The search for a rose without painful defense mechanisms often leads to thornless varieties, though many are more accurately described as “nearly” thornless. The presence or absence of these sharp structures is a stable, genetically determined trait that breeders have selected for over centuries. These smooth-stemmed cultivars offer the beauty and fragrance of a traditional rose without the handling difficulty. This characteristic results from natural genetic mutations that suppress the development of sharp epidermal outgrowths.
The Botanical Distinction: Prickles, Not Thorns
The sharp appendages on a rose stem are incorrectly called thorns in common language; botanically, they are known as prickles. A true thorn, such as those found on honey locust trees or citrus plants, is a modified stem or branch containing internal vascular tissue. This means a true thorn is deeply integrated into the plant’s structure.
In contrast, a rose’s prickle is merely a hardened outgrowth of the epidermis and the underlying cortex. Because prickles lack vascular bundles, they are superficial and can often be snapped off cleanly without tearing the stem. This superficial origin allows some rose varieties to completely eliminate these structures through genetic variation without compromising the plant’s health.
Specific Varieties of Nearly Thornless Roses
Gardeners looking for smooth stems have excellent options, ranging from climbers to miniature shrubs. One of the most famous truly prickle-free roses is the ‘Zephirine Drouhin,’ a historic Bourbon climber cherished for its strong fragrance and vibrant raspberry-pink blooms. This rose is genuinely smooth from the base of the cane to the flower stem, making it ideal for planting near walkways or arches.
For warmer climates, Rosa banksiae, commonly known as Lady Banks’ Rose, is a vigorous, completely prickle-free species that produces masses of small yellow or white flowers in the spring. Another reliable, low-prickle option is the popular ‘Iceberg’ floribunda, which produces clusters of white flowers on largely smooth canes. Its sports, such as ‘Burgundy Iceberg,’ share this desirable trait.
The heirloom Hybrid Perpetual ‘Reine des Violettes’ offers rich purple blooms on nearly smooth canes, and the light pink Bourbon rose ‘Kathleen Harrop’ is another well-known low-prickle cultivar. For many of these varieties, “nearly thornless” means the larger, older canes are smooth, while the newest growth may still possess a few smaller prickles. These cultivars are preferred for their ease of pruning and handling in high-traffic garden areas.
The Genetics Behind Prickle Reduction
The transition from a prickly stem to a smooth one is controlled by specific genes that regulate the development of epidermal outgrowths. Prickle density and presence are considered a quantitative trait, meaning they are influenced by multiple genetic loci. Research has identified a major quantitative trait locus (QTL) on Linkage Group 3 (LG3) of the rose genome associated with the absence or presence of prickles.
The trait for prickle-free stems often arises from natural mutations that suppress the genetic pathways responsible for their initiation. Breeders select for these naturally occurring mutations to propagate the smooth-stemmed characteristic. This intentional selection process has allowed for the stable introduction of prickle-free genes into commercial rose lines.
Recent genomic studies have pointed to candidate genes, such as RcTTG1 and RcTTG2, which are homologs of genes involved in trichome (hair) development in other plants. This suggests a shared regulatory mechanism. The genetic control is complex, involving the expression of these genes and the activation of hormones like cytokinin, which drives the cell division necessary for prickle formation.