Controlling aggressive bedding plants with PGR plug dips

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

Spring bedding plant production is well underway and many crops are being planted now for sales in Michigan later this spring. Controlling stem extension of aggressive annuals continues to be a struggle for some growers, especially for plants grown in mixed containers. In some situations, growers cut back aggressive species that have outgrown their container before the shipping date. Although this strategy is effective, it can be both labor-intensive and costly.

One height control technique that has been used successfully by an increasing number of growers is a plant growth retardant (PGR) “plug dip” or “liner dip.” The concept is simple: the plug tray is soaked in a PGR solution before transplant and the chemical is absorbed by the media. This strategy allows for many plants to be treated at once and can offer several weeks or more of height control depending on the rate, volume and growing environment. PGR plug dips are particularly useful when producing mixed containers that contain species with different growth rates and PGR responses. In mixed containers, aggressive species grow faster than less vigorous ones, thus dominating the container quickly (Figure 1). A plug dip on aggressive plants before transplant can allow the less vigorous plants time to develop in the mixed container and improve crop quality.

Figure1
Figure 1. Example of a mixed container planted with
an aggressive species (sweet potato vine) and a less
aggressive species (geranium). The sweet potato vine
is out-growing the geranium in the center of the container.
A PGR plug dip to the sweet potato vine before transplant
would have suppressed its vigor, allowing the geranium
to become more established.

Over the past several years, university researchers have performed experiments to determine appropriate application rates and to evaluate factors that may influence PGR plug dip effectiveness. Below are three key factors that can influence the response of a PGR plug dip. Controlling these factors can improve success and reduce the possibility of obtaining inconsistent responses.

1. Application rate

The application rate influences the amount of PGR active ingredient absorbed by the media and thus, the magnitude of the response. For example, plugs of calibrachoa ‘Callie Dark Blue’ dipped in Sumagic (uniconazole, Valent Professional Products) at 2, 4, or 8 ppm were 37, 41, and 59 percent shorter than nontreated plants when measured four weeks after application (Figure 2 below). Table 1 provides a list of suggested application rates for bedding plants that we have studied at Michigan State University. These rates are based on growing conditions in Michigan during mid- to late spring. Growers are advised to perform their own trials on a small scale to determine appropriate application rates for their growing conditions and desired response.

Application rate for a plug dip may be comparable to a rate used with a PGR drench application. However, an advantage of a plug dip compared to a drench is that there may be less potential for a flowering delay with a plug dip because the PGR does not contact the stems or leaves.

Table 1. Suggested plug dip rates for growers in northern climates.
Growers should perform their own trials on a small scale and adjust
these rates based upon the desired response.

Species Paclobutrazol
(Bonzi, Piccolo)
Uniconazole
(Concise, Sumagic)
Argyranthemum 6 to 8 ppm 3 to 4 ppm
Bacopa 4 to 8 ppm not tested
Calibrachoa 6 to 8 ppm 2 to 4 ppm
Coleus 6 to 10 ppm not tested
Nemesia not tested 2 to 4 ppm
Osteospermum 4 to 8 ppm not tested
Petunia Cascadias 4 to 6 ppm
Scaevola 4 to 6 ppm 2 to 4 ppm
Sweet potato vine 1 to 3 ppm not tested
Verbena 8 to 12 ppm 2 to 4 ppm
Wave petunia 6 to 8 ppm not tested


2. Duration of plug dip

The duration of the plug dip can affect the volume of solution that is absorbed by the media. We recommend that growers begin with a 30-second dip and adjust the time if necessary. If the media is saturated at the time of application, there may be little difference in response between a 30-second versus a 2-minute dip. However, if the plug is dry at application, a 2-minute dip will absorb more solution than a shorter duration and therefore increase the response.

3. Moisture content of media

The moisture content of the plug media will affect how much solution is absorbed during the dip. A plug that is dry at the time of application will absorb more of the PGR solution than a saturated plug and therefore produce a greater response. For example, moist plugs of verbena ‘Escapade Bright Eye’ dipped in Piccolo (paclobutrazol, Fine Americas) for 30 seconds were 6.5 inches taller when measured six weeks after application than plugs with dry media at the time of the dip (Figure 3 below). We recommend that plugs are moderately dry at the time of application. This can be accomplished by irrigating the plug trays during late afternoon on the day prior to the dip and performing the application on the following morning. In summary, a PGR plug dip is a very effective height control strategy for controlling stem extension in aggressive species. There are several factors to consider when using this technique to maintain consistency between applications. Each greenhouse operation should develop a repeatable application protocol that provides the desired height control. We encourage growers that are not using this PGR application technique to give it a try.

Figure 2
Figure 2. The effects of a 30-second plug dip in Sumagic
or Piccolo before transplant on stem extension of
calibrachoa ‘Callie Dark Blue’. Plugs were
moderately dry at application. Please click on image
to view larger version.

Figure 3
Figure 3. The effects of a Piccolo plug dip with moist
or dry media on stem extension of verbena ‘Escapade
Bright Eye’. Plugs were dipped in Piccolo at 12 ppm for
30 seconds before transplant. Please click on image to
view larger version.

Dr. Runkle’s work is funded in part by MSU‘s AgBioResearch

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