State Journal of Science: 35: 255-260 (1960)
Some notes on the use of Rosa laxa as a source of hardiness in rose breeding1
Department of Horticulture
Iowa State University of Science and Technology Ames, Iowa
The major objective of rose breeders since the introduction into Europe in 1792 of the everblooming clones of the southern Chinese R. chinensis-R. gigantea complex has been the development of everblooming roses capable of surviving in the coldest winters without protection. To further this goal, various rose species indigenous to the colder areas of the northern hemisphere have been combined with the everblooming clones. The current classes of everblooming garden roses are the result.
Although hardier than their everblooming ancestors, current garden roses are not dependably winter-hardy in many sections of the country. Little improvement in this respect can be expected within existing rose groups, for the ancestral species do not contain factors for superior winter hardiness. The only recourse is the introduction into breeding programs of one or more rose species carrying factors for superior hardiness.
The tetraploid species, Rosa laxa, Retzius (4), which is indigenous to the dry steppes of central Siberia, is such a species. It has never been a popular garden subject, for it is lacking in many of the qualities demanded of ornamentals. The plants of R. laxa which are currently available in the United States have been derived by seed and asexual propagation from plants grown from seed collected by Hansen in the Altai region of Siberia in 1913. This collection was given the name 'Semi' to distinguish it from earlier ones.(3) This species is closely allied to R. cinnamomea and is included in the section Cinnamomeae. The name 'laxa' has been given erroneously to two other roses; one, a form of R. blanda and the other, a form of R. canina.
Seedling populations of R. laxa show variation in plant height, degree of armature, presence and degree of pubescence on the foliage, and degree of remontance. Plant habit, flower color, foliage retention, disease tolerance, and hardiness are remarkably constant characteristics. The plants are very hardy, having survived, without protection of any kind, -30 °F. without injury. The plants are erect, vigorous, and range in height from four to eight feet. The young canes are yellow-green, changing to greenish-yellow with maturity. Bristles are plentiful on the basal portion of the canes. The flowering canes are free of bristles but have paired, curved infrastipular spines. Suckering is moderate. The large foliage is moderately susceptible to rose blackspot and tolerant of powdery mildew. However, the foliage matures and is lost early so that the plants are bare, except for the immature growth at the tips of the canes, by mid-September. The two-inch, five-petalled, white flowers are borne in clusters of five, terminally on lateral shoots arising from canes of the previous season's growth. Flowers later in the season are borne on secondary laterals from stems bearing the June bloom. Although the plants flower profusely in late May and continue intermittently until frost, the small size of the flower and flower cluster contribute to the effect of sparseness.
Beginning with 1951, the clone listed in the literature as R. laxa Morden was used in combination with garden roses of several classes, both June-blooming and everblooming. Although Skinner has used R. laxa in combination with R. spinosissima to produce June-blooming shrub roses, no reports have been found in the literature on the extensive use of this species in rose breeding to develop everblooming garden roses. It is thought that observations on the characteristics of these F1 and later hybrids and a comparison with both the garden rose and species parents may be of interest to other breeders.
Although R. laxa has been used as a parent in crosses in most of the years since 1951 in combination with other species, shrubs, and ever-blooming garden roses, this report is based upon crosses in which the Hybrid Tea cultivars 'Crimson Glory,' 'Mrs. Sam McGredy,' and 'Happiness' and a June-blooming rose, 'Josef Rothmund,' a derivative R. eglanteria (5), were used as seed parents. Like most of the species comprising the Cinnamomeae, R. laxa does not set seed readily with foreign pollen.
Plant and Flower Characteristics of the F1 Hybrids
The F1 seedlings resulting from the use of R. laxa pollen on the cultivars 'Crimson Glory,' 'Mrs. Sam McGredy,' 'Happiness' and 'Josef Rothmund' have, with the exception of certain seedlings, been fully hardy in normal Iowa winters. Some nonhardy seedlings coming from one of the hybrid tea cultivars, 'Crimson Glory,' display none of the characteristics of the pollen parent, leading to the assumption that they resulted from accidental self-pollination of the seed parent. The cultivars of the hybrid tea class survive winters here only with careful winter-protection. 'Josef Rothmund' is half-hardy, retaining sufficient wood in most winters, without protection, to permit a limited flowering the following June (1). Resistance to high summer temperatures and drought have been satisfactory. The plants, as well as both parents, continue normal growth and retain their foliage through periods of deficient rainfall. The development of disease tolerance has received considerable attention in the breeding program and is perhaps the greatest problem, next to hardiness, in areas where roses are grown. Most garden roses, including the ones used as seed parents in these crosses, are very susceptible to blackspot and powdery mildew. R. laxa is highly resistant to powdery mildew and is moderately susceptible to the strains of blackspot peculiar to the Ames area. The F1's have proved to be no more resistant to blackspot infection than the garden rose parents. Powdery mildew has not been noticed on the F1's, even when conditions resulting heavy infection on plants growing in adjacent rows are present.
Premature defoliation is a serious fault in R. laxa, which limits its use as an ornamental to those areas with a short growing season. The foliage of the F1's is intermediate in appearance to that of the parents. With the exception of a few seedlings, the foliage is lost as early as that of the species parent. In the case of the few individuals which retain their foliage until frost, the leaves take on autumnal coloring in bright scarlet, maroon and orange tones. This characteristic has not been noted in either parent.
In plant growth, the F1's are vigorous to very vigorous, more closely approaching their R. laxa parent in this respect. In growth habit and height they are intermediate to the parents with no individuals closely approaching either parent.
