By:Lois Ann Helgeson,firstname.lastname@example.org ,Consulting Rosarian,Minneapolis/St. Paul,MN
The American Rose Society’s North Central District includes the states of Minnesota,Wisconsin and North and South Dakotas,situated in USDA plant hardiness zones 3,4,and 5. Rose lovers here face a unique set of extreme conditions in which to grow their favorite flower. Temperatures in the winter can drop to -40° F and rise to over 100° F during the summer. The growing season is shorter than that of much of the rest of the country. While there are variations within the different states included in the district and even within the individual states themselves,the first Hybrid Tea roses generally can come into bloom late in May and the growing season is really considered over by the end of October.
In spite of the difficulties placed on us by Mother Nature,people in the upper Midwest successfully grow and enjoy rose varieties from the full range of the available rose classes. Growers wanting to experience the enjoyment of the most tender varieties have learned to protect them from the effects of winter extremes. Often they use a method called the Minnesota Tip which was developed specifically for zone 4.
The Minnesota Tip
It is known that the tender varieties of roses are severely or even fatally damaged at temperatures below approximately 20° F. It was considered imperative to find a method where these lower temperatures could be avoided. In addition there was concern for the late winter/early spring temperature freeze/thaw fluctuations that can also bring damage.
Midwest growers can thank Albert Nelson,an avid local gardener,for the development of the Minnesota Tip method of winter protection for tender roses. Having grown roses since the 1920′s,Nelson was determined to find a better method to winter-over tender roses than what was then being used. In the early 1950′s he heard about local raspberry growers who were tipping their plants and covering them with soil to protect them over winter. About the same time he learned about a lady in Massachusetts who was having some success protecting roses by bending,pegging and covering with evergreen boughs and soil. In 1952 Nelson first tried bending and then half covering roses with soil here in Minnesota. However,it was in 1954,when he bent the roses and totally covered them with soil,that he realized that he had found the right combination. It was to be a significant contribution toward successfully growing tender roses in cold climates.
The process was refined in the next few years to the method used today. In 1966,Jerry Olson and Carl Holst demonstrated the Minnesota Tip at the ARS convention held in Omaha,Nebraska. For that presentation,Jerry Olson and Dorothy Campbell wrote what was the first of the Minnesota Rose Society’s guide sheets on rose care. Charles Campbell named the process the Minnesota Tip.
Roses are dormant sprayed in mid to late October at the time when you are doing general fall cleanup. It is recommended that old mulch be removed to control a prime source of disease infection for the following season.
Tie the rose canes together using a synthetic twine that will not decay over winter. This process can be described as lacing up the plant –generally starting from the bottom and working up. It is important to have an extra length of twine either left at the top of the plant or added around the mid section of the tied plant. This will be allowed to extend above the ground to help the gardener locate and lift the plant in the spring.
A trench is dug on one side of the plant and then the soil is loosened all around the plant,using a garden fork to minimize root damage. The plant is tipped into the trench,using the garden fork,and taking advantage of the plant’s flexibility just under the graft union. With planning,the roses have been planted so that they will bend toward the side where the graft is attached,reducing the chances of breakage.
The plants are covered with the soil that was removed,being careful to leave the end of the extra length of twine exposed.
It is a good idea to water the bed well at this point to help settle the soil and to simply keep the canes and roots in good shape over the winter. Growers understand the importance of summer watering their roses and having rose beds with good drainage,however,the plants can also be stressed if they enter the cold weather season too dry.
As the temperatures drop in early November,a blanket of leaves 12″–18″deep is added. Watering will help keep the leaves from blowing around. An alternative is to simply place bagged leaves on top of the bed. Containers containing rodent bait are tipped on their sides and placed in the leaves or between the bags of leaves.
Early in April the leaves are removed. By the middle of April,the rest of the process is reversed.
Container grown roses,including trees,can be successfully protected by laying them on their sides and burying them,without removing the plants from their containers. Roses may also be dug and bundled bareroot,and then the bundle buried,much as in the Minnesota Tip. Healthy roses,protected by burying over winter,generally survive with very minimal cane damage. Growers,especially those in the portion of the NCD that lies within zone 5,have found that there are other methods that provide sufficient,successful winter protection.
Zone 3 and 4 growers may not find that these methods provide an acceptable level of protection from winter dieback and damage. Some of these methods are described here.
Additional Methods of Winter Protection Mounding
The base of tender rose bushes can mounded or hilled with 10″to 12″of soil. This is particularly important if the rose is grafted. The bud union is the source of all new cane growth for these plants,and if it is damaged over winter,the plant may be lost. A wire cylinder can be used to help contain the soil. Tall plants can be pruned and tied to prevent whipping in the wind. As with the Minnesota Tip,the roses should be dormant sprayed. The mounded roses are also covered with leaves and containers of rodent bait should be added.
