Skip to main content

Main menu

  • Home
  • Content
    • Ahead of Print
    • Current Issue
    • Special Issues
    • All Issues
  • Contribute
    • Submit to AUF
    • Author Guidelines
    • Reviewer Guidelines
  • About
    • Overview
    • Editorial Board
    • Journal Metrics
    • International Society of Arboriculture
  • More
    • Contact
    • Feedback
  • Alerts

User menu

  • Log in

Search

  • Advanced search
Arboriculture & Urban Forestry
  • Log in
Arboriculture & Urban Forestry

Advanced Search

  • Home
  • Content
    • Ahead of Print
    • Current Issue
    • Special Issues
    • All Issues
  • Contribute
    • Submit to AUF
    • Author Guidelines
    • Reviewer Guidelines
  • About
    • Overview
    • Editorial Board
    • Journal Metrics
    • International Society of Arboriculture
  • More
    • Contact
    • Feedback
  • Alerts
  • Facebook
  • Twitter
  • YouTube
  • LinkedIn
ArticleArticles

Regulation of Tree Growth and Development with Triazole Compounds

Tim D. Davis
Arboriculture & Urban Forestry (AUF) June 1991, 17 (6) 167-170; DOI: https://doi.org/10.48044/jauf.1991.043
Tim D. Davis
Associate Professor, Texas A&M University Research and Extension Center, Texas Agricultural Experiment Station, 17360 Coit Rd.,Dallas, TX 75252-6599
  • Find this author on Google Scholar
  • Search for this author on this site
  • Article
  • Figures & Data
  • Info & Metrics
  • References
  • PDF
Loading

During the past ten years, much research has been conducted on the response of plants to triazole-type plant growth regulators. This research has included work with a variety of shade and fruit trees. The objective of this brief article is to summarize results of triazole growth regulator research conducted with trees during the past several years. Rather than presenting a detailed catalogue of past research, major findings are highlighted.

Much of what is known about the response of trees to triazoles is based on research with fruit trees. There are at least two reasons for this. First, fruit crops have a clearly demonstrable dollar value that provides considerable economic incentive to agrichemical companies for development of growth regulators. Second, fruit trees for research purposes are available in uniform orchard blocks which facilitates experimental design and reduces variability. Unfortunately, it is more difficult to find uniform blocks of shade trees for experimental purposes. For these reasons, some extrapolation of fruit tree data to shade trees is needed.

Before discussing tree responses to triazoles, a brief description of the general properties of these compounds is in order. Triazoles such as paclobutrazol (trade name = Clipper) and uniconazole (proposed trade name = Prunit) work primarily by inhibiting gibberellin biosynthesis and are among the most active and persistent of all plant growth retardants. Only relatively small dosages are needed to control growth for an extended period of time. These compounds are xylem-mobile; little transport seems to occur in the phloem. Once inside a tree, triazoles are broken down quite slowly. Although the carriers that are used to deliver triazoles to trees (e.g. alcohol) may cause some phytotoxicity, the triazoles themselves seldom cause damage. In many cases, leaves on treated plants are darker green than controls. For these reasons, triazoles have considerable potential as tree growth regulators.

Shoot Growth Inhibition

Listen

There is ample evidence to show that triazoles are potent inhibitors of shoot growth in a wide range of plant species (5). The challenge with trees is to deliver the growth retardant to the site of active growth (i.e. the meristems) in a timely, uniform manner. Because trees are large in stature and have a relatively complex vascular system, delivery is not as simple as with other plants. If applied too late in the growing season, the growth retardant effects may not be evident until the following growing season. The exact time that is “too late” has not been critically determined for any species and will probably depend somewhat upon tree size, method of application, location, and weather.

Once the triazoles begin inhibiting tree growth, their effect may persist anywhere from one to several years (Table 1). This depends upon the dosage administered. Unfortunately there is not much information available regarding the long-term effects of a wide range of triazole dosages on shoot growth. Few of the many published studies on the effects of triazoles on tree growth have been carried out long enough to determine when growth inhibition subsided. This type of information is needed in formulating dosage recommendations.

