Cool Campanula


October 29, 2005 - Author: Sonali Padhye, Cathy Whitman, <>, and Art Cameron

Cool Campanula

Cooling and daylength can regulate flowering of some campanula species and cultivars. By Sonali Padhye, Cathy Whitman, Erik Runkle and Art Cameron


The genus campanula contains more than 300 species, many of which are of Northern origins. Campanula species are commonly referred to as bellflowers and can add a great splash of blue, white or red to any perennial garden. Many campanulas such as C. carpatica, C. portenschlagiana and C. poscharskyana thrive in cool temperatures and high light conditions, making them very suitable for the springs and summers in Northern Europe. Therefore, it is not surprising that in Northern Europe campanulas are extremely popular and almost a staple in any perennial garden. Several campanulas, especially many noteworthy cultivars such as C. punctata ‘Cherry Bells’ and campanula ‘Kent Belle’ are reportedly adaptable to high heat and humidity. A few campanulas, including C. rotundifolia, are native to the United States and offer great untapped potential to gardeners and plant breeders looking for new plant material. Smaller-sized campanulas such as C. carpatica form charming mounds of flowers in small containers and are important potted flowering crops. Several campanulas suitable for domestic conditions have been significant to the American herbaceous perennial market.

Cool Campanula fig1

Flowering campanulas

When perennials are offered in flower, their marketability is boosted considerably. Forcing perennials and scheduling them to flower on a specific date requires an understanding of perennial flower induction mechanisms. Our research program has studied several campanulas, and it is intriguing to review unique strategies to force them.

Flowering or sexual reproduction is considered the culmination of a plant’s developmental process. In order to ensure reproductive success, plants have evolved highly regulated mechanisms that operate on environmental cues. Thus, plants flower when the environment is most conducive for flowering and pollination. Two important environmental signals that plants, especially in Northern origins, respond to are exposure to cool temperatures, known as vernalization, and response to daylength, known as photoperiodism.

Their similar botanical characteristics cause campanulas to be grouped in the same genus, but we have found that campanula species and cultivars vary greatly in their flowering requirements. We will overview the flowering requirements of campanulas that enable these plants to be forced into bloom for subsequent market sales.

Vernalization Requirements

We have found that campanula species vary widely in their vernalization requirements (see Figure 1, page 72). Campanula carpatica has no vernalization requirement. In our studies, plants that were not cooled and plants that were cooled for short or long durations were similar in their flowering percent, time to flower and other flowering characteristics such as bud number and height at flowering. It is interesting that C. carpatica is very cold tolerant and can be held at low temperatures for extended periods, but exposure to cold is not required for flower induction.

Campanula glomerata and C. poscharskyana showed a facultative or quantitative vernalization response. During our research only a few plants flowered without cooling, but after cooling for 15 weeks at 41° F flowering percent increased significantly. Although cooling did not affect the flowering time of C. glomerata, C. poscharskyana flowered approximately three weeks sooner following vernalization compared to non-vernalized plants.

Campanula garganica, C. persicifolia, C. portenschlagiana and C. punctata have an obligate or qualitative vernalization requirement. These species did not flower unless cold was provided; however, vernalization induced these species to flower. It is essential to overcome the juvenile phase before beginning the cooling period. Some C. persicifolia cultivars reportedly require bulking, but in our preliminary screen the vegetatively propagated cultivar ‘Chettle Charm’ flowered readily without bulking.

‘Kent Belle’ reportedly has C. punctata, C. latifolia and C. takesimana in its parentage. In our studies, ‘Kent Belle’ has a facultative vernalization requirement. Percent flowering of non-vernalized ‘Kent Belle’ plants ranged from 10 to 40 percent depending on the light regime provided, but following vernalization all plants bloomed completely, regardless of the light regime tested. ‘Birch Hybrid’ is reportedly a hybrid between C. portenschlagiana, having obligate vernalization requirement, and C. poscharskyana, having facultative vernalization requirement. ‘Birch Hybrid’ has “inherited” an obligate vernalization requirement. We have maintained stock plants of ‘Birch Hybrid’ for the last 2-3 years. In rare instances, overgrown stock plants initiate a few flowers; however, flowering is non-uniform and very sparse. Therefore, horticulturally speaking, ‘Birch Hybrid’ requires vernalization for flowering.

Vernalization and Hardiness

The observed diverse vernalization responses prompted us to explore whether there is an obvious link between vernalization requirements and a plant’s native range or reported USDA Cold Hardiness Zone. Campanula carpatica, which is of very Northern origin and cold hardy to Zone 3, has no vernalization requirement; whereas, C. glomerata and C. poscharskyana, which are also native to the North and cold hardy to Zones 2 and 3, respectively, have a facultative vernalization requirement (see Figure 1, page 72)

Finally, C. garganica, C. persicifolia, C. portenschlagiana and C. punctata have obligate vernalization requirements yet varying cold hardiness between Zones 3 and 6. Thus, for campanula there does not appear to be a direct relationship between cold hardiness and vernalization requirement.

