Chemical plant growth regulators - active ingredients

Chemical plant growth regulators (PGRs) have differing modes of action by inhibiting natural plant growth hormones, an understanding of the active ingredients will help to get the most from the various products.

Chlormequat, daminozide, paclobutrazol, prohexadione-calcium and trinexapac-ethyl all act by inhibiting the biosynthesis of gibberellin (a natural plant growth hormone), although generally at different stages of its production resulting in slightly different effects within the plant.

Ethephon (2-chloroethylphosphonic acid) is an organophosphate, it penetrates plant tissue and is translocated around the plant, where it breaks down generating ethylene. Ethylene affects various plant growth processes including flowering and lateral branching in ornamentals. 

An understanding of the mode of action of the active ingredients is important to get the best out of each product in terms of plant response.

An understanding of chemical plant growth regulators is important to achieve the desired effect

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The active ingredients currently authorised within the UK for use as PGRs on ornamental crops under protection are included in the table below as an indcation of the options avaialble.

N.B. compiled May 2021 - always check on the latest approval status before using products.

Table 1. Plant growth regulators authorised for use in the UK

*Adjuvants: A suitable non-ionic surfactant should be used with chlormequat. 

** EAMU specifies only for use on crops grown in pots less than 10 cm. 

• The information in this table has been collated using information from the Health and Safety Executive (HSE) website (www.pesticides.gov.uk) and from suppliers’ labels and product technical information. Important - regular changes occur in the approval status of plant protection products, arising from changes in the legislation or for other reasons. For the most up to date information, please check the HSE website or with a professional supplier or BASIS-qualified consultant, as information could have changed since this table was produced 
• EAMU – Extension of Authorisation for Minor Use 
• Growers must hold a paper or electronic copy of an EAMU before using any product under the EAMU arrangements. Any use of a plant protection product with an EAMU is at grower’s own risk 

• Always follow approved label or EAMU recommendations, including rate of use, maximum number of applications per crop or year and where crop safety information is not available, test the product on a small number of plants to determine crop safety prior to widespread commercial use 

Application of chlormequat results in shorter internodes and thicker, darker green leaves. It is used to improve plant habit and increase branching early in the production phase, particularly during Pelargonium and geranium production. Use can also advance flowering (by up to 10 days) and marketing, depending upon the crop. Chlormequat is commonly used on geranium, Osteospermum, pansy and poinsettia. 

Chlormequat is absorbed via leaves and roots and so may be applied either as a foliar spray or drench. It is systemic and highly mobile within the plant and is passively absorbed by all plant tissues. However, as chlormequat is water soluble, uptake through the waxy cuticle of leaves can be relatively slow and only occurs when the foliage is wet. Use of a non-ionic wetting agent generally improves foliar uptake, enabling application rates to be reduced, thereby reducing the potential for leaf yellowing (chlorotic spotting/marginal leaf chlorosis). Activity is reduced by low temperature, and crops are less responsive below 10°C. 

Chlormequat is authorised for use on ornamentals as Stabilan 750 under permanent protection with full enclosure (EAMU 1416/17) (Table 2). Application should be carried out late in the day or when leaves are likely to stay wet for several hours. As chlormequat is only active in the plant for a short period, a second application may be necessary after 10–14 days. 

Table 2. Commonly used rates of chlormequat (as a foliar spray) 

* Adjuvants: A suitable non-ionic surfactant should be used with chlormequat. 

** Always obtain a copy of the EAMU for reference. 

See footnotes in Table 1. 

Three products containing daminozide are authorised for use on protected ornamentals within the UK: B-Nine SG, Dazide Enhance and Stature. Daminozide application reduces internode elongation, producing more compact plants with stronger stems, darker green foliage (due to increased chlorophyll concentration), better developed root systems and reduced leaf area in some plant species. Daminozide also increases lateral branching, resulting in increased flower production; application can delay flowering for up to five days, although this does differ between species and varieties. 

As with chlormequat, daminozide is water soluble, therefore uptake through the waxy leaf cuticle is slow and products should be applied during periods of high humidity or at the end of the day, onto dry foliage. However, it is highly mobile and once it has been absorbed it quickly moves throughout the plant. 

AHDB-funded work (project HNS 103b) demonstrated that daminozide is not only absorbed by leaves as previously thought, but also via roots. Drenches of daminozide reduced the plant growth of some plants (such as Achillea and Salvia) by almost 50%. Activity is less at high temperatures or where growth is vigorous. Daminozide is generally used to fine tune growth as it provides only moderate growth control and the effect is less persistent than chlormequat or paclobutrazol (Table 3). Products are also relatively expensive. 