View this table:
  • View inline
  • View popup
Table 1

Duration of growth retardation in various paclobutrazolÂtreated trees.

A less well-known triazole-induced phenomenon in trees is that of increased shoot growth following subsidence of growth inhibition. This has been observed in paclobutrazol-treated peach (1, 2) and apple (21) trees. Although the basis for this accelerated growth is unclear, it may be related to the accumulation of carbohydrates and minerals during the period of shoot growth inhibition which thereafter fuels rapid growth when the retardant dissipates. The consequences of this type of escape growth to the long-term health and value of the tree is not yet clear. Nevertheless, if this phenomenon is widespread, arborists will need to carefully observe treated trees and re-apply the growth retardant before such a growth flush occurs.

Several delivery methods have been devised for trees. At present, trunk injection is generally the most favored method. Hole angle, depth, and distribution are important variables in determining the efficacy of trunk injection (1 5). A hole angle of 30-45° is generally recommended; greater angles increase the chance of missing the xylem and lower angles may result in high pressure which can damage the tree. Judging the optimum hole depth and distribution is challenging. The holes should be deep enough to penetrate the xylem tissue and distributed so that the growth regulator can be evenly delivered to the canopy. The recommended hole spacing depends upon the species but is generally between 4 and 8 inches (7, 24).

A disadvantage of injection is that it requires that a hole be drilled in the trunk. Although the extent to which this damages the tree is a point of controversy, public perception of the hole is generally negative. One solution to this problem is to seal the hole with a vinyl plug (24). Bark splitting may also be reduced by avoiding injection during the dormant season. Another drawback to injection is that the time required to inject the growth regulator solution varies dramatically among species and time of year. For example, Watson (24) reported that only about 4 minutes were required to inject a black cherry tree but about 28 minutes were needed for white ash. Furthermore, injection times for a vairety of trees ranged from an average of 4 minutes in August and September to 45 minutes in February.

Soil injection and root collar drenches have been used to effectively apply growth retardants to trees (20) but consequent soil residues may be an ecological concern. In theory, bark paints may be useful for applying growth retardants, but thus far success has been limited. Solvents tried have either not been effective in evenly distributing the growth retardant or have caused considerable damage to the bark. Foliar sprays are inefficient and environmentally unacceptable for shade trees, particularly in populated areas.

Even after several years of research, it is still difficult to accurately predict an appropriate triazole dosage for any given tree. Dosage formulas based upon trunk diameter are generally considered the most useful but considerable species and environmental variability still occur. Hence the development of better methods for estimating optimal dosages is needed. A better knowledge of the underlying factors that contribute to variability in tree response to triazole treatment would also be helpful.

In addition to reducing shoot elongation on trees, triazoles sometimes alter shoot orientation (4, 9). In particular, pear shoots have exhibited a horizontal or even downward direction of growth. Although this has not been widely observed in other species, it may result in a “weeping” appearance. Triazoles have not been found to influence leaf coloration or retention in the fall.

Other Effects

Listen

In addition to their well-known effects on shoot growth, triazoles have been found to promote flowering in a variety of woody species including some fruit crops (10, 13, 21, 22, 25) and ornamentals (11, 12, 23, 26) (Table 2). This response does no always occur (6, 14) and is probably strongly influenced by dosage and timing of application. In addition to increasing the number of flowers, triazoles have sometimes advanced flowering by several days. This could increase the chances for spring frost damage and suggests that triazoles may influence tree dormancy.

