Abstract
Clematis (Clematis ×hybrida) has not traditionally fit into the standard production system for vegetatively propagated herbaceous perennials because of the lack of commercially available unrooted cuttings and relatively poor rooting success. We investigated strategies to improve stock plant production and propagation of clematis. The first experiment compared the propagation performance of four cultivars (H.F. Young, Reiman, Little Duckling, and Pinky). The second experiment examined cutting productivity and propagation performance of clematis cultivars when stock plants were grown at 21 or 27 °C and propagated with or without the application of rooting hormone. Stock plants grown at 27 °C resulted in greater cutting numbers and greater dry weights in the rooted cuttings after propagation. The third experiment demonstrated the effects of the origin of the cuttings of the stock plant on cutting productivity and propagation performance. When shoots emerged from underground buds, as compared with axillary buds, the numbers of cuttings and fresh and dry weights of the rooted cuttings were increased by nearly 50%. The promotion of shoot emergence from underground buds on the stock plants led to continuous cutting production for five cycles, with cutting number increasing from 67 to 128 cuttings/plant. Year-round cutting supplies can be achieved by trimming stock plants to the substrate surface to promote juvenile shoot development while maintaining stock plants under long-day photoperiods and warm temperatures (27 °C).
Clematis (Clematis ×hybrida) is a perennial ornamental vine that is well-recognized by the retail market. However, production has not traditionally fit into the vegetatively propagated greenhouse production schedules used for other herbaceous perennials. The reasons are the lack of availability of unrooted cuttings, poor rooting success compared with most annual and perennial species, and a production cycle from propagation to market that requires ≈2 years. Recently, a 1-year production schedule for clematis from propagation to finish has been developed (Samarakoon and Faust, 2020). Successful completion of this schedule relies on the availability of high-quality unrooted cuttings year-round. Most other floriculture crops’ cuttings are produced offshore because of the cost of production and favorable growing conditions. If stock plant production could be understood better, then unrooted cuttings of clematis could be produced offshore, and propagation could be performed by propagation specialists in the United States. Rooting percentages of propagation also need to be considerably higher than those currently observed for propagators to expand production of this crop. These improvements would allow clematis to fit into the array of perennial species propagated and flowered in the United States (Faust et al., 2016). The current study focused on developing strategies for stock plant management, cutting production, and propagation success for the supply of high-quality, rooted cuttings of clematis.
The limited number of previous studies of clematis have described cultivar differences, propagation media (Erwin et al., 1997), and micropropagation techniques (Kreen et al., 2002). Based on our preliminary investigations, protocols were developed for propagation techniques for clematis. Cuttings can be propagated with single-node cuttings with one leaf removed because the presence of both leaves did not affect rooting. Use of phenolic foam (Smithers-Oasis, Kent, OH) as the propagation medium resulted in better rooting compared with peat-based germination mix. Propagation under a tent with no fertilizer produced better root numbers and fresh weights compared with propagation under mist with or without fertilizer applied as a drench. However, rooting percentages were highly variable, ranging from 20% to 95%. Compared with many annual species, poor rooting is a common phenomenon for perennial species (Pijut et al., 2011). This has been attributed to juvenility (Amri et al., 2010; Kibbler et al., 2004; Rasmussen and Hunt, 2010), seasonality (Kibbler et al., 2004), bud dormancy (Smart et al., 2002), and cultivar (Erwin et al., 1997; Smart et al., 2002).
As evident for annuals, such as petunia (Petunia ×hybrida) (Ahkami et al., 2013), auxin, which is the key growth hormone in adventitious root formation, accumulates at the base of a cutting within 24 h after propagation, leading to an increase in sugar accumulation. Therefore, the basal stem becomes a sink causing cell division and meristem formation, and root emergence can be observed after ≈8 d. Therefore, if a cutting has received the optimum propagation environment, then it is hypothesized that failure in adventitious root formation could be attributed to the lack of auxin, sugar, or anatomical barriers for root formation. These factors could be influenced by the stock plant. For example, rooting percentages were greater for juvenile than for mature American elm (Ulmus americana) cuttings (Couvillon, 1987). When plants were cut back to promote shoots, the resulting cuttings had better propagation percentages when they originated from shoots of lower cutting positions because of more juvenility in oak (Quercus sp.) (Naalamle Amissah and Bassuk, 2009). Therefore, during the current study, we investigated techniques to manipulate stock plants to produce a greater number of juvenile cuttings.
