Some plants in the landscape need a little extra protection to make it through the winter months. Here is what you need to know to successfully overwinter plants in Iowa.
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.
Overwintering allows you to keep plants from year to year that normally wouldn’t survive multiple years in your garden. It allows you to grow plants with unique character and impressive size. It can also save you money since you don’t have to purchase new plants the following spring or invest money in large plants. In addition, overwintering allows you to grow unique or unusual plants normally not seen in an Iowa garden.
Of course for tropical and semi-tropical plants overwintering is necessary as they are not adapted to survive cold temperatures. But there are many other reasons why protection over the winter months may be necessary.
For plants such as Japanese maple (Acer palmatum), overwinter protection is needed because they are marginally hardy in Iowa and during extreme winters may suffer extensive damage or die from cold temperatures without extra protection.
Other plants, such as strawberries, are winter hardy, but need overwinter protection to prevent damage to flowers or above-ground stems that would inhibit flowering (and fruits) the subsequent year.
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.
Some broadleaf evergreens or conifers, such as boxwood (Buxus), may need protection from drying harsh winter winds since their leaves are able to lose water over the winter months.
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.
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.
For those plants that are typically winter hardy, there may be other reasons why they see damage or death over the winter.
In the winter, the top few inches of soil can see some 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. This is called frost heaving and when crowns and roots are exposed to cold air temperatures and drying winds, they can die. This same swing in temperatures caused by the radiant heat from the sun during the day and the rapid chilling during the night can also cause the smooth bark on some young trees to expand and contract causing sun scald damage.
Underdeveloped Root Systems
Those plants that are planted late in the growing season may not have had enough time to become fully established in the garden before winter. These plants have smaller root systems that are primarily located in the original root ball of the plant and have not yet fully grown into the surrounding soil.
Plants with small or underdeveloped root systems are more likely to frost heave allowing the crown and/or root mass to work up out of the ground during the freeze-thaw cycle that happens to the upper layer of soil throughout the winter. Frost heaving can lead to excessive drying and death of the plant.
Additionally, 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.
Dry Soil Conditions
Winter is a very dry time of the year since for part of the season water is not available to the plant because it is frozen. If plants go into the winter season with inadequate water due to underwatering, drought, or dry soil conditions, they 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.
Winter damage from dry soil conditions 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 water makes it impossible for the water lost through the leaves to be replaced. 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.
There are several approaches to overwintering plants to consider.
Utilize Mulch, Covers, or Protective Structures
One approach to overwintering plants is providing a mulch "blanket" or building a structure to protect plants from extreme and fluctuating cold temperatures.
Straw, Mulch, and Soil
Placing a layer of straw, mulch, or soil can 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.
Structures and Coverings
After plants have gone fully dormant but before the ground freezes, build a structure to protect plants from harsh winter winds and help moderate temperature fluctuations. Structures can be simple. Utilize 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. The material should allow for some air exchange to prevent heat build-up on a sunny day. Avoid wrapping burlap or other materials directly on the plants, in particular on evergreens, as it can damage the leaves.
More complex structures can also be built from a wide range of framing materials and covered with things like plastic sheeting, Styrofoam, bubble wrap, and other insulating materials. You are only limited by your imagination and ingenuity! The structure needs to be large enough to accommodate the plant without 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. As with simpler setups, these structures need to allow for air exchange. A lack of air circulation and small size are two of the reasons why products like Styrofoam cones can be difficult to use.
Move Plants to Protected Locations
Marginally hardy plants, unplanted hardy plants, as well as other plants that require cold dormancy and are easily moved 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 months. Soil moisture must be monitored carefully in this situation. 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 than 45°F on sunny winter days and much colder than 20°F 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, space and materials available, and the preference of the gardener.
When space permits, one of the most straightforward methods of overwintering a plant indoors 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 window or under artificial lighting. Avoid sites near cold drafts or heat sources. Water the plant when the soil surface becomes dry to the touch. Inspect plants for insect pests carefully and monitor them frequently for several weeks after bringing them indoors. Mealybug, scale, spider mites, aphids, and other pests can quickly get out of control if not treated as soon as they are noticed.
Typically, plants do not need a lot of fertilizer over winter but you can begin fertilizing every 2 to 4 weeks in late winter/early spring as you prepare to take them back outside. Move plants back outside after the last frost in spring. Acclimate them to new light levels by moving them first to a shady location and then gradually introducing them to more light over a two to three-week period.
When plants are large and indoor space is limited, it may not be practical to overwinter plants as houseplants. For many garden annuals like coleus, begonia, and impatiens, it is possible to propagate the plants in fall and root new smaller plants 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. More information about taking cuttings can be found in this article: How to Propagate Annuals from Cuttings
Dormant Bulbs, Corms, and Tubers
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 after the foliage is killed by frost in the fall. Carefully pull the bulbs (or tuber, rhizome, corm, etc.) out of the ground and remove any excess soil. Allow bulbs to dry and then store them in a cool, dark, and dry location. Temperatures between 40°F and 55°F work well for most species. Cardboard boxes, plastic crates, paper bags, mesh bags, or containers of peat moss, newspaper, sand, or vermiculite are all options for winter storage. Bulbs must be allowed to stay dry enough to remain dormant and free from moisture-loving mold or fungus, but damp enough to not dry out and shrivel up.
Check on bulbs periodically throughout the winter. Remove any rotting, soft, or dried-out bulbs that may develop. If conditions are too dry, very lightly moisten the material they are stored in. Be careful to not over water as it will lead to rot, mold, and fungal growth.
Plant bulbs outside once the danger of frost has passed and the soil has warmed – usually mid- to late-May in much of Iowa. Alternatively, these tender perennials can be potted up in late winter/early spring and grown in a bright and warm indoor location. Transplant them into the garden in mid- to late-May. More details about overwintering specific tender perennials can be found in this article: How to Overwinter Tender Perennials
Some tropical or semi-tropical plants, such as banana (Musa) and angel's trumpet (Brugmansia), do not readily form bulbs, tubers, or other geophytes. These species can still be overwintered in a dormant or semi-dormant state. Before the first frost in fall, bring the entire plant indoors. If not already in a container, dig it up from the garden and place it in a container or wrap the root ball with burlap or plastic. Do not cut the leaves or stems back. Store the containerized or uprooted plant in a cool, dark location (40 to 50°F). Allow the leaves and stems to dry down naturally. On woody plants, the leaves will yellow and drop off. Keep the dormant plants dry, but not so dry that the soil becomes powdery. Check soil moisture often and if very dry, lightly water. In most situations, light watering will be needed every two to four weeks.
In spring, remove the dry, brown stems and prune any damaged or dead woody stems before replanting outdoors after the danger of frost has passed. Transition plants gradually to brighter light outdoors over 2 to 4 weeks and resume regular watering and fertilizing.
Learn more about how to successfully overwinter many different types of garden plants utilizing the articles listed below.
- Containerized (unplanted) Perennials and Woody Plants
- Tender Perennials (Dahlia, Canna, etc.)
- Trees & Shrubs
- Newly-Planted Trees
- Fruit Trees