Time of bloom is late May and early June, as in the R. laxa parent. Neither the everblooming habit of the Hybrid Tea parent nor the remontant bloom habit of R. laxa has been recovered in the F1's. In the crosses with the hybrid teas, the flowers are intermediate to the parents in size, with none as small as those those of the R. laxa parent or as large as the flowers of the Hybrid Tea parent. There is a complete range in petalage from single flowers with five petals to very double blooms having more than 75 petals and petaloids. Flower form and texture are usually poor. Malformed pistils and stamens are the rule. Pollen, when it can be collected, is viable and is instrumental in obtaining a seed set on a wide range of garden rose cultivars.
The cross with 'Josef Rothmund' produced, with one exception, F1 progeny whose appearance can be best described as dwarf forms of R. laxa. The influence of 'Josef Rothmund' is evident in the reduced size of the plant, freedom from suckering, smaller leaf size, and the presence of red pigmentation in the immature foliage and stems. The primocanes are sparsely bristly at the base. The infrastipular spines on the floracanes are lacking. Those spines which are presnt are straight or slightly hooked, as in Rosa laxa. Although it has the plant habit of the rest of the F1's of this cross, the exception mentioned above has the light salmon-pink, double flowers of 'Josef Rothmund.' This seedling was selected for further use in breeding.
Plant and Flower Characteristics of Advanced R. laxa Hybrids
Several selections were made from the F1 hybrids of the R. laxa-garden rose combinations. These were used in further breeding with garden roses. The progeny obtained by backcrossing two of the F1 selections to garden roses are typical of the results obtained from the F1-garden rose backcrosses and are described below. The first is derived from the cross of 'Crimson Glory' x R. laxa and is typical of those F1's coming from crosses with the Hybrid Teas. Growth is vigorous and erect. The green canes are thickly covered with spines at the base and relatively free of spines on the flowering stems, as in the R. laxa parent. The foliage is large, leathery, free from powdery mildew and very susceptible to blackspot. Armature is intermediate in placement and type to that of the parents. The flowers are pale rose-pink, moderately double, with deformed pistils and stamens, but viable pollen is produced occasionally. The flowering period lasts for six weeks in late May and June. The second selection is the one referred to previously in the discussion of the F1 progeny of 'Josef Rothmund' and R. laxa. The progenies of these two selections back-crossed to garden roses are typical of those which result from back-crossing other F1's from crosses of this type to garden roses.
Pollen from the 'Crimson Glory'-R. laxa seedling was effective in producing viable seed on a wide range of cultivars of the Hybrid Tea, Floribunda, and Grandiflora garden classes. The seedlings segregate into two sharply defined groups of approximately equal numbers. One group bears a pronounced resemblance to R. laxa in growth habit, foliage and prickle characteristics. The flowers, which are borne only in June, are single, two to three inches in diameter, and are in the lighter tones of pink, salmon, and yellow. All the plants in this group are as hardy as the species parent. The plants of the second group resemble the garden rose parent in floral and foliage characteristics. The plants are June-blooming; the everblooming habit of the Hybrid Tea and the remontance of R. laxa being absent. All plants of this group winter-killed during the winter of 1958-59, even though they had been given winter protection. All the seedlings retained the freedom from powdery mildew and the susceptibility to blackspot of the 'Crimson Glory'-R. laxa parent.
The 'Josef Rothmund'-R. laxa seedling produces abundant pollen, which is effective in producing seed pods on a wide range or garden and shrub roses. Of chief interest are combinations involving everblooming roses of the Hybrid Tea and Floribunda roses classes. The Hybrid Tea cultivars 'Crimson Glory,' 'Happiness,' 'Dean Collins,' 'Pink Princess,' and 'Lady Alice Stanley' were the principal seed parents involved. The floribunda cultivars used as seed parents were 'Kordes’ Harmonie,' 'Florence May Morse,' 'Feuermeer' and 'Herrenhausen.' The first three are derivatives of R. eglanteria; the last, a floribunda of the European type.
The seedlings resulting from combining this selection with the Hybrid Teas bear a strong resemblance to that group in appearance of foliage and floral characters. The flowers are large, ranging from four to five inches in diameter, and double with 25-35 petals. Dark colors are absent, in spite of the use of the cultivars 'Dean Collins,' 'Happiness' and 'Crimson Glory' which are noted for producing progeny in the dark pink and red color range. The effect of the species parent can be seen in the semi-folded leaflets, the placement and type of armature, and early defoliation. Although the plants range in hardiness from those which kill back to within six inches of the ground to those which lose only the immature tips of the canes, they are hardier than the Hybrid Tea parent. Plant height ranges from 4 to 6 feet. Remontant segregates appear in a ratio of 8 June-flowering plants to 1 remontant.
The progeny from the Floribunda combinations are vigorous shrubs ranging in height from four to six feet. The abundant foliage is large and is held until late in the season. There is the same level of resistance to foliar diseases noted in the F1 populations. The flowers, ranging from five to 45 petals in the lighter tints of pink, apricot, salmon and yellow vary in sizes from 3 1/2 to 5 inches and are borne in clusters of five to seven. The plants continue growth until late in the season, resulting in tip killing of the immature wood. While the general appearance of the plants is that of the floribunda parents, R. laxa influence is evident in the location and shape of the spines, shape of leaflets and stipules, and superior hardiness. Remontant segregates, which retain the basic hardiness of the pollen parent, are obtained from the garden rose-'Josef Rothmund'-R. laxa progeny.
It is believed that the most valuable characteristics of R. laxa which appear to be transmitted to its progenies in crosses with selected garden rose varieties are (a) cold-hardiness, (b) resistance to powdery mildew, and (c) adaptability to areas of high temperatures and humidities and low summer rainfall. It should be possible to incorporate these features of R. laxa into our everblooming garden roses and develop cultivars adapted to culture in those areas where most cultivars now in existence can be grown only with careful attention to winter protection.