Table 3. Commonly used rates of daminozide (as a foliar spray) 

* Always check individual product labels as rates of individual products and application numbers can vary. 

See footnotes in Table 1. 

Recent AHDB-funded work (project HNS 103b) has demonstrated that daminozide is absorbed by both leaves and roots

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Cerone is the only product authorised for use in the UK containing ethephon alone via Extension of Authorisation for Minor Use (EAMUs) 2366/12 and 3091/19. It was previously marketed as Ethrel C, which was traditionally used to increase branching, stimulate basal bud production and abort flower buds, thereby increasing cutting yield on stock plants (Table 4). Cerone may be a suitable alternative to chlormequat early in the production cycle where plants do not respond to chlormequat. Cerone is most effective at temperatures over 10°C. 

Table 4. Commonly used rates of ethephon (as a foliar spray) 

* Always obtain a copy of the EAMU for reference. 

See footnotes in Table 1. 

Mepiquat chloride is related to chlormequat. It is a gibberellin inhibitor that reduces growth by slowing cell elongation, enhancing cell division and strengthening cell walls: improving plant habit and increasing branching. It is only formulated in combination with a range of other active ingredients, for example in Terpal it is formulated with ethephon (Table 5); and in Canopy (Table 6) with prohexadione-calcium. 

Terpal is authorised for use in ornamental plant production on container-grown crops under EAMU 0151/18. It contains a lower rate of ethephon relative to Cerone. 

Canopy is authorised for use on protected ornamental crops under EAMU 4484/19. Canopy contains a lower rate of prohexadione-calcium relative to Regalis Plus. 

Table 5. Commonly used rates of ethephon + mepiquat (as chloride) 

* AHDB projects PO 019a, 019b and HNS 103b (refer to the reports for more detailed information on cultivars and rates used). 

** Always obtain a copy of the EAMU for reference. 

See footnotes in Table 1. 

Table 6. Commonly used rates of prohexadione calcium + mepiquat (as a foliar spray) 

* Always obtain a copy of the EAMU for reference. 

** Based on applications in 1,000 litres of water/ha. 

See footnotes in Table 1. 

Bonzi and Pirouette are the only products containing paclobutrazol authorised for use on ornamentals within the UK. Application results in reduced cell division and elongation, reduced leaf area and improved leaf colour (and bract colour in some red poinsettia varieties) but has little effect on leaf production, branching or root growth. Paclobutrazol is upwardly systemic within the plant. It is rapidly taken up, primarily through stems (but also leaves and roots) but is not readily transported within the plant, therefore good coverage of stems and leaves is necessary for effective growth control. It is effective once the spray dries and will not wash off the foliage after approximately 30 minutes. 

Applications to maturing plants can reduce both flower size and flower stem length impacting on appearance. However, this product is particularly useful for holding plants back which are close to the point of marketing (Table 7). 

Table 7. Commonly used rates of paclobutrazol (as a foliar spray) 

* Always check individual product labels. 

See footnotes in Table 1. 

Prohexadione, marketed as Regalis Plus, is authorised for use on protected ornamental crops under EAMU 2153/19. Although in the same chemical group as daminozide and trinexpac-ethyl, it is more active. Limited trials have indicated that Regalis Plus produces compact growth with some bedding plant species. However, it also affects anthocyanin formation, which can result in flower petal bleach in some plant species. Early application in the production phase may limit this negative side effect and is likely to achieve the best growth regulation (see Table 8). As an alternative, the lower rate of prohexadione-calcium in Canopy may also reduce any potential of petal bleach. 

Table 8. Commonly used rates of prohexadione (as a foliar spray) 

* Always obtain a copy of the EAMU for reference. 

** Based on applications in 1,000 litres of water/ha. 

See footnotes in Table 1. 

Trinexapac-ethyl is marketed as Moddus, which posseses an EAMU (3062/10) for use in ornamental plant production, and Primo Maxx II, which has approval for use in ornamental plant production (EAMU 0621/18). (See Table 9). Chemically related to prohexadione and daminozide, it can cause more severe petal bleach than Regalis Plus in some species. 

Table 9. Commonly used rates of trinexapac-ethyl (as a foliar spray) 

* Always obtain a copy of the relevant EAMU for reference. 

** Based on applications in 1,000 litres of water/ha. 

See footnotes in Table 1. 

For more information on trials undertaken on PGRs on bedding plants, follow the links below:

Authors – Chloe Whiteside, Jill England and David Talbot. ADAS Horticulture.

Original authors – Jill England and David Talbot. ADAS Horticulture.

Webpage content correct as of May 2021.