Both the physiology of the stock plant and the propagation environment can have a role in propagation success. For example, the seasonal differences in rooting of hybrid aspen (Populus tremula × P. tremuloides) were related to the seasonal decrease in the sugar content in leaves and stems (Stenvall et al., 2009). Reductions in the sugar content were attributable to underground growth becoming sinks under short-day photoperiods (Sivaci, 2006). Long days and supplemental lighting promoted shoot growth and development of clematis when grown in a greenhouse (Samarakoon and Faust, 2020). Buds were apparently nondormant, and plants were not exposed to short days. Seasonal differences in rooting could also be attributable to the activity of buds and/or quantity of auxin present. As evident for apple (Malus ×domestica), the auxin content gradually decreased in apical buds from spring to summer, with axillary buds having greater auxin present during summer as the bud outgrowth occurred (Sivaci, 2006). Therefore, faster shoot development on the stock plant attributable to temperature can lead to better rooting (Maynard and Bassuk, 1992). Based on these reports, we concluded that the stock plant environment is critical for rooting success, and that growing in a greenhouse allows for better control over seasonal environmental differences for clematis stock plants. Because long days are conducive for shoot development, the influence of temperature of the stock plant environment on cutting production and propagation was investigated.
As discussed, low levels of endogenous auxin led to poor rooting. Use of the exogenous application of auxin to promote rooting is common for perennial propagation (Amri et al., 2010; Kesari et al., 2009). However, exogenous auxin can be inhibitory if endogenous auxin levels are high (Nanda and Anand, 1970); therefore, the application of auxin is not effective for some perennials (Kibbler et al., 2004). Rooting differences among clematis cultivars Comtesse de Bouchard, Gypsy Queen, and Jackmanii were reported, and variable responses to indole-3-butyric acid were noted (Erwin et al., 1997). During the current study, the effects of exogenous auxin on several cultivars were investigated. This project aimed to develop cultural techniques for cutting production and propagation of clematis.
Materials and methods
Growing conditions
Clematis stock plants were grown in a glass greenhouse (lat. 35°N, long. 83°W) with an environment controlled by a climate-control computer (Argus Control Environmental Systems, White Rock, BC, Canada). Heating and ventilation set points during day and night were 22 °C and 27 °C, respectively. Plants were shaded with retractable curtains [55% reduction of photosynthetic photon flux density (PPFD)] when the solar radiation measured outside the greenhouse exceeded 500 W⋅m−2. To avoid dormancy and produce cuttings continuously, plants were grown under long days (≈16 h) provided through supplemental lighting with metal halide lamps when solar radiation measured outside was <200 W⋅m−2 from 10:00 to 24:00 hr. A peat-based substrate (Fafard 3B; Sun Gro Horticulture, Agawam, MA) was used as the growing medium. A constant liquid fertigation schedule was supplied with a 15N–2.2P–12.5K water-soluble fertilizer (Peters Excel Cal-Mag; Scotts-Sierra, Marysville, OH) to provide 200 mg⋅L−1 N at each irrigation event. The substrate, greenhouse growing conditions, and fertilizer schedule were the same for all experiments unless stated otherwise.
Expt. 1: Cultivar differences in propagation
Two-year-old plants of ‘H.F. Young’, ‘Reiman’, ‘Little Duckling’, and ‘Pinky’ clematis were received on 31 Mar. from a commercial nursery. Thirty-six plants per cultivar were transplanted into 3-qt round containers (diameter, 6.7 inches; height, 6.9 inches). Entire shoots were harvested after flowering in May. From each shoot, the two lowest nodes on the stem and any nodes with flowers were removed; from the remaining shoot section, single-node, semi-hardwood cuttings were taken. After the removal of one leaf from each cutting and a dip application of rooting hormone powder (0.3% indole-3-butyric acid; Hormodin® 2; OHP Inc., Mainland, PA) to the cut stem, cuttings were inserted into a phenolic foam propagation medium (34-count strip cells with subirrigation trays; Oasis Wedge®; Smithers-Oasis) on 22 May. Trays were placed under a spunbond polyester fabric tent (Reemay®; Reemay Inc., Old Hickory, TN) and sub-irrigated with water every day for 4 weeks, and then every other day after that for an additional 4 weeks. The purpose of the tent was to maintain humidity around the cuttings while keeping water off the leaves to reduce fungal infections. After 6 weeks of propagation, 100 mg⋅L−1 N was provided at each irrigation. Rooting percentages were based on the visual appearance of roots on the edges of the foam cube after 8 weeks of propagation.
