Plant Growth Control Workshop – Plant Growth and Physiology: Growth Control Through Mechanical and Environmental Manipulation
A presentation summarising the use of mechanical and environmental cultural measures to control crop growth.
Options for Managing Crop Growth and Development
‘Summary of the HTA Grower Technical Workshop: ‘Cultural control of plant growth and optimising the use of plant growth regulators to manage crop growth and development’
Managing crop growth to meet marketing dates and customer specifications
A range of growth control measures, beyond the application of plant growth regulators (PGRs), can be employed to manage crop growth. Some, such as temperature, irrigation, and nutritional manipulation are often used as standard as part of any cultural production programme. As part of these, there are targeted actions which can also be considered, such as regulated deficit irrigation or DROP, where the temperature within the growing structure is reduced rapidly just after sunrise. Other measures need to be employed specifically, such as regularly brushing the growing points of plants. The key point to remember is ‘to take your foot off the accelerator before applying the brake’, so for example avoid applying inputs which encourage growth, such as ammonium nitrogen fertilisers, while employing mechanisms to hold the crop back, such as PGR applications.
Mechanical and environmental manipulation
The usual mechanical method of manipulating growth is to trim back plants, either manually via secateurs and shears, or automatically via various types of trimming machine, which can either trim plants individually, or entire crops set out on production beds. However, removing excessive growth from crops once it has been generated, is not ideal as it adds to cost while increasing the production timings of crops.
A better approach is to control growth before it becomes excessive. Brushing the tops of crops with sheets or strips of polythene, or applying an air current over the crop to agitate foliage, are not new ideas and have been trialled on several occasions. The main issue is the need to apply the treatment several times a day, so it requires some form of automatic application. Like many of the treatments featured in this summary, brushing/agitation doesn’t perform uniformly over a wide range of plant species, some plants do respond, but many show only a marginal response or none at all.
Environmental growth control can be achieved either by temperature or light manipulation. In its simplest form, cool temperatures can be used to slow growth or harden off plants, but there are also two specific procedures, DIF, where night temperatures are held below daytime temperatures to reduce growth, and DROP, which involves a drop of 7-8°C after sunrise for around two hours. DROP is usually the more appropriate approach for UK production, as it becomes costly and difficult to achieve DIF from spring onwards.
In terms of light manipulation, this can be achieved either by the use of LED growth lighting for intensive production, perhaps over propagation benches for instance, or via the use of spectral filter polythene, glasshouse screens, or shading materials to amend the light spectrum reaching plants. Previous trials have shown that, rather like brushing, not all plant species respond in the same way to light amended in its UV, blue, or red spectral content.
Irrigation and nutrient manipulation
The application of a controlled water stress (regulated deficit irrigation) to manipulate plant growth in container-grown crops has been trialled over many years following the development of commercial sensors to accurately monitor volumetric moisture content in the growing medium. The ability to determine and monitor the exact level at which water stress will slow growth without impacting quality is key to the application of the technique. For the technique to be applied consistently over a crop, both the irrigation levels applied to the crop, and the drainage of excess water away from the crop, need to occur uniformly.
The wider commercial application of such an approach is still in its infancy and will probably require the development of cheaper digital sensors, so that sufficient numbers can be deployed on nurseries to monitor the range of plant species and different products typically grown on UK nurseries.
In terms of nutrition, matching the type of fertiliser used to the crop being grown is essential. The use of soluble fertilisers permits a very dynamic approach to nutrition; it means crops cannot grow excessively due to high background feed levels in the growing medium, and that crop needs can be met instantly with a soluble feed. However, such feeds need to be applied at every watering, or at least weekly, and depending on the precision of the application system there may be considerable wastage; it also means the provision of adequate nutrition through marketing and shelf life may be limited.
The use of base fertilisers, with or without controlled release fertilisers, provides longer term nutrition but can result in a build-up of nutrients which stimulate softer growth on crops. Incorporating the appropriate levels of these fertilisers is also key to getting balanced growth.
Other measures for consideration include feeding with non-ammonium types of nitrogen fertiliser, such as calcium nitrate, or the use of fertilisers high in potassium to add to the growing medium electrical conductivity to 'stress' and harden plants.
Use of chemical plant growth regulators
The key plant growth regulator (PGR) active substances include chlormequat (Stabilan 750), daminozide (B-Nine SG and Dazide Enhance), and paclobutrazol (Bonzi and Pirouette). Other approved PGR substances include ethephon, ethephon + mepiquat, prohexadione and trinexapac-ethyl. Each substance has a slightly different impact on plant growth, but most of them work by impacting gibberellic acid synthesis within the plant.
They all need to be applied prior to any excessive growth, generally from post-potting establishment onwards. Products containing chlormequat and daminozide are often used to help control plant shape and habit, paclobutrazol can be used to stop growth close to the point of marketing, or on vigorous or woody plant species.
Depending on the product, they can be applied as a spray, sprench, or drench, alone or mixed with other PGRs or foliar fertilisers. Where adjuvants are needed, for example with chlormequat, selection is important to avoid potential crop damage.
The impact of various plant growth regulator treatments on Salvia ‘Hot Lips’
Presentations and Handout
Plant Growth Control Workshop – Plant Growth and Physiology: Growth Control Through Nutrition and Irrigation Management
A presentation covering fertiliser selection and use, and the management of irrigation levels to control crop growth.
Plant Growth Control Workshop – Getting the Best from Bonzi: Application Methods, Adjuvants, Timing of Applications, and Rates
A presentation by Syngenta UK covering the use of Bonzi on ornamental crops.
Plant Growth Control Workshop – Plant Growth Regulators: Available Products and Approvals, Best Practice and Use, PGR Mixtures, and Integration into Production Programmes
A presentation focusing on the getting the best from chemical plant growth regulators.
Plant Growth Control Workshop – Handout
The handout collating all the presentations from the ‘Cultural control of plant growth and optimising the use of plant growth regulators to manage crop growth and development’ workshop.