When trying to identify the forcing requirements of new campanula species and cultivars, new plants should be tested under controlled conditions since there have been no obvious relationships between a species’ native range and its vernalization response type. Although the vernalization response types of hybrids tend to be similar to that of a parent, without conducting experiments it would be impossible to guess whether a hybrid would have a vernalization requirement or not.

Temperature and Duration

Most of our vernalization recommendations are based on cooling plants at 41° F. Our research results indicate that ‘Birch Hybrid’ flowers in response to a very broad range of temperatures — 32-55° F, depending on the duration of exposure. Note that vernalization at 55° F only delayed flowering 1-2 weeks compared to the quickest flowering treatment, which was 41° F. Complete flowering of ‘Birch Hybrid’ was achieved after five weeks of vernalization at 32-46° F, and plants vernalized at 32 and 37° F flowered approximately 10 days later than those at 41 and 46° F. 

When we moved actively growing 4-week-old ‘Birch Hybrid’ plugs from 68 to 28° F the plugs often died during vernalization. Since ‘Birch Hybrid’ is very cold tolerant in the garden and is cold hardy to Zone 4 (-30 to -20° F), we believe exposing actively growing plant material to 28° F does not allow it to harden, resulting in plant death. Therefore, growers who wish to vernalize plants without pre-hardening should avoid vernalizing at or less than 28 °F.


We have also found that the photoperiodic requirements for flowering Campanula species vary greatly (see Figure 3, page 76). Campanula persicifolia and C. portenschlagiana are day neutral and, therefore, flower under any photoperiod. Also, the photoperiod does not affect flowering characteristics such as plant form, height, flower number, etc.

Campanula garganica, C. glomerata and C. poscharskyana are facultative longday plants. Under long days, C. garganica and C. glomerata flowered completely and faster, and C. garganica also had more buds. On the other hand, C. poscharskyana flowered under short and long days but flowered faster under and had more buds when forced under long days.

Campanula carpatica and C. punctata are obligate long-day plants. When grown under short days, these plants form rosettes; when grown under long days, plants elongate and bolt during flowering. The critical photoperiod of C. carpatica ‘Blue Clips’ is 14 hours. Therefore, at any photoperiod less than 14 hours plants form rosettes, and at or greater than 14 hours plants elongate and flower. It should be noted that ‘Blue Clips’ flowers faster under photoperiods of 16 or more hours rather than 14 hours.

Both ‘Birch Hybrid’ and ‘Kent Belle’ are day-neutral plants and can be forced under any photoperiod (see Figure 4, page 76).

For the different campanula species tested in our program, we did not observe any apparent relationship between the native ranges, cold hardiness, vernalization requirements and photoperiodic requirements for flowering. Thus, along with vernalization requirements, the photoperiodic requirements for flowering campanulas must be tested in controlled experiments to ensure successful forcing.

Forcing Campanulas

Campanulas can effectively be forced into flower by manipulating their photoperiod, vernalization requirement or both (see Figure 5, page 77). Along with the forcing requirement, cultural requirements should also be considered prior to campanula production.

Growing temperature. The primary effect of forcing temperature is the regulation of plant growth rate. As the forcing temperature increases time to flower decreases. However, forcing temperature also has secondary effects on flowering characteristics. Growing campanulas under high temperatures not only induces them in flower quickly but also reduces plant height, decreases flower number and size, and reduces the intensity of flower color. As mentioned earlier, most campanulas prefer cool seasons and should be forced under cool temperatures to obtain more aesthetically appealing plants. We have produced campanulas of good quality under 68° F in our research program.

Light quantity. Most campanulas respond favorably to light quantity, regardless of their photoperiodic requirements. The most commonly observed effect of supplemental lighting is increased bud number, as reported in several cultivars including all carpatica cultivars tested and ‘Birch Hybrid’. The only exception to this was C. glomerata, which showed no response to light quantity. Often times, adding supplemental lighting also hastens flowering. However, supplemental lighting increases plant temperature, and therefore, quicker flowering may be due to the increase in plant temperature and not the light quantity.

Plant growth regulators. Typically, plant growth regulator applications are effective at reducing the height of the campanulas we tested, though under our experimental conditions, the actual response of different cultivars varied. For example, A-Rest (SePRO), B-Nine (Chemtura Corporation), Bonzi (Syngenta Professional Products), Cycocel (OHP) and Sumagic (Valent USA) were all effective in height control of C. carpatica. Whereas, ‘Kent Belle’ responded well only to A-Rest, BNine, Cycocel and Sumagic. The rates, timing and choice of chemical depend on growing conditions and should be tested prior to use.


In summary, there are many campanulas suitable for production as potted crops. Scheduling campanulas is easier when manipulating photoperiod, vernalization requirement or both. Most campanulas tested favored high light conditions and responded well to most plant growth regulators tested.




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