View this table:
  • View inline
  • View popup
Table 2

Woody species in which flowering has been promoted by paclobutrazol treatment

An example, research with cherry indeed has demonstrated altered dormancy characteristics in paclobutrazol-treated trees (22). The number of growing degree (°C) hours (18) required to reach full bloom was reduced by 700-900 in paclobutrazol-treated trees. Rest intensity (based upon the amount of gibberellic acid needed to induce vegetative budbreak under favorable environmental conditions) was decreased by paclobutrazol. This suggests that paclobutrazol-treated trees were in a less-dormant state than the non-treated controls. It is not clear if this is a widespread phenomenon in triazole-treated trees and work with shade trees is needed. Nevertheless, this work demonstrates that triazoles may do more than simply reduce shoot elongation in trees.

In the previously-mentioned study with cherry trees (22), the altered dormancy characteristics in the paclobutrazol-treated trees were accompanied by reduced mid-winter cold hardiness of the flower buds. The treated trees had mid-winter T50 (temperature required to kill 50% of the buds) values that were about 2°C (ca. 4°F) higher than non-treated controls. A similar observation has been made with several other Prunus species that were treated with paclobutrazol (16). These findings are in sharp contrast to research with a variety of herbaceous species where paclobutrazol increased cold hardiness (reviewed in 5). Clearly more research is needed to fully understand the basis of triazole-induced alterations in cold hardiness. If reduced cold hardiness is a common tree response to triazoles, damage to flower buds could occur on treated trees that have marginal hardiness for a given locale. There seems to be little evidence that triazoles alter cold hardiness of vegetative buds.

Preliminary observations by two independent research groups suggest that triazole-treated pear trees have lower insect population densities than their untreated counterparts (3, 1 7).

To my knowledge no such observations have yet been made with shade trees and it is unclear why triazole compounds should reduce insect populations. It is interesting to note, however, that triazoles have been found to increase hydrocyanic acid potential in sorghum seedlings (19). Hydrocyanic acid potential has been linked to plant defense against predators. Thus triazoles may influence insect resistance by altering host plant defense mechanisms. Much more work is needed to substantiate this possibility, however.

Acknowledgment

Listen

The author expresses appreciation to the International Society of Arboriculture for providing funding which supported a portion of the work upon which this article is based.

Contribution No. 25902 from the Texas Agricultural Experiment Station. Mention of a proprietary product or vendor does not constitute an endorsement.

  • © 1991, International Society of Arboriculture. All rights reserved.