Roses may be protected using rose cones. The plants are sprayed,tied and pruned to fit inside the cone. Soil and leaves may be added to protect the graft union. If the graft union is below ground level,only leaves may be necessary. Cones with detachable tops are best as they allow the tops to be removed during warm spring days and replaced at night. If the tops are not detachable,four 1″ventilation holes can be added on the sides near the top. The bottom of the cone should be sealed with dirt and the top weighed down with bricks.
A rose bed may be protected by constructing an oversized cold frame or rose house over it. Plants are sprayed and pruned to about 2′or to fit the rose house. A simple wooden frame is constructed that will hold sheets of building styrofoam that make up the sides. Additional sheets of styrofoam are used as covers for these boxes. In the spring the covers can be slid open during warm days to provide ventilation and closed again at night. The box must be constructed in a manner to insure against the weight of the snow and rain as well as strong winds. In the spring these rose houses are dismantled and stored until the next fall.
An alternative method of protecting miniatures –and other container grown roses –is available to those with either an unheated garage or room where there is a reasonable degree of control of the winter temperatures. Keep in mind that most tender roses must be maintained at temperatures above 20 degrees,preferably in the 40s during the winter months. An alternative source of heat may be necessary during extreme cold periods. The potted plants are sprayed,tied and watered. To keep roses from drying out,the pots are placed into plastic garbage bags,two to a bag with the miniatures. The tops of the bags are tied. The bags are placed on pallets or platforms to separate them a few inches from the floor. This method is used by growers in all three growing zones in the North Central District. Some report,as a negative,that plants may respond to warm spring temps and began to grow before it is warm enough to move the pots back outdoors.
Factors in Cold Hardiness
Cold hardiness has three factors –plant acclimation to cold in the fall months,actual mid-winter hardiness and de-acclimation in the spring.
Acclimation in plants takes place in response to shortening day lengths and declining temperatures. Biochemical and physiological changes gradually occur that make plants more cold tolerant. Plants,including the different classes of roses,and the varieties within each of these classes,differ in their ability to make these changes. In fact,this ability can change somewhat from year to year for any particular plant due to changes in plant health and by annual variations in temperature patterns.
Mid-winter hardiness refers to the actual lowest temps that a plant will tolerate,without damage,once it has acclimated during the fall months.
De-acclimation occurs in the late winter and early spring. This basically is a decrease in hardiness in response to warming temperatures. It is a process that is opposite to that of fall acclimation.
Good Health is Important
Plants,including roses,that acclimate too slowly,can be damaged by early cold temps. In an unusual year an early cold snap that occurs before acclimation or hardening off can injure plants that normally are considered winter hardy. A plant weakened by poor health may never reach its normal maximum mid-winter hardiness level and thus may suffer tissue damage at considerably warmer temps than expected. Additionally,plants that deacclimate too rapidly during late winter thaws may suffer damage due to late spring frosts.
To at least some extent,the rose varieties that people choose to grow and the methods that they use to protect them will be influenced by these hardiness factors. The choices will also depend upon how much dieback –and subsequent reduced bloom that follows and in some cases increased risk of plant loss –that a particular grower is willing to accept. This is weighed against the amount of additional work that they might be willing to do in the spring and fall for seasonal protection.
There has been a term coined,dieback hardy,for those roses that generally can be grown with minimal winter protection,that are likely to sustain considerable winter damage,yet are known to be able to regrow the following spring and bloom quite well by June. It should be acknowledged,however,that while this,with certain varieties,is acceptable to most growers,that these same varieties will perform substantially better in the years when there are milder winters and reduced damage. Additionally,growers must understand that healthy plants –of any class and variety –will be better able to survive winter weather using any of the available protection methods. As an example,plants,defoliated by black spot or stunted by heat and water stress,face winter with a significantly reduced chance of survival.
Some marginally hardy varieties can be helped by careful choice of planting location –such as the east side of buildings or in areas where snow accumulates. Some areas are subjected to drying winter winds,and if without reliable natural snow cover,this can inflict additional damage on rose canes. Protection by shrubbery or buildings may create areas where the desiccating effect of the winds is substantially reduced. In my yard,the city snowplows push snow around the fence bordering a long perennial bed near the street providing the necessary additional protection for a number of Hybrid Perpetuals. Most years these,otherwise zone 4 marginal plants,have experienced little dieback and reward me with outstanding bloom.
Hardiness can also be improved by fall watering. Additionally,stopping nitrogen fertilizers and discontinuing deadheading after August will encourage the hardening off,or maturing,of the canes. Mounding the base of the plant with extra soil and mulching in the fall can provide extra protection. If a plant is grafted,the bud union should be placed 2″–4″below the soil surface when planting (not necessary when using the Minnesota Tip). This will provide additional protection for the bud union and may also result in the plant growing roots from the area above the graft –turning the plant own rooted –which is generally desirable for roses in colder climates.
