Chapter 18

Plant Growth Regulators for Greenhouse Crops

Plant Growth Regulators

Most plant growth regulators (PGRs) used in the greenhouse or nursery are used to regulate shoot growth (i.e., stretch) of containerized crops. These PGRs are referred to as “growth retardants.” These PGRs control plant height by inhibiting the production of gibberellins, the primary plant hormones responsible for cell elongation. Therefore, these growth-retardant effects are primarily seen in stem, petiole, and flower stalk tissues. Growth retardants do not reduce plant size, just the plant’s growth rate. Lesser effects are seen in reductions of leaf expansion, resulting in thicker leaves with a darker green color. Another group of PGRs used in floricultural crops are those that enhance branching. These PGRs are frequently called “chemical pinchers” because they generally inhibit the growth of the terminal shoots or enhance the growth of lateral buds, thereby increasing the development of lateral branches. They can be used to replace mechanical pinching of many crops. Often, this increased branching will also reduce the overall height of the plant. Plant growth regulators can be used to enhance flowering and are often used for flower removal.

Ancymidol

Ancymidol is a more active compound than daminozide or chlormequat chloride. Ancymidol inhibits gibberellin synthesis and activity, resulting in shortened stem internodes and a more compact plant. Secondary effects that result from ancymidol application include lower plant water requirements, better plant tone, and darker green leaf color. Ancymidol readily moves through the plant and is usually used on crops where other chemicals are not effective (most notably in bulb crops) or on very high-value crops (i.e. plugs).

Benzyladenine BA

Benzyladenine BA is used to promote branching and increase flower set in certain ornamental crops. Although the primary objective with BA is to increase branching, it has resulted in growth reduction in some crops. Plant response to benzyladenine is strongly influenced by cultural and environmental variables.

Chlormequat Chloride

Chlormequat chloride is another very popular PGR. As with daminozide, treated plants are more compact with shorter internodes, stronger stems, and greener leaves. Unlike daminozide, chlormequat chloride inhibits gibberellin production early in the process. Chlormequat chloride has its greatest effect on final plant height when applied at the beginning of rapid stem elongation and will have little or no effect if applied when shoots are not elongating or at the end of the elongation phase. Chlormequat chloride has activity when applied to both the leaves and the roots, but it is primarily applied as a foliar spray due to the higher concentrations required for adequate control when applied as a drench.

Daminozide

Daminozide is one of the most common PGRs used in the floriculture industry. This material is applied only as a foliar spray because it’s rapidly broken down when applied to the substrate. It’s highly mobile in the plant and will rapidly move from the point of application to all parts of the plant. It reduces ornamental plant size by reducing internode length. This internode shortening is the result smaller, more compact cells. In addition to compact plants the leaves are also a deeper green color.

Dikegulac Sodium

Dikegulac sodium temporarily stops shoot elongation, thereby promoting lateral branching. It’s thus a pinching agent for ornamental crops including azaleas, bougainvillea, clerodendron, fuchsia, grape ivy, geranium, lantana, lipstick vine, verbena and some of the herbaceous perennials.

Ethephon

Ethephon is a compound that breaks down in plant tissue after application to release ethylene, a natural plant hormone. As with ethylene, its effects can vary depending on the species and the stage of growth at time of application. Ethephon should be applied to actively growing plants prior to flower development. If flowers are present at the time of application, they are likely to abort.

Flurprimidol

Flurprimidol reduces internodal elongation through the inhibition of gibberellic acid biosynthesis. It has been shown to increase plant quality, without growth reduction. These characteristics include enhanced leaf and bloom color, darker foliage, increase leaf thickness, stronger stems, and decreased water use.

Methyl Esters

Methyl esters are labeled for chemical pinching of actively growing azalea, cotoneaster, juniper, ligustrum, Rhamnus, and Taxus. Ensure good coverage of growing points to physically burn soft tissue. Do not apply to the same plants more than once, and do not apply to herbaceous plant material.

Paclobutrazol

Paclobutrazol has a broad label for ornamentals that includes use on greenhouse or outdoor-grown, containerized crops. All of the paclobutrazol products are labeled for application through the irrigation system, including ebb/flow or flooded floor systems.

Uniconazole

This chemical is commonly used on perennials because it’s highly effective on a very broad range of plant species. Uniconazole is applied as a foliar spray or as a substrate drench. As a drench, uniconazole is applied at rates 50 percent lower than those recommended for paclobutrazol.

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