Literature Cited

Listen
  1. 1.
    1. Blanco, A.
    1987. Fruit thinning of peach trees (Prunus persica (L.) Batsch.): the effect of paclobutrazol on fruit crop and shoot growth. J. Hortic. Sci. 62:147–155.
    OpenUrl
  2. 2.
    1. Blanco, A.
    1988. Control of shoot growth of peach and nectarine trees with paclobutrazol. J. Hortic. Sci. 63:201–207.
    OpenUrl
  3. 3.
    1. Campbell, C.A.M.,
    2. M.A. Easterbrook, and
    3. A.J. Fisher
    . 1989. Effect of plant growth regulators paclobutrazol and chlormequat chloride on pear psyllid (Cacopsylla pyricola [Foerster]) and pear rust mite. J. Hortic. Sci. 64:561–564.
    OpenUrl
  4. 4.
    1. Curry, E.A.,
    2. E.A. Stahly, and
    3. M.W. Williams
    . 1984. Apparent change in gravimorphism on ‘Anjou’ pear shoots on trees treated with trlazole and pyrimidine-type growth retardants. Proc. Plant Growth Reg. Soc. Amer. 11:62. (Astract).
    OpenUrl
  5. 5.
    1. J. Janick
    1. Davis, T.D.,
    2. G.L. Steffens, and
    3. N. Sankhla
    . 1988. Triazole plant growth regulators, pp. 65–103. In J. Janick (ed.) Horticultural Reviews, Vol. 10. Timber Press, Portland, OR.
    OpenUrl
  6. 6.
    1. Elfving, D.C. and
    2. J.T.A. Proctor
    . 1986. Long-term effects of paclobutrazol (Cultar) on apple tree shoot growth, cropping, and fruit-leaf relations. Acta Hortic. 179:473–480.
    OpenUrl
  7. 7.
    1. Fuchs, A.D.
    1988. A comparison of three different trunk injection systems for use with plant growth regulators. J. Arboric. 14:94–98.
    OpenUrl
  8. 8.
    1. Hield, H.
    1983. Early responses with EL-500 and paclobutrazol trunk banding of ornamental tress. Proc. Plant Growth Reg. Soc. Amer. 10:182–190.
    OpenUrl
  9. 9.
    1. Huang, H.,
    2. S.Y. Cao,
    3. X.S. Qiao, and
    4. R. Lu
    . 1989. The effect of paclobutrazol on growth of some asian pears. Scientia Hortic. 38:43–47.
    OpenUrl
  10. 10.
    1. Iwahori, S. and
    2. S. Tominaga
    . 1986. Increase in first-flush flowering of ‘Meiwa’ kumquat, Fortunella crassifolia Swingle, trees by paclobutrazol. Scientia Hortic. 28:347–353.
    OpenUrl
  11. 11.
    1. Keever, G.J.,
    2. W.J. Foster, and
    3. J.C. Stephenson
    . 1990. Paclobutrazol inhibits growth of woody landscape plants. J. Environ. Hort. 8:41–47.
    OpenUrl
  12. 12.
    1. Kristensen, L.N. and
    2. E. Adr?ansen
    . 1988. Growth and flowering in Hebe x franciscana ‘variegata’ treated with plant growth regulators. Scientia Hortic. 36:139–149.
    OpenUrl
  13. 13.
    1. Lever, B.G.
    1986. Cultar’—a technical overview. Acta Hortic. 179:459–466.
    OpenUrl
  14. 14.
    1. Looney, N.E. and
    2. J.E. McKellar
    . 1987. Effect of foliar-and soil surface-applied paclobutrazol on vegetative growth and fruit quality of sweet cherries. J. Am. Soc. Hort. Sci. 112:71–76.
    OpenUrl
  15. 15.
    1. Orr, J.W.,
    2. S. Leonard, and
    3. J. Lentz
    . 1988. Field observations of tree injection. J. Arboric. 14:269–273.
    OpenUrl
  16. 16.
    1. Proebsting, E.L. and
    2. H.H. Mills
    . 1985. Cold resistance in peach, apricot, and cherry as influenced by soil-applied paclobutrazol. HortScience 20:88–90.
    OpenUrl
  17. 17.
    1. Raese, J.T. and
    2. E.C. Burts
    . 1983. Increased yield and suppression of shoot growth and mite populations of d’Anjou pear trees with nitrogen and paclobutrazol. HortScience 18:212–214.
    OpenUrl
  18. 18.
    1. Richardson, E.A.,
    2. S.D. Seeley, and
    3. D.R. Walker
    . 1974. A model for estimating the completion of rest for ‘Redhaven’ and ‘Elberta’ peach trees. HortScience 9:331–332.
    OpenUrl
  19. 19.
    1. Sankhla, N.,
    2. T.D. Davis,
    3. A. Upadhyaya,
    4. R.H. Walser, and
    5. R.G. Cates
    . 1990. Triazole plant growth regulators increase hydrocyanic acid potential in sorghum seedlings. J. Agron. & Crop Sci. (in press).
  20. 20.
    1. Sterrett, J.P. and
    2. T.J. Tworkoski
    . 1987. Response of shade trees to root collar drenches of inhibitors flur-primidol and paclobutrazol. J. Plant Growth Reg∪l. 5:163–167.
    OpenUrl
  21. 21.
    1. Tromp, J.
    1987. Growth and flower-bud formation in apple as affected by palcobutrazol, daminozide, and tree orientation in combination with various gibberellins. J. Hortic. Sci. 62:433–440.
    OpenUrl
  22. 22.
    1. Walser, R.H. and
    2. T.D. Davis
    . 1989. Growth, reproductive development and dormancy characteristics of paclobutrazol-treated tart cherry trees. J. Hortic. Sci. 64:435–441.
    OpenUrl
  23. 23.
    1. Wang, Y.T. and
    2. L.L. Gregg
    . 1989. Uniconazole affects vegetative growth, flowering, and stem anatomy of hibiscus. J. Am. Soc. Hortic. Sci. 114:927–932.
    OpenUrl
  24. 24.
    1. Watson, M.R.
    1987. Use of tree growth regulators at Potomac Edison. J. Arboric. 13:65–69.
    OpenUrl
  25. 25.
    1. Webster, A.D.,
    2. J.D. Quinlan, and
    3. P.J. Richardson
    . 1986. The influence of paclobutrazol on the growth and cropping of sweet cherry cultivars. I. The effect of annual soil treatments on the growth and cropping of cv. Early Rivers. J. Hortic. Sci. 61:471–478.
    OpenUrl
  26. 26.
    1. Wilkinson, R.l. and
    2. D. Richards
    . 1988. Influence of paclobutrazol on the growth and flowering of Camellia x Williamsii. HortScience 23:359–360.
    OpenUrl
  27. 27.
    1. Williams, M.W.
    Use of bioregulators to control vegetative growth of fruit trees and improving fruiting efficiency. Acta Hortic. 146:97–104.
PreviousNext
Back to top

