Gibberellic acid can improve cut flower production

A good cut flower must be productive, have long, strong and easy-to-harvest stems, have manageable insect and disease problems, flower in a relatively short period of time and have a long postharvest. This is quite a long list of requirements and, unfortunately, few cut flowers have them all.

For many cut flowers, the problems are straightforward: their stems aren’t long enough, they have too long of a production period or they require time-consuming procedures to initiate flowering. Fortunately, growers may have an option to solving these problems other than discarding the species.

For the last four years, researchers at North Carolina State University studied the use of gibberellic acid (ProGibb) on 12 field- and greenhouse-grown cut flower species. Here are the results of the NCSU research.

Crops tested and the results

Stem length increased

Of the 12 species tested, gibberellic acid increased the stem length of seven species.

Achillea. While Achillea filipendulina is generally tall enough, many other species and hybrids are often too short for commercial cuts. Although most species flower the first year, the percentage of plants that flower can be low.

Results: Gibberellic acid increased stem length of spring-planted ‘Summer Pastels’ with 800 parts per million providing optimum results. Higher rates did not increase stem length more. Maximum results were obtained 48 days after application.

Celosia. Crested varieties are the most popular types of celosia, but often tend to be too short. Celosia is a short-day plant. If transplants are started too early (late winter to early spring) under natural short days, plants may initiate flowers too soon and stems may be too short. Celosia seedlings tend to initiate flowers rapidly and become stunted if held for too long in a flat.

Results: Gibberellic acid increased stem length of celosia ‘Toreador Red’ with 1,600 ppm providing optimum results. Maximum results were obtained 26-33 days after application.

Dianthus. Many species in this genus are useful for cut production. ‘Bouquet Purple’ is exceptionally productive and flowers rapidly the first year from seed, but the stems are often too short for commercial sales. ‘Amazon Neon Duo’ produces large flower heads of intense red and purple flowers the first year from seed and has longer stems than ‘Bouquet Purple.’ However, when produced outdoors, even ‘Amazon Neon Duo’ is often too short.

Results: Gibberellic acid increased stem length of spring-planted dianthus ‘Bouquet Purple’ with 400 ppm providing optimum results. Higher rates did not increase stem length more. Maximum results were obtained 19 to 26 days after application.

The stem length of spring-planted dianthus ‘Amazon Neon Duo’ was increased by 2.8 inches with two applications of 200 ppm gibberellic acid or by 3.6 inches with one application of 400 ppm. One or two applications of 200 to 400 ppm gibberellic acid can be made to field-grown ‘Amazon Neon Duo’ starting when plants are established. Gibberellic acid had no effect on flower appearance or harvest date other than the foliage was lighter green for seven to 10 days after treatment.

Digitalis. Most cultivars are biennial, requiring two growing seasons to produce tall-enough stems. Camelot, a new series, flowers rapidly the first year from seed but stems are often too short to be useful in the first year.

Results: While gibberellic acid applications of 800 ppm increased spring-planted stem length, noticeable flower malformations rendered many of the flowers unusable. Other cultivars should be tested to determine if they are also susceptible to flower malformations.

Larkspur. This well-known cut flower is traditionally planted in fall in mild climates and in early spring in cold climates. While the best cut stem quality is obtained with fall planting and overwintering, harsh winter weather can result in plant loss. Spring plantings tend to produce shorter stems.

Results: The stem length of spring-planted ‘Imperial Giants Pink Perfection’ was increased by 2.6 inches with one application of 200 ppm gibberellic acid, by 3.3 inches with two applications of 200 ppm or by 6.8 inches with one application of 400 ppm. One or two applications of 200 to 400 ppm gibberellic acid can be made starting when plants are established.

Ornamental pepper. While ‘Nippon Taka’ is one of the taller cut cultivars, even it is only minimally tall enough (24 inches) for commercial cut production. A long crop time (usually all summer) is needed to produce sufficiently tall stems.

Results: Gibberellic acid increased stem length of ‘Nippon Taka’ with 1,600 ppm providing optimum results.

Zinnia. This field-grown cut flower often requires a lot of time for stems to become long enough to harvest. Once plants reach production size, stem length is usually not a problem.

