A study of factors that promote root formation and growth of Leyland cypress (X Cupressocyparoa leylandii dallim. and A. B. Jackson) stem cuttings

Abstract

A number of experiments were conducted to investigate factors influencing rooting and growth of Leyland cypress cuttings. Bottom heat was supplied to semi-hardwood cuttings of cultivars 'Ferndown', 'Leighton Green' and 'Naylor's Blue' at 16, 23 and 30°C during the whole rooting period or as a split treatment with 25°C until root emergence and 15°C during root growth. 'Ferndown' was more sensitive to bottom heat temperatures than 'Leighton Green' or 'Naylor's Blue' (82.5%, 99.0% and 95%, respectively). Survival was significantly lower (min. of 13.4%) in the 30°C treatment than at all other temperatures. Bottom heat hastened callus formation and only cuttings in the control treatment (no bottom heat) had not produced callus after four weeks. However, 15 weeks after insertion, all cuttings had produced callus with cuttings in the 30°C treatment having the lowest percentage of callused cuttings, due to the high mortality. Mortality also affected analysis for rooting parameters and no significant effect of cultivar or temperature was detected for any parameter.

The effect of moisture availability of six different ratios of pumice and peat on rooting of Leyland cypress 'Ferndown' and 'Leighton Green' cuttings was investigated. Overall cutting survival was low in this experiment (38.9%) and was affected by cultivar (50.8% for 'Leighton Green' and 26.9% for 'Ferndown') and propagation medium independently. Survival was highest in straight pumice (90.8%) and 3 parts pumice: 1 part peat (72.5%). High cutting mortality in other propagation media affected analysis for callus formation and rooting parameters.

The influence of synthetic auxin (IBA) and wounding of basal tissues were examined for their ability to promote root formation in callused, non-rooted stem cuttings of cultivar 'Leighton Green'. Survival of the cuttings was 100%. The percentage of cuttings with additional callus formation was significantly higher in wounded (99% with single and 99.5% with double wound) than in unwounded cuttings (94.9%). The combination of a double wound and 10,000 ppm IBA resulted in the highest percentage of rooted cuttings (63.9%) and the highest percentage of cuttings with a root system judged suitable for transplanting (42.8%). The number of roots formed per rooted cutting was significantly affected by IBA with 4.5 roots were formed on average by cuttings supplied with 10,000 ppm IBA compared to 2.6 roots in non-treated cuttings Root dry weight was significantly higher in cuttings treated with 10,000 ppm IBA (2.23 g) than in cuttings treated with 5,000 ppm or no IBA (1.35 and 0.78 g, respectively).

To evaluate the response of aboveground growth to different levels of nitrogen (0, 50, 100. 150 and 200 g m⁻³ month⁻¹), phosphorus (0, 30, 60, 90 and 120 g m⁻³ month⁻¹) and lime (0 and 5000 g m⁻³ ), rooted Leyland cypress 'Haggerston Grey' stem cuttings were grown in 3.9-litre planter bags for 36 weeks. All plants that were fertilised with nitrogen reached minimum specifications for outplanting. Maximum height increase was obtained with N applications from 50 to 200 g N m⁻³ month⁻¹ and 30, 60 or 90g P m⁻³ month⁻¹. Maximum increase in root collar diameter was achieved with applications of 150 g N m⁻³ month-⁻¹ at either lime levels or 200 g N m⁻³ month⁻¹ at 5000 g lime m⁻³ . Height increase over time for individual plants followed an exponential shape for most plants, with an average correlation coefficient of r = 0.94.