Growing Shrub Roses in Montana

This guide tells how to choose plants, prepare a site and care for shrub roses. Includes a chart of roses that do well in Montana’s climate.

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MONTANA’S CLIMATE IS ONLY MARGINALLY

suitable for growing most types of roses, but many shrub roses are hardy and vigorous, require little care under our severe conditions, and do well here. The American Rose Society recognizes five main divisions within the shrub rose class, and three have adapted cultivars: hybrid kordesii, hybrid rugosa, and shrub. They have a sprawling habit, and typically grow from 2 to 6 feet or more in every direction, given appropriate conditions. Shrub roses produce large quantities of clusters of flowers with varying degrees of fragrance.

Purchasing Plants

Buy plants from a reputable nearby nursery. If no local nursery carries the desired cultivars, purchase them from a reputable Northern nursery. Plants produced by Southern nurseries, though they may be the same cultivar as those offered by a Northern nursery, are not acclimated to our conditions and may not do as well in the garden. Bare root plants on their own roots and graded as “Number 1” are the best. These have at least three vigorous canes.

Choosing the Best Site

A good planting site receives at least six to eight hours of sun daily during the growing season. The flowers of roses planted on a site with a southeast exposure that receives some afternoon shade retain their color longer than those exposed to full, hot afternoon sun. Morning shade promotes the development of some diseases such as blackspot, since the leaves remain moist with dew for a longer period of time. The site needs good air circulation to reduce the chance of disease.

Soil structure is not as important as its drainage capacity. Roses will not thrive where water does not readily drain or where the water table can rise to within a couple of feet of the soil surface. If improved drainage is needed, work peat moss and/or sand into the soil before planting.

Soil should be slightly acid to neutral with a pH of about 6.5 to 7.0. Roses in strongly alkaline soils, with a pH near 8.0, will develop chlorosis, or yellowing of the leaves, due to the unavailability of soil iron.

Shrub roses can become much larger than hybrid tea roses, so be sure to allow enough room for each plant to develop to its full size.

Preparing the Soil

Prepare the soil in the fall or in the spring at least a month before planting. Work in five or six bushels per 100 square feet of organic matter such as compost, leaf mold or peat moss. Composted or dehydrated manure is also acceptable. If manure is used, add one-half pound of superphosphate or equivalent to each bushel to supply additional phosphorus. Spread the organic matter about four inches deep over the entire bed and spade it in.

Roots of many roses will extend at least two feet into the soil, so work it at least to that depth. If individual planting holes are being prepared, mix the soil from the holes thoroughly with organic matter, using one part of organic matter to four parts of soil.

Planting

Plant shrub roses in early spring as soon as the ground can be worked and while the plants are still dormant. If stock has arrived but planting must be delayed for a couple of days, unpack the roses, moisten the packing material, then repack them. To hold them for a longer period of time, bury them completely in a trench outside (heel them in).

Plant spacing depends upon which species and cultivars are being planted and upon the desired effect. For example, the Red Leaf Rose (Rosa rubrifolia) can reach a height of several feet. If plants are to remain as individual specimens, space them six to eight feet apart, or closer if a hedge is desired. To get a rough idea of spacing, allow as much space for the spread of the plant as the ultimate height of the plant. Decide on spacing, then dig the planting holes at least 12 inches deep and 18 inches in diameter. Loosen the soil in the bottom and at the sides of the hole.

Prior to planting, trim any dead or injured roots, remove diseased, dead or injured canes, and cut the remaining canes back to about a foot in length. Flowering is often delayed if they’re trimmed shorter than that.

Don’t expose the roots to the sun or wind during planting, but keep them wrapped in damp sphagnum moss or burlap until they are placed into the planting hole. Never wrap long roots around the inside of the planting hole, as this can cause them to grow in a circle, eventually girdling and killing the plant. Instead, trim them to fit the diameter of the hole.

If the plants were grafted, set them with the bud union about two inches below the soil surface. Spread the roots so they slope in a natural position along a cone of soil. Be careful not to break any roots. Fill the hole with prepared soil and firm it about the root system. This helps eliminate air pockets that could damage the roots.

When the hole is half full of soil, fill it to the top with water. When the water has drained, fill the remainder of the hole with soil and water thoroughly again. Finally, mound about eight inches of soil over the bush to protect it against late frost. Remove this when danger of frost has passed.

Caring for Shrub Roses

Don’t rely on sprinkler irrigation to water shrub roses; thoroughly soak the soil to a depth of about 10 inches once each week, as roses require a large amount of water. To discourage diseases, keep water off foliage. Apply no fertilizer the year of planting. Begin the year after planting by applying a complete fertilizer, such as 5-10-10, at the rate of about three pounds per 100 square feet per year. Apply half the fertilizer when the new growth is a few inches long in spring and the rest near the end of June. Don’t apply any after the Fourth of July.

Leaves may turn yellowish-white from lack of iron, particularly if the soil is strongly alkaline. If so, spray plants with ferrous sulfate at the rate of one ounce to two gallons of water, or with chelated iron, mixed according to directions on the label. Also, acidifying the soil should help. If the soil pH is between 7.0 and 7.5, apply one pound of garden sulfur per 100 square feet. If it’s near 8.0, use two pounds, and if it’s above 8.5, use three pounds per 100 square feet. Purchase these products at the local garden supply store.

The plants usually benefit greatly from several inches of organic mulch. Be sure the soil is wet before applying the mulch. A dry mulch can absorb moisture from the soil.

Shrub roses require little pruning. Simply remove the dead, unhealthy or crowding branches to maintain the desired shape.

Rake and discard all fallen leaves and fruit to help keep the site as pest-free as possible. If planting in a windy location, erect a snow fence or burlap barrier to the windward side of the bush to reduce winter drying of the shoots. Erect the barrier in fall, and remove it in spring. Also, water the plants well in fall after the leaves have turned color but before the ground freezes. This will ensure that the plants have as much water as they need going into the drying winter.

There are several diseases that can affect roses, including powdery mildew, rust, and the most important, blackspot, caused by the fungal organism Diplocarpon rosae (Figure 1). Blackspot causes black spots on rose leaves, which then turn yellow and fall. Complete defoliation can occur in severe infections. The disease overwinters on stems, and spores germinate in the wet weather of spring. Locations that receive overhead irrigation in shade can also be conducive to blackspot infection.

Figure 1: Blackspot causes black spots on rose leaves, which then turn yellow and fall. BY WARD UPHAM, KANSAS STATE UNIVERSITY, BUGWOOD.ORG #5511595

While blackspot is typically not a problem in dry climates, it can be fairly prevalent in the more moist areas of western Montana. Susceptibility and resistance are relative and variable, depending on environmental conditions. Prevention is key; keep roses in a healthy, active state of growth to reduce vulnerability. See Table 1 for resistance comparisons.

Flowering

The fullness of rose flowers is measured by the petal count, ranging from the “single” flower with 5 petals to “very full” flower with over 41. See Table 1 for details.

Shrub roses don’t necessarily bloom all summer. Many cultivars are repeat or continuous bloomers, while others bloom only in spring or summer. See Table 1.

To encourage strong root growth of young plants, remove spent blooms to discourage formation of hips. After the bush is well-established, let hips form to add color to your fall and winter garden.

TABLE 1. Shrub roses that do well in Montana with little or no care.