Project title: Hardy nursery stock: Examination of techniques to raise humidity in mist houses Project number hns 76 Project leader Dr R. S. Harrison-Murray Annual report Feb, 2002

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Project title: Hardy nursery stock:

Examination of techniques to raise humidity in mist houses
Project number HNS 76
Project leader Dr R.S. Harrison-Murray
Annual report Feb, 2002

(year 2)
Previous reports March 2001
Key workers: Mrs W. Oakley

Location of project: HRI-East Malling, West Malling, Kent

ME19 6BJ

Phone: 01732 843833; Fax: 01732 849067
Project co-ordinator John Hedger, New Place Nurseries Ltd.,

London Road, Pulborough, W. Sussex, RH20 1AT

John Woodhead, Hilliers Nursery Ltd.,

Ampfield House, Romsey, Hants, SO51 9PA
Date project commenced: 1 March, 1999
Date completion due: 28 February, 2002
Key words: humidity, mist, fog, evaporation, transpiration, environment, water balance, hardy nursery stock, propagation, rooting, cuttings

The results and conclusions in this report are based on an investigation conducted over one year. The conditions under which the experiment was carried out and the results obtained have been reported with detail and accuracy. However because of the biological nature of the work it must be borne in mind that different circumstances and conditions could produce different results. Therefore, care must be taken with interpretation of the results especially if they are used as the basis for commercial product recommendations.



Commercial benefits of the project 1

Background and objectives 1

Summary of results and conclusions 2

Action points for growers 4

Anticipated practical and financial benefits 4


Introduction 6

Purpose 6

Basic requirements of the environment for stress-sensitive cuttings 6

General background 7

Previous work in this project 8

Objectives of phase two 10

Materials and Methods 10

Design and construction of the large-scale polythene-enclosed mist system 10

Twin-span polythene house 10

The propagation bed 10

Supporting framework 11

The polythene chamber 12

The mist system 13

Sand heating 13

Rooting experiments 17

Plant material – main subjects 17

Plant material - additional subjects at reduced replication 18

Rooting media 20

Records of cutting responses 20

Propagation environments 20

Monitoring environmental conditions 22

The experimental enclosures 24

Results and Discussion 26

Rooting experiments 26

The main subjects 26

Appearance of cuttings 27

Additional subjects 38

Comparison of conditions in the different propagation environments 40

Humidity 40

Temperature 42

Radiation 42

Effect of misting frequency on humidity 49

Water deposition rates 53

Combining mist and fog 53

Modelling the energy balance of enclosed systems 55

The original model 55

Effect of the type of polythene 56

Under-bed insulation 57

Thermal buffering effect of the substrate 59

Effect of EFAR 61

Long-wave radiation balance 61

Effect of the location of shade 64

Conclusions 65

Future work 67


GLOSSARY : terms, abbreviations and products used 70




This project is investigating ways in which nurserymen can achieve the sort of environmental conditions required for the cuttings of stress-sensitive species which require maximum environmental support to achieve high rooting percentage (e.g. soft cuttings of Garrya elliptica 'James Roof' or large cuttings of Cotinus coggygria 'Royal Purple' suitable for 'designer liner' production).

Commercial benefits of the project

Progress in this project is expected to have the following commercial benefits:

  • Reduced wastage: about 200 million HNS cuttings are taken every year and while failure rates vary from nursery to nursery and from crop to crop, it has been estimated to be at least 25% overall. Optimising the rooting environment could contribute to reducing this wastage.

  • Cost savings: See cost benefit analysis below.

  • Increased ability to respond to a sharp rise in demand when a particular plant becomes fashionable.

Background and objectives

The purpose of this project is to provide reliable information on how to achieve optimal environmental conditions for stress sensitive cuttings. Such cuttings need a combination of generous leaf wetting and an atmosphere that is almost saturated with water vapour (i.e. 100% rh). A good fog system can achieve this but in practice most growers have found it hard to manage and it is consequently not in widespread use. Polythene-enclosed mist can achieve almost as good results but tends to suffer from excessively high temperatures. Furthermore, it is not favoured by many growers because polythene covers over each mist bed makes it difficult to monitor the condition of cuttings.

This project is investigating a number of alternative approaches, including the scaling up polythene-enclosed mist whilst learning how to avoid the excessive temperatures referred to above.

