Some Hardy Perennials Need Winter Protection in the North and Midwest, and Tender Perennials Require Special Protection Methods.
Many perennials hardy for your zone make it through winter just fine with no special attention. But in northern and midwest gardens, some count on at least a blanket of snow early in the season to moderate soil temperature and protect from harsh drying winds. With the unusual weather patterns in recent years, bitter cold often precedes the snow leaving your perennials unprotected.
Just before the ground freezes, heaping on 4 to 6 inches of mulch, compost, or shredded leaves in and around the base of your plants will prevent the soil temperatures from rising and falling rapidly. This is essential for perennials not quite hardy for your zone, and will prevent frost and thaw cycles from heaving your perennials up out of their bed. It also seems that perennials that are not cut down after the growing season not only provide winter interest in your garden, but also weather the winter better. This may, in part, be because the plants hold on to some leaves in fall and snow through the winter. Ever notice that plants that are left through the winter are the last areas to lose snow in the spring? That snow is preventing the soil from warming rapidly, keeping the perennials dormant until growing season has truly arrived. Protecting hardy perennials is good insurance against a harsh winter.
Cutting Back Perennials
The one thing all experienced gardeners can agree on regarding this subject, is never cut a perennial to the ground while the foliage is still green. The plant is still working on reserving energy for spring growth. Wait until all foliage has died back naturally or until after a hard freeze to remove top growth. Some plants that are cut back too soon may try to regrow in warm fall periods, not leaving enough energy to survive winter, or to produce a plant in spring.
The difference of opinion among gardeners relates to when foliage should be cut back, spring or fall. Much research has made it clear that, at least with marginally hardy plants, old growth should be left through winter. The stems catch and hold leaves and snow which are nature’s natural protective mulch. The old plants will also provide some winter structure and interest. Some are more attractive than others, with interesting seed capsules or dried seed heads. However, any diseased or damaged foliage should be removed and disposed of to help control the problem next year. Cutting back plants that were afflicted with insect problems such as borers or insects that lay their eggs in or on the plants will also reduce the likelihood of insect infestation next year.
The hardiest of perennials can be cut back to the ground in fall without much concern for survival. If you have had any problems with disease or fungus, it is best to remove all old topgrowth before winter to help reduce the possibility of the disease recurring. Even with marginally hardy plants, persistent disease may be eliminated if the infected foliage is removed. Be sure to heavily mulch instead, or mound protective soil at the base, and then mulch.
Just about the time the soil is freezing, it is time to pile on the mulch. Mulch does not keep your plant warm through winter, it helps the soil to maintain a more constant temperature and also helps it to retain moisture.. Soil that freezes, thaws, and freezes will eventually damage roots and may heave your plants up out of the soil. You can choose from a variety of bark products from your local garden center. Or apply compost, peat moss, or straw. Be careful about using hay that is often available for Halloween or autumn decorating, it is often loaded with seeds that will sprout up like crazy next spring. Pine boughs are excellent for covering evergreen shrubs to protect them from winter sun scald and wind burn. If you use a real Christmas tree, recycle it by protecting your living shrubs. Extra large leaves can work well for smaller shrubs and plantings.
Plants and perennials that require additional protection to survive your winter will need a deep layer of mulch added by mid November or later, when the ground is beginning to freeze. Do NOT add deep mulch too early or your plants will not be slowly exposed to colder temperatures, allowing them to acclimate for winter. Leaves, other than oak or beech, are not recommended for mulching. They tend to mat down and prevent air from reaching the soil, damaging your plants. Grass clippings are also less effective than other mulching materials.
Certain tender perennials just can't be resisted even in northern climates. Before selecting a tender perennial, shrub or tree for your garden, be sure to check special methods for protecting them through winter.
When considering tender perennials, as well as tender shrubs, for your northern or midwest garden it is important to understand the preparations required for winter. A perennial is considered tender if it is not fully hardy in your zone, or may not be recommended for your zone at all. Look for information on the garden tag such as "hardy in (your zone) with winter protection". What that winter protection is generally is not on the tag, so a little research may be required. Your garden center staff should have some basic information, and often some very good tips. Extra mulch or compost mounded around the base of the plant may be enough. Some will require up to 8 inches of soil mounded at the base. A little creativity and experimentation will often produce excellent protection methods. As in the photo above, mulching material covered and held in place by a porous (will let in air and moisture) covering is the primary objective. Soil mounded up several inches at the base of the plant first will add more protection. It is not advisable to select a plant more than one zone away from yours. Survival chances are diminished.