In this issue

Arboriculture & Urban Forestry (AUF)
Vol. 17, Issue 6
June 1991
  • Table of Contents
  • Index by author
Print
Download PDF
Email Article

Thank you for your interest in spreading the word on Arboriculture & Urban Forestry.

NOTE: We only request your email address so that the person you are recommending the page to knows that you wanted them to see it, and that it is not junk mail. We do not capture any email address.

Enter multiple addresses on separate lines or separate them with commas.
Regulation of Tree Growth and Development with Triazole Compounds
(Your Name) has sent you a message from Arboriculture & Urban Forestry
(Your Name) thought you would like to see the Arboriculture & Urban Forestry web site.
Citation Tools
Regulation of Tree Growth and Development with Triazole Compounds
Tim D. Davis
Arboriculture & Urban Forestry (AUF) Jun 1991, 17 (6) 167-170; DOI: 10.48044/jauf.1991.043

Citation Manager Formats

  • BibTeX
  • Bookends
  • EasyBib
  • EndNote (tagged)
  • EndNote 8 (xml)
  • Medlars
  • Mendeley
  • Papers
  • RefWorks Tagged
  • Ref Manager
  • RIS
  • Zotero
Share
Regulation of Tree Growth and Development with Triazole Compounds
Tim D. Davis
Arboriculture & Urban Forestry (AUF) Jun 1991, 17 (6) 167-170; DOI: 10.48044/jauf.1991.043
del.icio.us logo Twitter logo Facebook logo Mendeley logo
  • Tweet Widget
  • Facebook Like
  • Google Plus One
Bookmark this article

Jump to section

  • Article
    • Shoot Growth Inhibition
    • Other Effects
    • Acknowledgment
    • Literature Cited
  • Figures & Data
  • Info & Metrics
  • References
  • PDF

Related Articles

  • No related articles found.
  • Google Scholar

Cited By...

  • No citing articles found.
  • Google Scholar

More in this TOC Section

  • Using the CSR Theory when Selecting Woody Plants for Urban Forests: Evaluation of 342 Trees and Shrubs
  • Right Appraisal for the Right Purpose: Comparing Techniques for Appraising Heritage Trees in Australia and Canada
  • Urban Tree Mortality: The Purposes and Methods for (Secretly) Killing Trees Suggested in Online How-To Videos and Their Diagnoses
Show more Articles

Similar Articles

© 2025 International Society of Arboriculture

Powered by HighWire