Expt. 2: Effects of cultivar, stock plant temperature, and rooting hormone on cutting production and propagation
Using the stock plants from Expt. 1, shoots of ‘H.F. Young’, ‘Reiman’, and ‘Little Duckling’ were trimmed on 22 May at the substrate surface without leaving any visible, aboveground nodes; therefore, all shoot emergence occurred from underground buds. After the removal of shoots, 15 plants of each cultivar were moved to climate-controlled growth rooms with a constant temperature of either 21 or 27 °C and a 16-h photoperiod using metal halide lamps delivering a PPFD of 50 ± 15 μmol⋅m−2⋅s−1 at the top of the canopy. Cuttings were harvested when 90% of the shoots were flowering (e.g., 22 July for plants at 27 °C and 7 Aug. for plants at 21 °C). Cuttings were propagated with or without rooting hormone powder (Hormodin® 2) for 8 weeks under the same conditions as described for Expt. 1. Six stock plants from each cultivar were used for each treatment combination (cultivar × temperature × rooting hormone). The experiment was repeated with use of the same plants in the growth rooms (21 or 27 °C).
The number of shoots per plant, shoot length, number of nodes, and number of flowers were recorded on the stock plants grown under the two different temperatures. After cuttings were propagated for 8 weeks, data regarding the number of rooted cuttings, fresh and dry weights of roots, and the number of new shoots developing from the axillary buds of a cutting were collected.
Expt. 3: Influence of shoots developed from the underground versus axillary buds on cutting productivity and propagation
Two-year-old ‘H.F. Young’ clematis plants were received from a commercial greenhouse on 31 Mar., transplanted into 3-qt round containers, and grown as stock plants in a heated greenhouse. All existing shoots were trimmed to the substrate surface, and plants were grown outdoors from October to January to expose them to cold temperatures for vernalization and break dormancy. On 26 Jan., plants were moved to a greenhouse with long days (16 h). On 5 Mar., shoots that developed during forcing in the greenhouse were trimmed to leave two nodes on the shoots or trimmed at the substrate surface. Sixteen plants per treatment were used. Cuttings from trimmed shoots were propagated (as described in Expt. 1) to evaluate rooting. The same stock plants were used to produce cuttings continuously as described. The first round of cutting production started on 5 Mar. (cutting production cycle 1).
Data collection included shoot number per plant, shoot length, number of nodes/shoots, number of flowers, and number of cuttings. After data collection was performed, shoots were trimmed on 4 May at the substrate surface or aboveground, leaving two axillary shoots in their respective treatments (Fig. 1A and B) to evaluate further cutting production on the stock plant. The trimmed shoots were propagated (cutting production cycle 2). As shoots re-emerged from the stock plants, data collection was performed on 17 July and included shoot length, node number, and flower number. Cuttings were again propagated at this time (cutting production cycle 3).
Examples of shoots of ‘HF Young’ clematis (Clematis ×hybrida) plants trimmed to (A) two axillary buds or to (B) the substrate surface and (C) the subsequent shoot emergence and flowering from the plants trimmed to axillary buds (left) and underground buds (right) during cutting production cycle 2 of Expt. 3.
Citation: HortTechnology 32, 4; 10.21273/HORTTECH05029-22
Examples of shoots of ‘HF Young’ clematis (Clematis ×hybrida) plants trimmed to (A) two axillary buds or to (B) the substrate surface and (C) the subsequent shoot emergence and flowering from the plants trimmed to axillary buds (left) and underground buds (right) during cutting production cycle 2 of Expt. 3.
Citation: HortTechnology 32, 4; 10.21273/HORTTECH05029-22
Examples of shoots of ‘HF Young’ clematis (Clematis ×hybrida) plants trimmed to (A) two axillary buds or to (B) the substrate surface and (C) the subsequent shoot emergence and flowering from the plants trimmed to axillary buds (left) and underground buds (right) during cutting production cycle 2 of Expt. 3.