Popularity and availability of the hardier roses has grown in recent years. Successfully choosing varieties for colder climates requires some understanding of how cold hardiness relates to the various classes of roses. It is important to note that within even the most hardy classes of roses,there are some particular varieties that are more cold hardy and others that are more tender. The tenderness is often attributed to the amount of china or tea in their genetic makeup.
In the North Central District,hardy roses come from two general classes –Old Garden Roses and Shrub Roses. Generally OGR classes of Alba,Centifolia,Damask,Hybrid Perpetual (best in sheltered locations),Gallica and some of the Moss,Species and Species Hybrid roses are considered Minnesota zone 4 hardy. Most Alba and Centifolia roses are zone 3 hardy. Shrub classes of Kordesii and Hybrid Rugosa are generally zone 3 hardy. The Shrub subclass contains roses of varying degrees of hardiness. Polyanthas are often considered dieback hardy. Growers in the warmer portions of the North Central District,zone 5,find that they can grow the English roses,the Hybrid Musks,and even some of the Bourbons,with a minimum of winter protection. However,for most of the district,growing zones 3 and 4,we find that these perform better with significant winter cover –including the Minnesota Tip method.
Hardy Old Garden Roses for Our District
SPECIES AND SPECIES HYBRIDS:Species roses are those that occur naturally in the wild. Some of these spring/early summer bloomers are outstanding.
GALLICA:These roses can be traced back to the Greeks and Romans,who brought them to England and France. They are some of the oldest roses in existence. Gallicas bloom once heavily in the spring,on shorter,bushy plants,with flowers that are red,deep pink,mauve,striped or splashed with spots. The fragrance is intense and spicy.
ALBA:Albas were introduced by Roman traders before 77 A.D. These are tall,hardy,spring-blooming roses. The flowers have light,sophisticated fragrances,and are generally semi-double to double,pink or white. Foliage is grey-green,disease resistant,and shade tolerant.
DAMASK:These roses date back to biblical times,referred to by Pliny in ancient Rome and Virgil in 50 B.C. The arching canes are smaller in diameter than those on the Gallicas,but make taller plants. Flowers,often in clusters,are semi-double to double,white to deep pink,borne on short peduncles with intense,unusual fragrance. Autumn Damasks are known for repeat bloom in the fall,though it rarely occurs in the colder climates.
CENTIFOLIA:Known as cabbage or Provence roses,often depicted in old Dutch paintings,these intensely fragrant roses bloom once,generally later than other spring blooming types. They
are a hardy Alba-Damask hybrid with thorny arching canes and white to deep pink flowers.
MOSS:A fragrant sport of the Centifolias,these roses have moss-like growth on the sepals which exudes a sticky substance having a balsam scent. Some will repeat bloom.
HYBRID PERPETUAL:These roses were first recognized in Queen Victoria’s time. They have good June bloom with lighter repeat bloom later. Blooms are reds,pinks,whites and mixes,and are often quite fragrant. Some winter dieback is common,mulching or careful site selection is advised for best success. Lightly prune after spring bloom to encourage later summer bloom.
Hardy Shrub Roses for Our District
RUGOSA ROSES:These are the most shade,drought and poor condition tolerant roses. They have bright green heavily textured foliage that is disease resistant and that dislikes chemical sprays. These shrubs,with repeat bloom,come in reds,mauves,pinks and white. The plants have attractive hips in the fall.
EXPLORER SERIES/OTTAWA AGRICULTURE RESEARCH STATION,CANADA:Many of these roses have Rugosa roses in their genetic development which gives them extra hardiness and additional disease resistance. Included in these roses is the first truly hardy climber,William Baffin.
PARKLAND SERIES/MORDEN RESEARCH STATION,MORDEN,MANITOBA,CANADA:These are hardy roses with exceptional summer repeat bloom,especially if given attention similar to that which we give our tender repeat bloomers. Some have flowers that are similar to those of the tender Hybrid Teas and Floribundas.
DR. GRIFFITH BUCK/IOWA STATE UNIVERSITY,AMES,IOWA:Buck roses are complex hybrids of Species roses,Shrub roses,early English roses,Hybrid Teas and Grandifloras. The hardiness varies considerably. These roses were developed to be hardy in zone 5. Many of them can be considered dieback hardy in zone 4. More of these are becoming commercially available today. Eighty-seven of the Buck varieties have been registered with the ARS as of 1997.
OTHER TYPES:There are a number of other roses that have been introduced,such as the Meidiland roses and David Austin’s English roses. Most of these will perform best with winter protection,including the Minnesota Tip,in zones 3 and 4. Growers in zone 5 will grow these and also the Hybrid Musks and some of the Bourbons with less protection. Some others,like the shrubs Nevada and Lillian Gibson,are hardy in all of the North Central District zones.
To download the pdf version of this article click here.