Cold climate gardeners, being resistant to adhere to their zone recommendations, have been quite successful in getting favorite plants through winter. Zone 4 gardeners have been growing hybrid tea roses, Butterfly Bush, Rose of Sharon, and several varieties of Hydrangea like Nikko Blue, as well as many of the less hardy rhododendrons for years. The success often depends on the severity of any single winter, but with proper protection, the roots and crown are preserved to grow anew. Plants should be heavily mulched to ensure the roots are protected whenever you plant a shrub or perennial not quite hardy for your zone. In regions that drop below -38 degrees, further protection is advised, treating the plant almost like a hybrid tea rose. And as mentioned above, mound several inches of soil at the base of the plant, and mulch heavily
Butterfly Bush blooms on new spring growth, so cutting the bush back, then mounding soil at the base and heavy mulching will be enough protection to preserve your plant. The plant dies back nearly to the ground each year in zone 4 and north anyway. Spring will bring a whole new plant if the roots and crown are adequately protected. This method will work well for Annabelle hydrangea and Roayl Purple Smokebush, as well as nearly any tender perennial that dies back to the ground each winter.
When it comes to flowering shrubs, the same protections are generally successful if the shrub flowers on new wood. If they shrub forms flower buds on old wood in summer or fall, the possibility of success is diminished. The flower buds are not always evident, but they are there. The plant should be covered with mulch right up to the tips of the branches. The easiest way to do that is usually by forming a chicken wire cage around the plant. Then fill it to entirely cover the plant with leaves or other mulching material. Soil mounded at the base of the plant first will give you added protection to the roots. These are common methods for rose protection. You can use this method for Rose of Sharon, Rhododendrons, tender Hydrangea, as well as tender perennials.
Protecting larger shrubs and trees that bloom on old wood is more challenging due to size. It is difficult to build a large enough structure to protect an entire plant that grows beyond 2 or 3 feet. If you are determined to try a tender shrub or tree, be prepared to lose it. However, there are a few things you can do to help. First, select a planting site that offers some protection. Planting on the North or East side of a structure or screen of tall evergreens or a large tree will protect your plant from winter sun and wind. But be sure the site will allow enough summer sun. Planting close to a heated building will provide a warmer micro-climate, but beware of sun reflection from light colored surfaces. And sometimes to get close enough to the warmth, your large shrub or tree will be quickly overgrown for the space. Only a healthy plant has a chance of survival. Give the shrub or tree perfect growing conditions (proper soil, drainage, sun, water and fertilizer). Do not fertilize after mid August, and water thoroughly in fall, gradually decreasing the water in September to allow the plant to naturally prepare for winter. And take heart that if you can properly protect a young shrub or tree until it is well established, many will begin to acclimate to its’ growing conditions. If it adapts and thrives, your attention was worth the effort.
Many tender and not so hardy perennials can be protected in the ground, but the truly tender perennials need to be removed from the ground and stored. You may not want to bother with saving bulbs and roots that can be replaced inexpensively, but if you have chosen the more expensive perennials such as tropicals, it may be worthwhile.
There are many clematis hardy enough for the northern zones that need no winter protection other than, perhaps, some extra mulching. But clematis is often a special plant for a special spot. Finding the perfect one may take you out of your zone, and you may be quite willing to make an exception to a "no special care" policy.
Clematis twist their stalks around a support. If you use a mesh such as chicken wire around/over your support structure, the tender plant will be attached to the mesh rather than the structure. Before winter remove the mesh with the vine attached and lay it flat on the ground. Mulch the base heavily and cover the entire plant. If you need to lay your mesh out over the lawn, cover it with leaves and stake down burlap over. Or just pile your bagged leaves from the lawn on top. In spring, remove the mulch and reattach the mesh. When growth starts, prune back to a few strong shoots and remove the dead vines. (Note that pruning times can be different for hardy clematis, based on whether they flower on new or old growth.)
If your clematis was afflicted with a fungal disease, remove all infected vines and dispose of them by bagging or burning. Cover the crown with a few inches of soil so it is now “deep planted”. The most damaging fungus to clematis attacks at the soil line. Then cover with at least a few inches of mulch. Your clematis may regenerate, but could take a few years to fully recover. Apply a sulfur based fungicide in spring.
Hardy Mums are not always so hardy in a northern garden. Even when recommended for your zone winter can be hard on them. Do not cut them back in fall, and mulch heavily at the base with leaves or compost after the leaves have begun to turn brown from hard frosts. In zone 4 or colder, that may not be enough. Mound up soil about 8 inches deep around the base of the plant.