Citation: HortTechnology 32, 4; 10.21273/HORTTECH05029-22
After the third cutting production cycle, all shoots from all stock plants were trimmed at the substrate surface, and shoots were allowed to emerge. Shoot number and node number data were collected on 24 Sept. (cutting production cycle 4). Cuttings were harvested at the substrate surface, and plants were grown for 6 weeks. Emerging shoots were evaluated for cutting productivity for the fifth time on 30 Nov. (cutting production cycle 5). As described for Expt. 1, the two lowest nodes and nodes with flowers cannot be used for propagation. Therefore, the usable cutting number of a shoot was calculated by subtracting two from the total node count per shoot and also subtracting the number of nodes possessing flowers in the leaf axil.
Data analysis
All data were analyzed using an analysis of variance and statistical software (JMP Pro version 11; SAS Institute, Cary, NC). Expt. 2 was analyzed with the cultivar and temperature as the main effects for shoot length, number of nodes, and number of flowers. Data were pooled from the two experiments. Cultivar, temperature, and rooting hormone application were the main effects for parameters measured after propagation. Mean comparisons were conducted using Tukey's honestly significant difference test (Expt. 1 and Expt. 2) and Student’s t test (Expt. 3), with P ≤ 0.05 indicating statistical significance.
Results
Expt. 1: Differences in the propagation of the cultivars
Propagation percentages for rooted cuttings (mean ± se) varied among cultivars (P < 0.0001). ‘H.F. Young’ and ‘Reiman’ had the greater rooting (85% ± 2%), followed by ‘Pinky’ (50% ± 3%) and ‘Little Duckling’ (33% ± 3%). Although the rooting percentages of ‘H.F. Young’ and ‘Reiman’ were similar, the shoots that developed from the axillary buds of the cuttings of ‘Reiman’ were relatively small.
Expt. 2: Effects of cultivar, stock plant growing temperature, and rooting hormone on cutting production and rooting success
The number of shoots produced varied based on the stock plant growing temperature and the cultivar (Table 1). ‘H.F. Young’ averaged 5.7 shoots compared with ‘Little Duckling’ and ‘Reiman’, with only 2.1 shoots each (Fig. 2). Within plants of ‘H.F. Young’, a greater number of shoots was produced at 21 °C (6.7 shoots); only 4.7 shoots were produced at 27 °C. A significant interaction between cultivar × growing environment occurred for the number of nodes and flowers per shoot (Table 1). The number of nodes was greater in ‘H.F. Young’ and ‘Reiman’ at 27 °C. Hence, the numbers of cuttings were greater for ‘H.F. Young’ and ‘Reiman’ at 27 °C. The number of flowers was lower for ‘H.F. Young’ and ‘Little Duckling’ within the 27 °C growing environment. For ‘H.F. Young’, a 27 °C stock plant environment provided a better environment for cutting production. Therefore, even with the reduction in shoot number observed at 27 °C, more cuttings (50 cuttings) were produced per stock plant compared with the 21 °C environment, which produced 42 cuttings per stock plant.
Shoot number (A), node number (B), and number of flowers (C) of ‘HF Young’ (HFY), ‘Little Duckling’ (LD), and ‘Reiman’ (R) clematis (Clematis ×hybrida) stock plants grown under two temperatures [21 or 27 °C (69.8 or 80.6 °F); Expt. 2; n = 15]. Vertical bars represent ±1 se. Mean separation by Tukey’s honestly significant difference test at P ≤ 0.05. (1.8 × °C) + 32 = °F.
Citation: HortTechnology 32, 4; 10.21273/HORTTECH05029-22
Shoot number (A), node number (B), and number of flowers (C) of ‘HF Young’ (HFY), ‘Little Duckling’ (LD), and ‘Reiman’ (R) clematis (Clematis ×hybrida) stock plants grown under two temperatures [21 or 27 °C (69.8 or 80.6 °F); Expt. 2; n = 15]. Vertical bars represent ±1 se. Mean separation by Tukey’s honestly significant difference test at P ≤ 0.05. (1.8 × °C) + 32 = °F.
Citation: HortTechnology 32, 4; 10.21273/HORTTECH05029-22
Shoot number (A), node number (B), and number of flowers (C) of ‘HF Young’ (HFY), ‘Little Duckling’ (LD), and ‘Reiman’ (R) clematis (Clematis ×hybrida) stock plants grown under two temperatures [21 or 27 °C (69.8 or 80.6 °F); Expt. 2; n = 15]. Vertical bars represent ±1 se. Mean separation by Tukey’s honestly significant difference test at P ≤ 0.05. (1.8 × °C) + 32 = °F.