Summary of results and conclusions

The first stage of the project examined the factors determining the temperature and humidity achieved in a well-sealed polythene enclosure. The results showed that there was no reason why enclosed mist could not be scaled up to a large walk-in enclosure that would be acceptable to nurserymen. Such a facility has now been constructed and evaluated. In rooting trials it has been compared with a highly successful ventilated fog house and a conventional open mist system. Environmental conditions have been monitored and compared with many other experimental enclosures to gain insights into the factors controlling temperatures in well-sealed enclosures. Particular attention has focussed on the effect of the 'External to Floor Area Ratio' (EFAR), and the importance of thermal buffering by the sandbed or other material on which the propagation trays are stood.

The conclusions are summarised below:

  • Polythene-enclosed mist can be scaled up sufficiently to make it attractive to commercial nurseries

  • Humidity in the large-scale enclosed mist system was indistinguishable from that in a good fog system i.e. 100% rh

  • If only part of the enclosure was misted then the high humidity was localised to the misted area. Also, if the interval between mist bursts is too long then humidity can drop between bursts.

  • For the most sensitive subjects, fog was more supportive than enclosed mist. For example, in fog, Garrya elliptica ‘James Roof’ continued to elongate and achieved 100% rooting in four weeks; in enclosed mist, most cuttings did not elongate and only 67% had rooted after 8 weeks.

  • What makes fog more supportive than enclosed mist was not identified. It may be due to the circulation of water droplets in the air underneath the leaves. Such water would help maintain humidity saturation close to the under-surface of the leaves as the air warms up.

  • The addition of small quantities of fine droplet fog, such as can be produced ultrasonically, may allow that additional support to be reproduced in a mist + fog system. Since ultrasonic foggers have a low output it is envisaged that the mist will provide the main support system during stressful conditions, which are usually confined to quite a short period in the middle of the day.

  • Preliminary tests indicate that such a system will allow limited ventilation of the enclosure without the rh falling substantially below 100%.

  • The tendency for temperature lift to increase as EFAR decreases was confirmed but, in proportional terms, the change in temperature lift was less than half the change in EFAR.

  • Under-bed insulation tends to increase temperature lift in enclosures by reducing the loss of heat into the ground.

  • The increase in temperature caused by under-bed insulation depends on the heat storage capacity of the material above the insulation. A layer of concrete or sand can store a much of the heat received in one day, thereby limiting the maximum temperature reached.

  • Operating on benches and/or with a thin layer of capillary matting would provide minimal thermal buffering, leading to higher maximum temperatures.

  • Heat loss by radiation is not likely to account for much of the heat loss from polythene enclosures. Both water and modern UV-stabilised polythenes such as Polytherm AF absorb strongly in the infrared waveband.

Action points for growers

It is now possible to make tentative recommendations for growers interested in exploring the use of a large-scale enclosed-mist to provide a more supportive environment for stress-sensitive or ‘designer-liner’ cuttings:

  • Design a robust enclosure and ensure that it is well sealed. Even a small opening can dramatically reduce humidity.

  • Ensure that entire floor area, including access paths, is kept wet by occasionally hosing down areas that are non-misted.

  • Recognise that heavy condensation on the polythene provides no guarantee that humidity is high around the cuttings.

  • External reflective shading is essential to minimise heat load. It is believed to be most effective if supported away from the polythene of the enclosure so that each receives the maximum airflow to keep it cool.

  • Underbed insulation or propagation on benches rather than floor-level beds will tend to lead to higher daytime temperatures.

  • Use at least 5 cm of sand beneath the propagation trays. This will ensure that the rooting medium is well drained even during frequent misting and will also act as a thermal buffer that will limit maximum temperatures even if there is underbed insulation.

  • Remember that not all subjects will show any benefit of a more supportive environment. It is mainly subjects which are considered ‘difficult’ that are likely to show substantial benefit. Large ‘designer liner’ cuttings are likely to benefit, not only in terms of rooting but also subsequent growth and establishment.

Anticipated practical and financial benefits

This research should allow growers to exploit existing knowledge on the benefits of a highly supportive rooting environment for more difficult cuttings by making it possible for them to create, on a commercial scale, the sort of environments that have proved highly effective at a research level. In particular, it will benefit growers attempting to use larger than normal cuttings to shorten the time from cutting to saleable plant (i.e. the designer liner concept)

Cost benefit analysis

Estimate of number of cuttings that fail to make saleable liners

= 25% of 200M cuttings p.a.

= 50M cuttings p.a.

At an average price of £0.20, the value of this lost production

= 50M x 0.2 = £10M

Making the conservative estimate that improvement in propagation environment could reduce losses by 5% (equivalent to increasing average rooting percentage by just 1%) then the value of lost production saved

= 5% of £10M

= £500K

Total cost of the project is approximately £120K, therefore the cost of the project will be recouped in less than a year.

Over a ten year period, the ratio of benefit to total cost

= 500 x 10 / 120 = 41.7

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