Results: Gibberellic acid increased stem length of ‘Benary’s Giant Scarlet’ with 800 ppm providing optimum results. Maximum results were obtained 28 to 37 days after application.

More frequent applications should be tested, making applications every four to five weeks apart. Application of gibberellic acid, even at the lowest rates tested, reduced flower diameter. Assuming the same effect occurs on other cultivars, producers should only apply gibberellic acid to cultivars with large flowers. Gibberellic acid application resulted in lighter-green foliage than the control plants. Gibberellic acid slightly decreased days to harvest up to three days, but this is not likely to be commercially useful.

No effect on stem length

Of the 12 species tested, gibberellic acid had no effect on stem length on five species.

Ageratum. ‘High Tide Blue’ and ‘High Tide White’ are exceptionally productive, but do not produce long stems until very late in the season.

Results: Gibberellic acid applied at 400, 800 or 1,600 ppm did not have an effect on stem length of ‘High Tide Blue’ and ‘High Tide White.’

Delphinium. This well-established cut flower is traditionally planted in fall in mild climates and in early spring in cold climates. While the best cut stem quality is obtained with fall planting and overwintering, harsh winter weather can result in plant loss. Spring plantings tend to have shorter stems.

Results: One or two applications of 0, 50, 100, 200 or 400 ppm gibberellic acid did not influence stem length, harvest date or appearance of spring-planted ‘Aurora Deep Purple’ delphinium.

Lobelia. Most cultivars used for commercial cut flower production are seed propagated and vary in time to flower after sowing. Some plants produce stems that are quite long and other plants within the crop produce short stems.

Results: Gibberellic acid did not have an effect on stem length of lobelia Compliment Mix. It also tended to cause flower abnormalities, especially at higher rates.

Ornamental kale. For many growers it is a struggle to get enough height on ornamental kale, especially in outdoor production. It is also is fairly slow growing.

Results: No differences in height, color, foliage diameter and stem quality were noted when 10, 20, 40, 80 or 100 ppm gibberellic acid were applied three times to ‘Sunset’ plants when they were 12, 16 and 20 inches tall.

Stock. This well-known cut flower is traditionally planted in fall in mild climates and in early spring in cold climates. It is also commonly grown most of the year outdoors in California. While the best cut stem quality is obtained with fall planting and overwintering, harsh winter weather can result in plant loss. However, spring plantings tend to have shorter stems.

Results: One or two applications of 0, 50, 100, 200 or 400 ppm gibberellic acid to ‘Column Lilac Lavender’ did not influence stem length, harvest date or appearance of spring-planted stock.

Effect of gibberellic acid on stem length*

Gibberellic acid rate (ppm)	No. of applications	Dianthus ‘Bouquet Purple’ (inches)	Larkspur ‘Imperial Giants Pink Perfection’ (inches)
0	1	18.6 c	20.6 c
50	1	19.2 bc	19.9 c
50	2	19.4 b	21.2 bc
100	1	19.5 b	21.5 bc
100	2	19.5 b	20.2 c
200	1	19.2 bc	23.2 b
200	2	21.4 a	23.9 b
400	1	22.2 a	27.4 a

* Both species were planted in spring and field grown.

Means within a column followed by the same letter are not significantly different based on LSD (P<0.05).

An overview of the findings

Of the 12 species tested, gibberellic acid increased the stem length of seven species but did not have an effect on five. These species may be insensitive to gibberellic acid, or it may be effective on those species but did not show in our results because the concentrations and/or application number were too low.

For most species, 400 ppm or higher rates of gibberellic acid increased stem length. In most cases, only one application was tested and maximum results occurred about three to six weeks after application.

As with any plant growth regulator, be aware that undesirable side effects may occur, including weak stems, flower malformation and lack of uniformity. Always test a few plants first and follow label instructions.

- John Dole and Ingram McCall 

John Dole is professor and Ingram McCall is agricultural research technician, North Carolina State University, (919) 515-3537; john_dole@ncsu.edu.

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The authors thank Valent U.SA. Corp. for funding research; Benary Seed, American Takii, Goldsmith Seed, PanAmerican Seed, and Sakata Seed for providing seed, and Diane Mays, Lane Greer, Beth Harden, Aliya Donnell, Erin Possiel, Tina Krug, Leslie Tichner and Lee Davis for assisting in harvesting.