Citation: HortTechnology 32, 4; 10.21273/HORTTECH05029-22
Analysis of variance for parameters related to growth and flowering of three cultivars of clematis (Clematis ×hybrida) stock plants (‘HF Young’, ‘Little Duckling’, and ‘Reiman’) grown under two temperatures [21 or 27 °C (69.8 or 80.6 °F)] during shoot development (Expt. 2).
The rooting percentage was not influenced by the stock plant temperature. However, cultivars interacted with the application of the rooting hormone (Table 2). For ‘Little Duckling’, the propagation percentage was 25% better without the application of rooting hormone (89% without rooting hormone), whereas ‘H.F. Young’ and ‘Reiman’ were unaffected by the rooting hormone application, with average propagation percentages of 86% and 77%, respectively. The shoot dry weight per cutting was influenced by cultivar, growth environment of the stock plants, application of rooting hormone, and their interactions (Table 2). For ‘H.F. Young’, the greatest shoot weight occurred within nonhormone treatments and under the 27 °C stock plant temperature (Fig. 3). No differences were evident between stock plant temperature treatments for ‘Little Duckling’. For ‘Reiman’, the nonhormone treatment resulted in a greater shoot weight as compared with the application of rooting hormone when propagated with cuttings developed on plants from the 21 °C environment, and no differences were observed for cuttings from plants from the 27 °C environment. The root dry weight was influenced by the stock plant temperature; cuttings from the 27 °C environment produced 50% more root weight than cuttings from the 21 °C environment. Application of the rooting hormone did not influence the root dry weight of a cutting. ‘H.F. Young’ had a greater root weight than the other two cultivars.
Shoot (A) and root (B) dry weights of cuttings propagated from ‘HF Young’ (HFY), ‘Little Duckling’ (LD), and ‘Reiman’ (R) harvested from clematis (Clematis ×hybrida) stock plants grown under two temperatures [21 or 27 °C (69.8 or 80.6 °F)] and propagated with or without 0.3% indole-3-butyric acid as the rooting hormone (Expt. 2; n = 6). Vertical bars represent ±1 se. Mean separation by Tukey’s honestly significant difference test at P ≤ 0.05. 1 g = 0.0353 oz. (1.8 × °C) + 32 = °F.
Citation: HortTechnology 32, 4; 10.21273/HORTTECH05029-22
Shoot (A) and root (B) dry weights of cuttings propagated from ‘HF Young’ (HFY), ‘Little Duckling’ (LD), and ‘Reiman’ (R) harvested from clematis (Clematis ×hybrida) stock plants grown under two temperatures [21 or 27 °C (69.8 or 80.6 °F)] and propagated with or without 0.3% indole-3-butyric acid as the rooting hormone (Expt. 2; n = 6). Vertical bars represent ±1 se. Mean separation by Tukey’s honestly significant difference test at P ≤ 0.05. 1 g = 0.0353 oz. (1.8 × °C) + 32 = °F.
Citation: HortTechnology 32, 4; 10.21273/HORTTECH05029-22
Shoot (A) and root (B) dry weights of cuttings propagated from ‘HF Young’ (HFY), ‘Little Duckling’ (LD), and ‘Reiman’ (R) harvested from clematis (Clematis ×hybrida) stock plants grown under two temperatures [21 or 27 °C (69.8 or 80.6 °F)] and propagated with or without 0.3% indole-3-butyric acid as the rooting hormone (Expt. 2; n = 6). Vertical bars represent ±1 se. Mean separation by Tukey’s honestly significant difference test at P ≤ 0.05. 1 g = 0.0353 oz. (1.8 × °C) + 32 = °F.
Citation: HortTechnology 32, 4; 10.21273/HORTTECH05029-22
Analysis of variance for parameters related to propagation of three cultivars of clematis (Clematis ×hybrida) stock plants (‘HF Young’, ‘Little Duckling’, and ‘Reiman’) grown under two temperatures [21 or 27 °C (69.8 or 80.6 °F)] during shoot development (Expt. 2); n = 6.
