Plant growth regulators (PGRs) are excellent management tools for controlling excessive stem elongation. By far the major workhorse used by growers is paclobutrazol (Piccolo, Piccolo 10XC, Bonzi, Paczol, Downsize) because it has a high level of activity and it is cost-effective. Many large growers often apply paclobutrazol overhead by boom irrigation. In addition, some collect and re-circulate their irrigation water. This can lead to a build up of paclobutrazol and other crop protection chemicals in a re-circulating irrigation system.
Some growers with re-circulating irrigation systems have noticed that sensitive plants such as wax begonias (Photo 1), pansies, and vinca appear to be stunted. Tests run by Fine Americas on irrigation water samples from re-circulating systems confirmed the presence of a low level of paclobutrazol [<100 parts per billion (ppb)]. While this level is extremely low, it still can have a negative effect on growth of sensitive species. Growers are concerned and have asked for a solution to this stunted growth.
Fresco – A Growth Enhancer
It has been asked if Fresco can be used to counteract the paclobutrazol effect? Fresco is labeled as a treatment to overcome the effects of an over application of gibberellins-inhibiting PGRs such as paclobutrazol, uniconazole (Concise/Sumagic), flurprimidol (Topflor), and ancymidol (Abide/A-Rest). Fresco is a combination of 6-BA and GA4+7 and is used to promote growth.
Fresco can be applied as a foliar spray, substrate drench or through chemigation. Typical recommended spray rates are in the range of 1 to 5 ppm. One should begin with the lowest rate, make the application and then wait 7 days to determine if the desired level of growth is achieved. Reapplication can be made if additional growth enhancement is desired. The goal is to apply only enough Fresco to overcome the paclobutrazol effect. Too high of a rate will result in excessive stem stretch and a light yellowing of the newly developing leaves. Growers have found that the 1 to 5 ppm range works in most cases, but growers have reported that the response rate can vary significantly by cultivar. So it is best to start with a small trial to determine optimal rates.
Fresco Rate Trials to Counteract Paclo Residual in Re-circulating Irrigation Water
Fine Americas approached North Carolina State University to conduct a Fresco foliar spray trial to determine suitable rates for overcoming very low rates of paclobutrazol in the irrigation water. We surveyed a number of growers to determine the typical irrigation regime used with re-circulating water systems and overhead boom watering is the most common method. This provided a baseline of the degree of growth control from leaf, stem, and substrate uptake of paclobutrazol in re-circulating watering systems.
Wax begonia plugs (Super Olympia Rose) were potted into 1801 cell packs filled with Fafard 1P (a peat-based substrate). We mixed four different irrigation water treatments: (1) 0 ppb paclobutrazol + 100 ppm N, (2) 5 ppb paclobutrazol + 100 ppm N, (3) 10 ppb paclobutrazol + 100 ppm N, and (4) 50 ppb paclobutrazol + 100 ppm N. The paclobutrazol used was Piccolo 10XC and the fertilizer used was 17-4-17. The irrigation water treatments were applied overhead each time the plants required watering, with a total of 10 irrigations being made over the course of the experiment.
To counteract the effects of the paclobutrazol, Fresco foliar sprays of: 0, 1, 3, or 6 ppm were applied twice (2 and 3 weeks after the plugs were transplanted) to each of the paclobutrazol irrigation water treatments. Thus the plants received either 0, 2, 6, or 12 ppm Fresco in total.) This set-up gave a 4 paclobutrazol rate x 4 Fresco foliar spray rate factorial experiment. [Note we took the approach of applying Fresco as a foliar spray to target only sensitive plant species. Including Fresco in the irrigation water would not be as cost effective as a foliar spray to targeted paclobutrazol sensitive plants.]
What we found?
Even the low parts per billion dose of paclobutrazol in the irrigation water resulted in growth control (Photo 2). As the dose increased from 0 to 50 ppb, the wax begonia plants were more severely affected with reduced growth.
To counteract the paclobutrazol effect, the application of Fresco foliar sprays provided excellent results (Photo 3). When the paclobutrazol irrigation rate was 5 ppb, a 1 ppm Fresco foliar spray applied twice overcame the PGR effect. With 10 ppb of paclobutrazol in the irrigation water, the rate of 1 to 3 ppm of Fresco applied twice was sufficient. At the highest paclobutrazol rate studied (50 ppb), a Fresco rate of 3 ppm applied twice was optimal for promoting growth.
Rates higher than the optimal range listed above resulted in excessive stem stretch and upper leaf yellowing (Photo 4).
Take Home Message
Until a suitable technique can be found to remove chemical residues from re-circulating irrigation water, Fresco foliar sprays offer a cost effective method of counteracting the undesired affect of paclobutrazol residues in greenhouse re-circulating water systems on particularly sensitive plants. Individual growers will need to conduct their own tests to determine the level of paclobutrazol in their re-circulating irrigation water. Growers with their own trials can determine optimal Fresco foliar spray rates to enhance plant growth, using our results as a starting point, on sensitive plant such as wax begonias, pansies and vinca.