to go into winter). Woody shrubs can be gently tied up after they go dormant to fit inside the structure. Avoid wrapping materials, like burlap, directly on the plants as it can damage the stems and leaves, especially on evergreen plants.
The covering should allow for some air exchange to prevent heat build-up on a sunny day. A lack of air circulation and small size are two reasons why products like Styrofoam cones can be difficult to use.
Move Plants to Protected Locations
Plants in containers can be over-wintered by placing them in a moderately cold location such as an unheated or minimally-heated structure where temperatures can be maintained between 20°F and 45°F over the winter.
This approach works well for any marginally hardy or unplanted hardy plants, plants that require dormancy, but don't survive extreme low temperatures.
Check moisture levels of the soil often during the winter and irrigate if necessary. In an unheated structure, water may be needed as often as once every two to three weeks if temperatures are above freezing. Avoid over-watering plants.
An attached, unheated garage or a three-season porch is often a suitable over-wintering location if you are confident the temperatures can stay consistently in the 20 to 45°F temperature range. Many unheated structures can vary more widely in temperature than this getting both warmer on sunny winter days and much colder during the night. It is important to monitor and adjust temperatures inside the structure if needed.
Bring Plants Indoors
Plants that don’t require a cold dormancy period can be successfully overwintered by bringing them indoors. While many perennials and woody plants native to temperate climates cannot be overwintered in a warm home, most tropical and semi-tropical plants as well as many garden annuals are easily overwintered indoors.
There are several ways these plants can be stored or grown indoors over the winter months. Which you use depends on the species of plant, the space and materials available, and the preference of the gardener.
Live Plants
When space permits, one of the most straightforward methods of overwintering is simply bringing the plant indoors and keeping it as a houseplant. Bring the plant inside before a hard freeze. Place it in a sunny spot, avoid drafts, and water when the soil is dry. Check for pests regularly. To prepare plants for going back outside, begin fertilizing every 2-4 weeks in late winter/early spring. Move plants back outside after the last frost, acclimating them to the new light levels gradually over 2-3 weeks.
Learn more in this article: Moving Indoor Plants Outside for the Summer.
Cuttings
When plants are large and indoor space is limited, it may not be practical to overwinter plants as houseplants. For some plants, like coleus, begonia, and impatiens, it is possible to take cuttings and root over winter so they are ready to plant back outside in spring. Take stem or leaf cuttings in fall before the first frost – early to mid-September in much of Iowa.
Learn more in this article: How to Propagate Annuals from Cuttings
Dormant Plants
Tender perennials that have bulbs or other geophytes like corms or tubers, such as canna, gladiolus, and dahlia, can be dug up in the fall and stored indoors over the winter in a dormant state. Prune tender perennials back immediately before or after a killing frost in the fall. Pull bulbs and remove excess soil. Store in a cool, dark, dry location at 40-55°F. Check periodically for rot. Plant outside after the last frost or pot them up indoors in late winter/early spring.
For tropical or semi-tropical plants that do not form bulbs, corms, tubers, or other underground storage structures, such as banana (Musa), geranium (Pelargonium), and angel's trumpet (Brugmansia), they may still be overwintered in a dormant or semi-dormant state by relying on their thick or succulents stems.
Before the first frost, bring the plant indoors. Store in a cool, dark place (40-50°F) and let the leaves and stems dry naturally. Some species may benefit from a light watering every 2-4 weeks to prevent excessive drying. In spring, prune dead stems and replant. Gradually transition to brighter light over 2-4 weeks and plant outdoors after frost.
Learn more, including detailed information for specific species, in these articles:
Why Damage Occurs Over Winter
Cold Temperatures
For tropical, semi-tropical, and other non-hardy plants the damage over winter comes from the cold temperatures. These species do not have strategies to prevent cell breakage or death due to freezing temperatures and cannot survive an Iowa winter.
Temperature Fluctuations
In the winter, the top few inches of soil can see wide temperature fluctuations heating up in the bright sun during a winter day and cooling off quickly overnight. This wide fluctuation in temperature can thaw and freeze the upper soil layer causing shallow-rooted plants to heave out of the ground (called frost heaving). When crowns and roots are exposed to cold air temperatures and drying winds, plants that are typically winter hardy can die.
These temperature swings can also lead to a elongated, sunken, dried, or cracked area of dead bark, usually on the southwest side of a tree, especially young trees with smooth bark. This sunscald injury is not well understood but is typically attributed to temperature fluctuations.
Underdeveloped Root Systems
Plants planted late in the growing season do not have had enough time to become fully established in the garden before winter. These plants have smaller root systems that have not fully grown into the surrounding soil.
Plants with small root systems have less capacity to take up adequate amounts of water before the ground freezes making them more likely to suffer damage.
Additionally, plants with small root systems are more likely to frost heave allowing the crown and/or root mass to work up out of the ground leading to excessive drying and death of the plant.
Dry Soil Conditions
Winter is a very dry. For part of the season water is not available to the plant because it is frozen. Plants going into winter with inadequate water due to underwatering, drought, or dry soil conditions, are more likely to be damaged. Damage can also occur in spring if the ground thaws and there is not adequate moisture to support the plant as it comes out of dormancy.
This damage is particularly common on broadleaf evergreens and conifers. Because the foliage of evergreens is present all winter, water is lost through the surface of the leaves and needles even in the winter. Dry soil conditions and/or frozen soil moisture make it impossible for plants to replace lost moisture and browning and death of the leaves and shoots can occur.
Wet Soil Conditions
Overly wet or soggy soil conditions can lead to crown and root rot especially during the periods in late winter when the soil thaws and in early winter before the soil freezes. Some plants, such as coral bells (Heuchera) and lavender (Lavendula), are particularly sensitive and quickly die when conditions are too wet over winter.
Advice for Specific Species
Learn more about how to successfully overwinter many different types of garden plants utilizing the articles listed below.
- Roses
- Mums
- Herbs
- Tender Perennials (Dahlia, Canna, etc.)
- Geraniums
- Annuals
- Houseplants
- Trees & Shrubs
- Newly-Planted Trees
- Boxwood
- Fruit Trees
- Strawberries
- Asparagus
- Figs