Expt. 3: Influence of shoots developed from the underground buds versus axillary buds on cutting production and propagation
During cutting production cycle 1, 12 shoots developed per plant; from these, 67 cuttings were produced. The rooting percentage was 86%. During cutting production cycle 2, stock plants trimmed to the substrate surface developed three more shoots per plant, and the shoots were 22 cm longer than plants trimmed to produce axillary shoots (Table 3, Fig. 1C). The number of days to first flower was delayed by 7 d for underground shoots compared with aboveground axillary shoots. Flower numbers between treatments were similar. On average, plants trimmed to the substrate surface produced 77 cuttings per plant, whereas plants trimmed aboveground developed only 34 shoots per plant. Propagation percentages did not vary for cuttings, with 52% for cuttings from underground shoots and 47% for cuttings from axillary shoots. The rooting percentages were low compared with those of previous experiments because of the incidence of a fungal disease called phoma blight caused by Phoma sp. However, the fresh weight and dry weight of shoots and roots of a rooted cutting were greater when cuttings originated from underground shoots (Table 3).
Cutting productivity and propagation parameters of cuttings originating from axillary and underground buds of ‘H.F. Young’ clematis (Clematis × hybrida) stock plants (Expt. 3, cutting production cycle 2).
During cutting production cycle 3, plants trimmed to the substrate surface produced three more shoots compared with the plants trimmed to the aboveground axillary shoots (Table 4). Shoot length did not vary, with an average of 50 cm. The number of nodes was greater for underground shoots than for axillary shoots (Table 4), and a single flower or no flowers were present on shoots irrespective of the treatment. With the reduction of nodes with flowers, the number of cuttings suitable for propagation increased. Six cuttings per shoot were produced from underground shoots as compared with five cuttings per shoot from axillary shoots. Therefore, the total cutting production averaged 50 cuttings per plant for plants trimmed to the substrate surface, whereas only 35 cuttings per plant developed from the plants trimmed to aboveground axillary shoots (Table 5).
Cutting productivity and propagation of cuttings originating from axillary and underground buds of ‘H.F. Young’ clematis (Clematis ×hybrida) stock plants during cutting production cycle 3 (Expt. 3).
Cutting productivity per ‘H.F. Young’ clematis (Clematis ×hybrida) stock plant during five cutting production cycles as described for Expt. 3. Node number includes all vegetative and reproductive nodes.
During cutting production cycle 4, all stock plants were trimmed to the substrate surface, and 13.0 shoots per plant were produced (Table 5). The shoot length averaged 35 cm, and the node number per shoot was 7.4. With either one or no flowers on shoots, there were five usable cuttings per shoot; hence, 65 cuttings per plant were produced. During cutting production cycle 5, 128 cuttings per plant, 16 shoots per plant, and 8 nodes per stem were produced; however, flowers were not visible (Table 5). As indicated by the five cutting production cycles (Table 5), cutting production is greatest when the plants are stimulated to produce shoots from underground buds.
Discussion
The origin of the shoot, whether from axillary or underground buds, can influence the capacity to produce cuttings as well as subsequent propagation performance of clematis. Shoots from underground buds were more juvenile, leading to delayed flowering and increased node number. Juvenility of adventitious shoots/buds originated from underground structures have been previously reported (Del Tredici, 2001; Vesk and Westoby, 2004). For herbaceous perennial gentian (Gentiana sp.), shoots developing from below the substrate surface resulted in juvenile shoots and remained without flowers as compared with axillary shoots on the same plant (Samarakoon, 2012). For clematis, axillary shoot development is limited to buds located in the remaining nodes after trimming because axillary buds suppress underground shoot emergence, potentially because of correlative inhibition or paradormancy (Chao et al., 2006). As a result, trimming shoots to the substrate surface led to higher cutting productivity.
Underground buds result in shoots that produce cuttings with improved shoots and root weight. Better propagation performance of juvenile cuttings compared with mature cuttings has been reported for perennials (Maynard and Bassuk, 1992; Naalamle Amissah and Bassuk, 2009). Higher root numbers of hybrid pine (Pinus elliottii var. elliottis × P. caribaea var. hondurensis) (Rasmussen and Hunt, 2010) and greater root dry weight of carolina buckthorn (Rhamnus caroliniana) (Graves, 2002) were reported for juvenile cuttings as compared with mature cuttings. The juvenility of shoots may lead to the accumulation of hormone that promote root formation (Osterc et al., 2009). Propagation percentages were similar between underground and axillary shoots during the current study; however, because of the incidence of phoma blight, further investigations are warranted.