Around six weeks before you might expect the first frost, stop fertilizing so that growth is slowed down, new growth is not sturdy enough to handle frost and freezing and will not survive. Also stop deadheading and pruning roses, as both will encourage new growth. Do continue watering however, that is essential to keep your roses alive and healthy as it prepares itself for dormancy. Make sure the mulch covering the soil is still deep, several inches over the root areas.
Fall is a good time to add lime or some other amendment to adjust the soil if you need it. Roses prefer a pH between 6.0 and 6.9. Test your soil before adding lime. See Soil Amendments for more information about pH and testing kits.
If your roses suffered foliage diseases this summer, remove all foliage and discard it (NOT in your compost heap!) after a hard freeze. Diseased foliage allowed to fall to the soil may reinfect your roses next spring.
Check the detailed rose care instructions for more information, but if you are in or north of zone 6, even hardy roses will need extra protection. Hybrid and grafted roses may need some protection north of zone 8. Rose trees should be Minnesota tipped, heavily protected, or brought inside to a place that will remain about 30-40 degrees until mid spring.
Overwintering Tuberous Root Perennials
These would include Dahlias, Cannas, Calla, 4 o’clocks, Gladioli and Caladium, as well as tuberous root begonias. Winter protection in the garden is not possible. These rootballs need to be properly stored inside over the winter. Not an easy task, I have tried storing Cannas, only to have them rot. Unfortunately, these are not inexpensive to replace each year, but for the perfect accent in a bed, it may be worthwhile. Following are basic storage methods, but you will find that there are many different ideas out there. What works for each individual gardener may have something to do with conditions in their storage area - light, moisture, and temperature. Storing tender perennials may take a little experimentation.
Most gardeners will agree on the basics, beginning with: Dig up the bulbs or root balls carefully. Cut off foliage and brush off loose soil. For plants with corms, such as gladioli, separate the corms and discard old shriveled corms. Then place in a warm dry place to dry them out. If you try to dry them outside, rain will continually delay the drying process. The garage may get cool at night, but will just slow down the process. If you can, drying them indoors will be most efficient.
When the bulbs or tubers are dry, gently brush off excess soil being careful not to bruise them. Dust with an all purpose garden pesticide with fungicide. Most should be stored in a container layered with peat moss, vermiculite, perlite, cocoa hulls or sawdust. Store in a cool, but frost free spot until spring. The garage will often get too cold in the northern zones, and even an unheated basement may get too warm for some types. If you have anything resembling a root cellar, that might be the best. Perhaps a storage closet along an outside cement wall in the basement. If you are not sure, test your location with a thermometer this winter and check it frequently. The packing material you use will help keep the temperature of your bulbs a bit more stable.
Caladium - Dig up before frost and dry in a warm place for a week to ten days. Then store following the instructions above. Best storage temperature is 60 degrees.
Canna - Dig out after a hard frost. Cut the tops back to about 3 or 4 inches. Air dry in a warm place for a week or two. Store in an empty shallow box (this I haven’t tried yet - not packed into any medium). Many gardeners pack in one of the mediums mentioned above. Best storage temperature is 45 - 50 degrees.
Dahlia - Just about every Dahlia grower will give you a different storage method. You just need to find what works for you. After the first light frost cut back to a few inches. Lift these plants out carefully, leaving as much soil intact with the root ball as possible. Only allow to air dry a few hours, then pack following instructions above. Method 2: After gently removing from the ground, hose off all soil. Place in a plastic bag with holes for ventilation, packed with one of the above mediums. Store in a covered plastic container to keep dark, fill the container with crumpled newspaper. Method 3: Store in a slightly moistened peat moss. Check frequently throughout the winter - if peat moss has dried out, moisten again. Best storage temperature is 35 - 40 degrees.
Gladiola - Dig up after frost when the foliage is fading. Save even the smallest corms, they will grow large when you plant them next spring and eventually produce a flowering plant. Allow to dry up to four weeks in a warm, dry place with good circulation. About 75 - 80 degrees is best for curing the corms. Then they need to be stored a little differently then mentioned above. Find a dry, cold place, about 35 -40 degrees. Hang them from the wall or ceiling in open mesh bags for good circulation - use an old onion bag, nylon stockings, or a lingerie washing bag
Tuberous Begonia - Dig up before frost and cut the foliage back to a couple of inches. Dry the roots for two to three weeks in a warm place. Store following the instructions above. Best storage temperature is 45-50 degrees.
Also see additional winterizing information for lawn and garden - Preparing for Winter, and detailed information on winter protecting roses - Growing Roses in Cold Climates.
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