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PGR's - Best Practice

The principle that you should aim to use cultural methods before considering a chemical treatment applies to plant growth control as much as to protecting crops from pest and disease.

In most seasons, though, cultural methods based on manipulating irrigation, nutrition, temperature, and light levels and wavelengths are, on their own, rarely responsive or controllable enough to enable growers of protected pot and bedding plants to meet the specifications and schedules their customers demand. “Despite all the recent advances in crop nutrition and environmental control, crop growth and development are still largely influenced by the weather,” says Ant Surrage of Fargro. Everyone is looking to reduce the use of chemicals in ornamental crop production but at the same time, growers are having to meet ever tighter plant specifications and marketing schedules."

“It’s only by careful integration of plant growth regulators into the production regime that you can achieve the speed or precision of control needed. The aim should be to maximise the effectiveness of applications, for example to schedule a crop or hold back a batch in response to changes in demand or unseasonal weather.”

Plant growth regulators most commonly act by inhibiting the growth hormone gibberellin but because the different active ingredients each work on a slightly different aspect of gibberellin synthesis or activity, they differ subtly in their effects. For instance, paclobutrazol, the active ingredient in Bonzi, controls growth by inhibiting gibberellin production by the plant. “It causes cells in the stems and leaves to divide and expand more slowly,” says Glenn Kirby, technical manager for Syngenta Ornamental Controls. “The result is reduced stem growth and leaf expansion without affecting branching, the number of leaves or root growth. There are also useful improvements in leaf colour, and in bract colour in red poinsettia varieties.”

As well as its value for holding back crops that are close to being ready for marketing, a light ‘toning’ application just before despatch will help to harden quickly-grown plants in spring, says consultant Chris Need. “It will help them tolerate conditions in the supply chain,” he says. Small, frequent applications of plant growth regulators are often the best way to fine-tune crop growth according to conditions. Bonzi’s label, for example, allows a range of rates and spray intervals depending on the crop and on the effect you need.

The general advice for foliar-applied growth regulators is to treat well-established, well-watered and unstressed plants when the leaves are dry and light conditions are not too bright – using shade screens if necessary or spraying late afternoon or early evening to allow the active to be absorbed by the leaves overnight. “However, one of the advantages of Bonzi is that it’s absorbed quickly so most growers don’t have to go out of their way to ensure slow drying,” says Mr Need.

It’s also important to take into account what other treatments the crop has received, such as certain triazole fungicides which can retard growth as a side-effect.

“As with any crop spraying you need to apply enough for good even coverage, as too low a volume may give uneven results,” says Mr Surrage. “But avoiding run-off is also important as some plant growth regulators can be taken up by the roots, and persist in growing media or on capillary matting.

“Attention to spray volume is vital,” points out Mr Need. “Be aware that differences in volume can impact on the effect: a light mist of Bonzi would be enough for a ‘toning’ application, for instance, while a heavier spray would give a different result.”

Mr Surrage advises growers who want to apply growth regulators to a new crop, or to achieve a specific effect not tried before, to run a small-scale trial first. In the case of Bonzi, introduced about 30 years ago, there’s plenty of industry experience at hand, adds Mr Kirby. “And that means there’s a lot of confidence about how to get the best from it.”