Growing stock plants at moderate temperatures (27 °C) increased cutting numbers because of the increased node number per shoot and decreased flower numbers. Although shoot numbers were greater at 21 °C, that did not significantly increase the cutting number because the shoots had more floral nodes that could not be used for propagation. The reduction of flower numbers at warmer temperatures (29.8 °C) was reported for evening primrose (Oenanthera fruticose) (Clough et al., 2001), and an increased node number in response to increasing temperatures from 15 to 25 °C was noted for dahlia (Dahlia pinnata) (Brøndum and Heins, 1993). Stock plant management at 27 °C is beneficial for cutting productivity of clematis compared with 21 °C.
The quality of rooted cutting after propagation was greater when cuttings were obtained from 27 °C, as evidenced by the increased weight of shoots and roots. Cuttings from lemon myrtle (Backhousia citriodora) stock plants grown at 30 °C (Kibbler et al., 2004) and cuttings from Madagascar jasmine (Stephanotis floribunda) stock plants grown at more than 23 °C (Hansen, 1989) promoted adventitious root formation and axillary budbreak as compared with cuttings from stock plants grown in cooler temperatures. As reported for grape (Vitus vinifera), the cuttings taken from stock plants grown at high temperatures were associated with greater stem length and greater leaf dry weight (Buttrose, 1968). It is possible that increased growth associated with high temperatures led to better accumulation of resources, resulting in a better quality of cuttings. Therefore, clematis stock plant management at 27 °C, along with our previous research of long-day photoperiods on promoting shoot growth (Samarakoon and Faust, 2020), led to increased cutting production as well as better propagation performance.
Clematis plants grown in a greenhouse at 27 °C under long days and with the continuous trimming of shoots to the substrate surface to promote shoots developing from the underground buds showed that cutting production can be continued for five cycles (Table 5). With each successive cycle of cutting production, plants flower less and are more vegetative, thus allowing for increased cutting production. These results suggest that year-round cutting production can be achieved successfully for clematis with the proper temperature, photoperiod, and trimming techniques.
‘H.F. Young’ and ‘Reiman’ had better propagation percentages during Expt. 1 as compared with ‘Little Duckling’ and ‘Pinky’. Similar to the current study, distinct cultivar differences among cultivars have been reported for Comtesse de Bouchard, Gypsy Queen, and Jackmanii clematis (Erwin et al., 1997). As evidenced by the current study, rooting hormone did not increase the rooting percentage of ‘Little Duckling’. The better rooting cultivars, like H.F. Young and Reiman, can be propagated without rooting hormone. As previously reported (Erwin et al., 1997), rooting hormone promoted cutting survival of only one cultivar, Jackmanii clematis. Therefore, a lack of auxin may not be the main reason for poor rooting of some clematis cultivars, and factors such as juvenility and the stock growing environment warrant further investigation.
In summary, strategies to manage stock plants as well as propagation of clematis have been described. With the maintenance of juvenility of stock pants via stimulation of underground buds, clematis cuttings can be available over multiple crop cycles, and propagation can be performed irrespective of the time of the year. Cutting productivity and propagation performance could be further increased by growing stock plants at 27 °C. During propagation, responses to rooting hormone vary with the cultivar, and the cultivars we investigated can be propagated without rooting hormone. The application of rooting hormone incur additional costs for materials and labor in commercial greenhouses. Therefore, recognizing the cultivar response to rooting hormones will allow propagators to skip this step.
Vegetatively propagated ornamental plant production relies on the timely availability of unrooted cuttings. We have presented stock plant management guidelines for cutting continuity and quality either offshore or in the United States. The possibility of stock plant production offshore would allow possibilities for clematis to be more widely grown by growers in the United States and would improve the economics of growing clematis by reducing the losses experienced during propagation. For example, because propagation could be performed any time of the year, growers could schedule clematis successfully, depending on projected market dates. In combination with a 1-year production schedule for clematis (Samarakoon and Faust, 2020), the current research provides strategies to successfully propagate clematis within 8 weeks, thereby allowing for the integration of clematis into a more conventional greenhouse production schedule.
Units
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